John wiley sons synthetic methods in step growth polymers eds rogers long 2003

A collection of these original papers offers an interesting andinformative insight into the development of polymer science and the industry that fun-it spawned.7 1.1.2 Applications In ge

SYNTHETIC METHODS

IN STEP-GROWTH

POLYMERS

Courtesy of Lou Germinario, Eastman Chemical Company.

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Copyright  2003 by John Wiley & Sons, Inc All rights reserved.

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Library of Congress Cataloging-in-Publication Data Is Available

Synthetic methods in step-growth polymers / edited by Martin E.

Rogers and Timothy Long.

p cm.

Includes index.

ISBN 0-471-38769-X (cloth)

1 Polycondensation 2 Plastics I Rogers, Martin E.

II Long, Timothy E., 1969 –

QD281.P6S96 2003

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

A CAMERONCHURCH Department of Chemistry, University of Florida, ville, FL 32611-7200

Gaines-JEFFDODGE Bayer Corporation, Pittsburgh, PA 15205

ALAINFRADET Chimie des Polym`eres, Universit´e Pierre et Marie Curie, Paris,France

REINOUDJ GAYMANS Twente University, Chemistry and Technology of neering Plastics, 7500 AE Enschede, The Netherlands

Engi-S LIN-GIBSON Polymers Division, NIST, Gaithersburg, MD 20899-8543

QIAO-SHENGHU Department of Chemistry, City University of New York, lege of Staten Island, Staten Island, NY 10314

Col-TIMOTHYE LONG Department of Chemistry, Virginia Polytechnic Institute andState University, Blacksburg, VA 24061

R MERCIER LMOPS, 69390 Vernaison, France

J E MCGRATH Department of Chemistry, Virginia Polytechnic Institute andState University, Blacksburg, VA 24061

JAMESH PAWLOW Department of Chemistry, University of Florida, Gainesville,

FL 32611-7200

D PICQ LMOPS, 69390 Vernaison, France

MALCOLM B POLK Georgia Institute of Technology, School of Textile andFiber Engineering, Atlanta, GA 30332-0295

J S RIFFLE Department of Chemistry, Virginia Polytechnic Institute and StateUniversity, Blacksburg, VA 24061

MARTIN E ROGERS Luna Innovations, Blacksburg, VA

B SILLION SCA 69390 Vernaison, France

JASONA SMITH University of Florida, Department of Chemistry, Gainesville,

KENNETHB WAGENER Department of Chemistry, University of Florida, ville, FL 32611-7200

Gaines-SHENG WANG Department of Chemistry, Virginia Polytechnic Institute andState University, Blacksburg, VA 24061

1 Introduction to Synthetic Methods in Step-Growth Polymers 1

Martin E Rogers, Timothy E Long, and S Richard Turner

4.4 Synthetic Methods 246

5 Polyimides and Other High-Temperature Polymers 265

B Sillion, R Mercier, and D Picq

Sheng Wang and J E McGrath

6.7 Miscellaneous Poly(arylene ether)s, Poly(arylene

7 Chemistry and Properties of Phenolic Resins and Networks 375

S Lin-Gibson and J S Riffle

8 Nontraditional Step-Growth Polymerization: ADMET 431

A Cameron Church, Jason A Smith, James H Pawlow, and

Kenneth B Wagener

CONTENTS ix

8.4 Synthetic Methods: Silicon-Containing Polymers,

Step-growth polymerization continues to receive intense academic and industrialattention for the preparation of polymeric materials used in a vast array of appli-cations Polyesters used in fibers, containers and films are produced globally at arate of millions of metric tons per year Polyamides (1.7M metric tons) and poly-carbonates (1.6M metric tons) led the global engineering polymers marketplace in

2000 High temperature engineering liquid crystalline polyesters were projected

to grow an amazing 13 to 15% per year from 2001–2006 A step-wise ization mechanism serves as the fundamental basis for these polymer products,and future discoveries will require fundamental mechanistic understanding andkeen awareness of diverse experimental techniques

polymer-This text was not intended to be comprehensive, but serve as a long-standingresource for fundamental concepts in step-growth polymerization processes andexperimental methodologies Ten invited chapters provide a review of majorclasses of macromolecules prepared via step-growth polymerization, includingpolyesters, polyamides, polyurethanes, polyimides, poly(arylene ethers), and phe-nolic resins Moreover, recent advances in acyclic diene metathesis polymeriza-tion and transition metal coupling represent exciting new directions in step-growthprocesses The final chapter describes processes for subsequent recycling anddepolymerization of step-growth polymers, which are important considerations

as we attempt to minimize the negative impact of step-growth polymers on ourenvironment In addition to providing a literature review of this rapidly evolv-ing research area, special attention was devoted to the incorporation of detailedexperimental methodologies enabling researchers with limited polymerizationexperience to quickly impact this field We would like to express our gratitude

to the chapter authors for their valuable contributions, and we hope that this textwill cultivate new ideas and catalyze discoveries in your laboratory

MARTIN E ROGERS

TIMOTHY E LONG

xi

 2003 John Wiley & Sons, Inc ISBN: 0-471-38769-X

1

In these early days of polymer science, the correlation of structure and erty in the newly synthesized structures was a daunting challenge As Carotherssaid, “problem of the more precise expression of the relationships between thestructures and properties of high polymers is complicated by the fact that some

prop-of the properties prop-of this class prop-of substances which are prop-of the greatest practicalimportance and which distinguish them most sharply from simple compoundscan not be accurately measured and indeed are not precisely defined Examples

of such properties are toughness and elasticity” (ref 6, p 317)

Today, step-growth polymers are a multi-billion-dollar industry The basic damentals of our current understanding of step-growth polymers from monomerfunctionality to molecular weight distribution to the origins of structure–propertyrelationship all had their beginnings in the pioneering work of Carothers andothers at DuPont A collection of these original papers offers an interesting andinformative insight into the development of polymer science and the industry that

fun-it spawned.7

1.1.2 Applications

In general, step-growth polymers such as polyesters and polyamides possessmore robust mechanical properties, including toughness, stiffness, and highertemperature resistance, than polymers from addition polymerization processessuch as polyolefins and other vinyl-derived polymers Even though many com-mercial step-growth polymerization processes are done on enormous scale usingmelt-phase processes, most step-growth-based polymers are more expensive thanvarious vinyl-based structures This is, at least in part, due to the cost of themonomers used in step-growth polymerizations, which require several steps fromthe bulk commodity petrochemical intermediates to the polymerizable monomer,for example, terephthalic acid from the xylene stream, which requires oxidationand difficult purification technology These cost and performance factors are key

to the commercial applications of the polymers

Most of the original application successes for step-growth polymers were

as substitutes for natural fibers Nylon-6,6 became an initial enormous successfor DuPont as a new fiber Poly(ethylene terephthalate) (PET) also found itsinitial success as a textile fiber An examination of the polymer literature in the1950s and 1960s shows a tremendous amount of work done on the propertiesand structures for new fibers Eventually, as this market began to mature, theresearch and development community recognized other commercially importantproperties for step-growth polymers For example, new life for PET resultedfrom the recognition of the stretch-blow molding and barrier properties of thisresin This led to the huge container plastics business for PET, which, althoughmaturing, is still fast growing today

The remainder of this introductory chapter covers a few general but importantparameters of step-growth polymerization References are provided throughoutthe chapter if further information is desired Further details of specific polymersmade by step-growth polymerization are provided in subsequent chapters withinthis book

STRUCTURE–PROPERTY RELATIONSHIPS IN STEP-GROWTH POLYMERS 3 1.2 STRUCTURE –PROPERTY RELATIONSHIPS IN

STEP-GROWTH POLYMERS

1.2.1 Molecular Weight

Polymers produced by step-growth polymerization are composed of molecules with varying molecular weights Molecular weights are most oftenreported as number averages,Mn, and weight averages,Mw Rudin, in The Ele-

macro-ments of Polymer Science and Engineering, provides numerical descriptions of

molecular weight averages and the derivation of the molecular weight averages.8

Other references also define molecular weight in polymers as well as methodsfor measuring molecular weights.8 – 11Measurement techniques important to step-growth polymers include endgroup analysis, size exclusion chromatography, lightscattering, and solution viscometry

The physical properties of polymers are primarily determined by the molecularweight and chemical composition Achieving high molecular weight during poly-merization is critical if the polymer is to have sufficient thermal and mechanicalproperties to be useful However, molecular weight also influences the polymermelt viscosity and solubility Ease of polymer processing is dependent on the vis-cosity of the polymer and polymer solubility High polymer melt viscosity andpoor solubility tend to increase the difficulty and expense of polymer processing

documented.12 – 14Below a critical molecular weight, the melt viscosity increases

in proportion to an increase in molecular weight At this point, the viscosity isrelatively low allowing the material to be easily processed When the molecularweight goes above a critical value, the melt viscosity increases exponentially withincreasing molecular weight At higher molecular weights, the material becomes

so viscous that melt processing becomes more difficult and expensive

Several references discuss the relation between molecular weight and physicalproperties such as the glass transition temperature and tensile strength.15 – 17 Thenature of thermal transitions, such as the glass transition temperature and crystal-lization temperature, and mechanical properties are discussed in many polymertexts.8,17,18 Below a critical molecular weight, properties such as tensile strengthand the glass transition temperature are low but increase rapidly with increasingmolecular weight As the molecular weight rises beyond the critical molecularweight, changes in mechanical properties are not as significant When developingpolymerization methods, knowledge of the application is necessary to determinethe target molecular weight For example, polymers used as rigid packaging orfibers require high strength and, consequently, high molecular weights

Thermoplastic commercial step-growth polymers such as polyesters, carbonates, and polyamides are generally made with number-average molecularweights in the range of 10,000–50,000 g/mol Polymers within this molecularweight range are generally strong enough for use as structural materials yet lowenough in melt viscosity to be processable at a reasonable cost

poly-Thermosetting resins are combined with fibers and other fillers to form sites.19 Thermosetting resins with low viscosities are necessary to wet fibers or

compo-other fillers and to allow efficient processing and application prior to curing.When preparing thermosetting resins, such as unsaturated polyesters, phenolics,and epoxides, it is necessary to minimize viscosity by severely limiting molecu-lar weight.

For example, the molecular weight of unsaturated polyesters is controlled toless than 5000 g/mol The low molecular weight of the unsaturated polyesterallows solvation in vinyl monomers such as styrene to produce a low-viscosityresin Unsaturated polyesters are made with monomers containing carbon–carbondouble bonds able to undergo free-radical crosslinking reactions with styreneand other vinyl monomers Crosslinking the resin by free-radical polymerizationproduces the mechanical properties needed in various applications

Step-growth polymerizations can produce polymers with a wide range ofphysical properties Polysiloxanes made from the step-growth polymerization ofsilanols have among the lowest glass transition temperatures Polydimethyl silox-anes have a glass transition temperature near−125◦C On the other hand, step-growth polymerization produces polyimides and polybenzoxazoles with glasstransition temperatures of 300◦C to over 400◦C.20,21

Even within a particular class of polymers made by step-growth tion, monomer composition can be varied to produce a wide range of polymerproperties For example, polyesters and polyamides can be low-Tg, amorphousmaterials or high-Tg, liquid crystalline materials depending on the monomer com-position

polymeriza-The dependence of polymer properties on chemical compositions is reviewed

in basic polymer texts.9,10The backbone structure of a polymer defines to a largeextent the flexibility and stability of a polymer molecule Consequently, a greatrange of polymer properties can be achieved within each class of step-growthpolymers by varying the backbone structure using different monomers

The most common backbone structure found in commercial polymers is thesaturated carbon–carbon structure Polymers with saturated carbon–carbon back-bones, such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, andpolyacrylates, are produced using chain-growth polymerizations The saturatedcarbon–carbon backbone of polyethylene with no side groups is a relativelyflexible polymer chain The glass transition temperature is low at −20◦C forhigh-density polyethylene Side groups on the carbon–carbon backbone influencethermal transitions, solubility, and other polymer properties

Nearly all of the polymers produced by step-growth polymerization containheteroatoms and/or aromatic rings in the backbone One exception is polymersproduced from acyclic diene metathesis (ADMET) polymerization.22 Hydrocar-bon polymers with carbon–carbon double bonds are readily produced usingADMET polymerization techniques Polyesters, polycarbonates, polyamides, andpolyurethanes can be produced from aliphatic monomers with appropriate func-tional groups (Fig 1.1) In these aliphatic polymers, the concentration of thelinking groups (ester, carbonate, amide, or urethane) in the backbone greatlyinfluences the physical properties

STRUCTURE–PROPERTY RELATIONSHIPS IN STEP-GROWTH POLYMERS 5

C

O (CH 2 )x C

Increasing the methylene content increases the melting point, eventually ing toward theTmof polyethylene at low linking group concentrations The linearaliphatic polyesters and polycarbonates have relatively lowTg’s (−70 to−30◦C)and melting points below 100◦C The linear aliphatic polyesters and polycar-bonates are not used as structural materials due to the low melting temperaturesand limiting hydrolytic stability Aliphatic polyesters are used as soft-segmentpolyols in polyurethane production

tend-In contrast to the polyesters and polycarbonates, the linear aliphatic polyamidesand polyurethanes have high melting points and higher glass transition temper-atures as the amide and urethane linking groups participate in intermolecular

hydrogen bonding In Chapter 3 of Polymer Chemistry, Stevens discusses the

influence of hydrogen bonding in polyamides compared with polyesters.9Stevensnotes that poly(hexamethylene adipamide) melts at 265◦C compared to 60◦C forpoly(hexamethylene adipate).9

Aromatic groups in the polymer backbone bring rigidity and thermal stability

to the polymer molecule (Fig 1.2) Consequently, the demands of high-strengthand high-temperature applications are met by polymers with a high aromaticcontent in the backbone Polymers with a particularly high aromatic content canshow main-chain liquid crystallinity

Aromatic polymers are often more difficult to process than aliphatic polymers.Aromatic polyamides have to be processed from very aggressive solvents such

as sulfuric acid The higher melting temperatures and viscosity also make meltprocessing more difficult Thermal stability and processing of aromatic polymerscan be balanced by the use of flexible spacing groups in between aromatic rings

on a polymer backbone Hexafluoroisopropylidene, isopropylidene, oxygen, bonyl, and sulfonyl bridging groups between rings increase opportunities for bondrotation, which decreasesTg’s and increases solubility Also, incorporating non-symmetrical monomers with meta and ortho linkages causes structural disorder

car-in the polymer chacar-in, improvcar-ing processability Flexible groups pendant to anaromatic backbone will also increase solubility and processability

The following chapters will provide detailed discussions of the perty relations with various classes of step-growth polymers

structure–pro-1.2.2 Polymer Architecture

Block copolymers are composed of two different polymer segments that arechemically bonded.23,24The sequential arrangement of block copolymers can varyfrom diblock or triblock copolymers, with two or three segments respectively,

to multiblock copolymers containing many segments Figure 1.3 is a schematicrepresentation of various block copolymer architectures The figure also includesgraft and radial block copolymers Step-growth polymerization can be used effec-tively to produce segmented or multiblock copolymers and graft copolymers.Well-defined diblock and triblock copolymers are generally only accessible bychain-growth polymerization routes

A variety of morphologies and properties can be achieved with separated block copolymers Copolymers of hard and soft polymer segments have

microphase-STRUCTURE–PROPERTY RELATIONSHIPS IN STEP-GROWTH POLYMERS 7

A −B Copolymer

A −B−A Triblock copolymer

Radial or star copolymer

Graft copolymer

a variety of properties depending on their composition Copolymers with smallamounts of a soft segment will behave as a toughened glassy polymer whilecopolymers made predominately of the soft segment will act as a thermoplas-tic elastomer

The thermal properties of block copolymers are similar to physical blends

of the same polymer segments Each distinct phase of the copolymer displaysunique thermal transitions, such as a glass transition and/or a crystalline meltingpoint The thermal transitions of the different phases are affected by the degree

of intermixing between the phases

Segmented or multiblock copolymers can be made by combining a ally terminated oligomer or prepolymer with at least two monomers To form a

function-segmented copolymer, the backbone oligomer must not be able to participate ininterchange reactions with the monomers For example, combining a polyesteroligomer with a diacid and diamine in a melt polymerization might result ininterchange reactions between the monomers and the ester linking groups in theoligomer backbone In this case a random polyesteramide copolymer would beproduced instead of a segmented copolymer Commercial examples of segmentedcopolymers produced by step-growth polymerization include polyester–polyether,polyurethane–polyether, and polyurethane–polyester copolymers.

Multifunctional monomers with functionality greater than 2 can be used toform three-dimensional polymer structures during step-growth polymerization.Incorporating multifunctional monomers, Ax, with AA and BB monomers results

in crosslinking between polymer chains and eventual gelation The point at whichgelation occurs depends on the average functionality of the monomer mixture andthe conversion of functional groups.25

Adding small amounts of multifunctional monomers results in branching ofthe main polymer chain The branched polymer will have a higher polydispersityand melt viscosity than analogous linear polymers Branching agents are oftenused to modify the melt viscosity and melt strength of a polymer Branching instep-growth polymers also changes the relationship between melt viscosity andthe shear applied to a melt Branched polymers tend to undergo a greater degree

of shear thinning than unbranched linear polymers

Monomers of the type AxBy are used in step-growth polymerization to produce

a variety of polymer architectures, including stars, dendrimers, and hyperbranchedpolymers.26 – 28The unique architecture imparts properties distinctly different fromlinear polymers of similar compositions These materials are finding applica-tions in areas such as resin modification, micelles and encapsulation, liquidcrystals, pharmaceuticals, catalysis, electroluminescent devices, and analyticalchemistry

Dendrimers are characterized by highly regular branching following a strictgeometric pattern (Fig 1.4) Dendrimers are prepared in a multistep synthesisoften requiring purification between steps One method of producing dendrimers

is known as the divergent method.29 Using the divergent approach, dendrimergrowth starts at the core and proceeds radially out from the center Each layer isbuilt in a stepwise addition process

In the convergent method, dendrimer growth begins with chain ends of face functional groups” coupling with an ABy building block.30 This leads tothe next-generation dendron The process can be repeated to build larger den-drons Finally, the dendrons can be attached to a polyfunctional core producing

in a randomly branched macromolecule referred to as hyperbranch polymers

SYNTHESIS OF STEP-GROWTH POLYMERS 9

The hyperbranch polymers differ from dendrimers in that perfect branching isnot achieved and additional linear units are present in the molecule (Fig 1.5).The extensive branching in hyperbranched polymers prevents crystallization andresults in amorphous materials Hyperbranched materials are generally brittlewith low melt viscosity due to the lack of long chains to form entanglements.These properties can be exploited as functional modifiers in crosslinking resins,32

thermoplastic processing aids,33as well as components in adhesives and coatings

1.3 SYNTHESIS OF STEP-GROWTH POLYMERS

Many synthetic methodologies have been investigated for the synthesis of molecular-weight step-growth polymers However, only organic reactions thatproceed in a quantitative fashion (>99%) are suitable for the preparation ofhigh-molecular-weight linear polymers The susceptibility of the electrophiliccarbonyl to nucleophilic attack has received significant attention in step-growthpolymerization processes and is widely utilized in commercially important

high-Figure 1.5 Hyperbranch structure.

families of polymeric materials, including polyesters,34 polyamides, polyimides,polyurethanes, polycarbonates,35 epoxy resins,36 and phenol-formaldehydepolymers.37 Nucleophilic and electrophilic substitution reactions are alsoemployed in the synthesis of many other classes of step-growth polymers Forexample, poly(arylene ethers) are synthesized via the nucleophilic substitution

of an aryl halide with a diphenol in the presence of a basic catalyst Diversepolymer families are prepared using nucleophilic and electrophilic substitutionreactions in a step-growth polymerization, including aromatic poly(ketones),poly(arylates), poly(phenylene sulfides), poly(sulfones), and poly(siloxanes).38,39

Transition metal coupling has also received recent attention for the synthesis ofhigh-performance poly(arylenes) or poly(aryl alkenes).40,41 In addition, nonpolarpolymers are readily prepared via recent advances in step-growth polymerizationusing ADMET polymerization.42

A diverse array of polymeric compositions are attainable using step-growthpolymerization processes; however, many experimental criteria must be addressed

in order to achieve well-defined compositions and predictable molecular weights

In order to achieve high molecular weight in a step-growth polymerization cess, the synthetic methodologies described above must meet certain well-esta-

pro-blished criteria The following essential criteria are often cited for the successful

preparation of high-molecular-weight linear polymers:

SYNTHESIS OF STEP-GROWTH POLYMERS 11

1 high reaction conversions (>99.9%) as predicted using the Carothers’equation,

2 monomer functionality (f) equal to 2.0,

3 functional group stoichiometry equal to 1.0,

4 absence of deleterious side reactions that result in loss of monomer tionality,

func-5 efficient removal of polymerization condensates, and

6 accessibility of mutually reactive groups.43

Most introductory polymer textbooks discuss the growth of molecular weight for astep-growth polymerization process High molecular weight is not achieved untilhigh monomer conversions are reached.44 This is in sharp contrast to free-radicaladdition polymerizations where high-molecular-weight polymers are produced atrelatively low conversions

The Carothers equation relates the number-average degree of polymerization tothe extent of reaction and average functionality of a step-growth polymer In theCarothers equation, the number-average degree of polymerization,Xn, relates tothe extent of reaction,p, and average functionality,favg, of the polymer system:

2− pfavg

The molecular weight of a polymer will be reduced if either the extent of sion or the average functionality is decreased At 95% conversion of difunctionalmonomers, for example,Xn is only 20.25 The molecular weight is also related to

conver-a stoichiometric imbconver-alconver-ance,r, which is normally defined to be less than 1.0:

Xn− 1

Xn+ 1The number-average molecular weight of a polymer may be controlled by offset-ting the stoichiometry of two dissimilar mutually reactive difunctional monomers.The polymer will have the same endgroup functionality as that of the monomerused in excess For a generic polymer made from a difunctional monomer AAwith A functional groups and an excess of difunctional monomer BB with Bfunctional groups,r is defined as

NB

where NAis the moles of A functional groups and NB is the moles of B tional groups The amount of AA and BB monomer used is then 12NAand 12NB,respectively

func-The molecular weight can also be controlled by adding a monofunctionalmonomer The monofunctional endgroup, B, has the same functionality as mono-mer BB In this case, the moles of A functional groups in the difunctional

monomer, AA, is given as NA and the moles of AA is 1

2NA The moles of Bfunctional group in the difunctional monomer, BB, is given asNB and the moles

must be calculated To determineNBandN

B, two equations must be solved Thefirst comes from the above equation, which rearranges to

NB+ N

B= NA

By solving these two equations simultaneously, NB and N

B can be determined.Figure 1.6 summarizes the impact of the functional group conversion on themolecular weight.46 High reaction conversion (p) is required to achieve highmolecular weight for linear step-growth polymerization processes.47

Although most step-growth polymerizations involve the formation of a volatilecondensate, this is not a prerequisite for step-growth polymerization, and poly-urethane formation is a classic example of a step-growth polymerization that doesnot form a low-molar-mass condensate.48Thus, step growth defines the polymer-ization process in terms of the basic mechanism, and step-growth polymerization

is preferred terminology compared to earlier terms such as condensation merization However, in most instances when a condensate is formed, efficientremoval of the condensate using either low pressures (typically 0.1–0.5 mm Hg)

poly-or a dry nitrogen purge at high temperatures is required In addition, efficientagitation and reactor engineering have received significant attention in order tofacilitate removal of condensates and ensure accessibility of mutually reactivefunctional groups This is especially important in melt polymerization processeswhere the zero shear melt viscosity (η ) is proportional to the 3.4 power of the

SYNTHESIS OF STEP-GROWTH POLYMERS 13

Linear step-growth polymerizations require exceptionally pure monomers inorder to ensure 1 : 1 stoichiometry for mutually reactive functional groups Forexample, the synthesis of high-molecular-weight polyamides requires a 1 : 1molar ratio of a dicarboxylic acid and a diamine In many commercial pro-cesses, the polymerization process is designed to ensure perfect functional groupstoichiometry For example, commercial polyesterification processes often utilizedimethyl terephthalate (DMT) in the presence of excess ethylene glycol (EG) toform the stoichiometric precursor bis(hydroxyethyl)terephthalate (BHET) in situ.Step-growth polymerization processes must be carefully designed in order toavoid reaction conditions that promote deleterious side reactions that may result

in the loss of monomer functionality or the volatilization of monomers Forexample, initial transesterification between DMT and EG is conducted in thepresence of Lewis acid catalysts at temperatures (200◦C) that do not result inthe premature volatilization of EG (neat EG boiling point 197◦C) In addition,polyurethane formation requires the absence of protic impurities such as water toavoid the premature formation of carbamic acids followed by decarboxylation andformation of the reactive amine.50 Thus, reaction conditions must be carefullychosen to avoid undesirable consumption of the functional groups, and 1 : 1stoichiometry must be maintained throughout the polymerization process

As mentioned previously, the use of multifunctional monomers results inbranching The introduction of branching and the formation of networks aretypically accomplished using trifunctional monomers, and the average function-ality of the polymerization process will exceed 2.0 As the average function-ality increases, the extent of conversion for network formation decreases In

many instances, the trifunctional or higher functional monomers contain tive groups that are identical to the difunctional monomers For example, pen-taerythritol (f = 4) and 1,3,5-benzene tricarboxylic acid (f = 3) and trimelliticanhydride (f = 3) are commonly used in polyesterification Many novel fami-lies of step-growth polymers are attained through the judicious combination ofcontrolled endgroup functionality, extent of branching, and molecular weight.Hyperbranched step-growth polymers have received significant review in the lit-erature and are an exquisite example of controlled functionality and topologyusing well-defined monomer functionality.51

reac-REFERENCES

1 L H Beakland, U.S Patent 942, 699, 1907; L H Beakland, Ind Eng Chem., 1,

149 (1909).

2 W H Carothers, J Am Chem Soc., 51, 2548 (1929).

3 W H Carothers, J Am Chem Soc., 51, 2560 (1929).

4 W H Carothers, U.S Patents 2,130,947 and 2,130,948 (to DuPont), 1938.

5 J R Whinfield and J T Dickson, British Patent 578,079, June 14, 1946.

6 W H Carothers and F J van Natta, J Am Chem Soc., 52, 314 (1930).

7 H Mark and G S Whitby (Eds.), Collected Papers of Wallace Hume Carothers on High Polymeric Substances, Interscience Publishers, New York, 1940.

8 A Rudin, The Elements of Polymer Science and Engineering, 2nd ed., Academic, San

11 C S Wu, Handbook of Size Exclusion Chromatography, Dekker, New York, 1995.

12 M Alger (Ed.), Polymer Science Dictionary, 2nd ed., Chapman and Hall, London,

1997.

13 C K Schoff, in Encyclopedia of Polymer Science and Engineering, Vol 14, Wiley,

New York, 1988, p 466.

14 L J Fetters, D J Lohse, and S T Milner, Macromolecules, 32, 6847 – 6851 (1999).

15 J R Martin, J F Johnson, and A R Cooper, J Macromol Sci Revs Macromol.

Chem., C8(1), 57 – 199 (1972).

16 R W Nunes, J R Martin, and J Johnson, Polym Eng Sci., 22(4), 205 (1982).

17 R J Young, in Comprehensive Polymer Science: The Synthesis, Characterization, Reactions and Application of Polymers, Vol 2: Polymer Properties, Pergamon, New

York, 1989.

18 J E Mark, A Eisenberg, W W Graessley, L Mandelkern, E T Samulski, J L.

Koenig, and G D Wignall, Physical Properties of Polymers, 2nd ed., American

Chemical Society, Washington, DC, 1993.

19 S H Goodman, Handbook of Thermoset Plastics, 2nd ed., Noyes Publication,

West-wood, NJ, 1998.

23 A Noshay and J E McGrath (Eds.), Block Copolymers, Academic, New York, 1977.

24 R P Quirk, D J Kinning, and L J Fetters, Comprehensive Polym Sci., 7, 1 (1989).

25 G Odian, Principles of Polymerization, 3rd ed., Wiley, New York, 1991.

26 H R Kricheldorf, Pure Appl Chem, 70(6), 1235 (1998).

27 H R Kricheldorf, Macromol Symp 122, 12 – 23 (1997).

28 K Inoue, Prog Polym Sci., 25, 453 (2000).

29 C N Moorefield and G R Newkome, in Advances in Dendritic Macromolecules,

Vol 1, G R Newkome (Ed.), JAI Press, Greenwich, CT, 1994.

30 C J Hawker and K L Wooley, in Advances in Dendritic Macromolecules, Vol 2,

G R Newkome (Ed.), JAI Press, Greenwich, CT, 1995.

31 A Sunder, J Heinemann, and H Frey, Chem Eur J., 6(14), 2499 (2000).

32 R Mezzenga, L Boogh, and J E Manson, Composites Sci Technol., 61, 787 – 795

(2001).

33 P Froehling and J Brackman, Macromol Symp., 151, 581 – 589 (2000).

34 I Goodman and J A Rhys, Polyesters, Elsevier, New York, 1975.

35 D G LeGrand and J T Bendler, Handbook of Polycarbonate Science and ogy, Marcel Dekker, New York, 2000.

Technol-36 B Ellis, Chemistry and Technology of Epoxy Resins, Blackie, Glasgow, United

King-dom, 1993.

37 J Ferguson, Macromol Chem., 3, 76 – 92 (1984).

38 I Yilgor and J E McGrath, Adv Polym Sci., 86, 1 (1988).

39 R W Dyson (Ed.), Siloxane Polymers, Chapman and Hall, New York, 1987.

40 E E Braunsteiner and J K Stille, Macromolecules, 12, 1033 (1979).

41 G K Noren and J K Stille, J Polym Sci (D), Macromol Rev., 5, 209 (1984).

42 K B Wagener and T A Davidson, “Non-Conjugated and Conjugated Dienes in

Acrylic Diene Metathesis (ADMET) Chemistry,” in New Macromolecular ture and Functions Proceedings OUMS ’95, M Kamachi and A Nakamura (Eds.),

Architec-Springer Verlag, New York, 1996.

43 M Chanda, Advanced Polymer Chemistry Marcel Dekker, New York, 2000.

44 P C Hiemenz, Polymer Chemistry: The Basic Concepts, Marcel Dekker, New York,

47 H R Allock and F W Lampe, Contemporary Polymer Chemistry, 2nd ed.,

Prentice-Hall, Englewood Cliffs, NJ, 1990.

48 D L Lyman, Rev Macromol Chem., 1, 191 (1966).

49 J R Fried, Polymer Science and Technology, Prentice-Hall, Upper Saddle River, NJ,

1995.

50 D J Sparrow and I G Walton, Macromol Chem., 3, 93 – 97 (1984).

51 P E M Allen and C R Patrick, Kinetics and Mechanisms of Polymerization tions, Wiley, New York, 1974.

Reac-2 Polyesters

Alain Fradet and Martine Tessier

Chimie des Polym`eres, Universit´e Pierre et Marie Curie, Paris, France

2.1 INTRODUCTION

Polyesters are defined as polymers containing at least one ester linking group perrepeating unit They can be obtained by a wide range of reactions, the most impor-tant being polyesterifications between dibasic acids and diols or their derivatives(Scheme 2.1)

•••

R2 •••

+ R 3 COOH ↑ C

Many other reactions have been reported for the synthesis of polyesters, such

as reactions between dicarboxylic acid salts and dialkyl halides, reactions between

 2003 John Wiley & Sons, Inc ISBN: 0-471-38769-X

17

chlorocarbonyloxy-terminated monomers and diacids, or reactions betweenbisketenes and diols These reactions, however, cannot be applied to the synthesis

of high-molar-mass polyesters under economically viable conditions and arelimited to very specific laboratory-scale syntheses Two notable exceptions arethe ring-opening polymerization of lactones and lactides for the production ofdegradable polyesters and the biosynthesis of aliphatic polyesters by bacteria orgenetically modified plants

The polyester family is extremely large and, depending on the nature of R1

and R2 (Scheme 2.1), exhibits an enormous variety of structures, architectures,properties, and, therefore, applications Aliphatic polyesters comprise (i) linearlow-molar-mass hydroxy-terminated macromonomers used in the synthesis

of polyurethanes, (ii) biodegradable and bioabsorbable thermoplastic linearpolyesters such as polylactides and polylactones, and (iii) the recently describedhyperbranched polyesters applied as crosslinkers in coatings or as rheologymodifiers in thermoplastics Aliphatic –aromatic polyesters such as poly(ethyleneterephthalate) (PET) and poly(butylene terephthalate) (PBT) are processedinto economically important fibers, films, and engineering thermoplastics.Wholly aromatic copolyesters exhibit superior mechanical properties and heatresistance and have found a number of applications as high-performancethermoplastics Unsaturated polyesters and alkyd resins (glyptal resins) arethermosetting resins widely used in the coating and composite industries Thepolyester family comprises not only thermoplastic polymers and thermosettingresins but also rubberlike polymers: Polyester thermoplastic elastomers (esterTPEs) — copolyesters of terephthalic acid, 1,4-butanediol, and dihydroxyaliphatic polyethers — associate the mechanical properties of rubber withthe processing characteristics of thermoplastics Many other ester-containingpolymers could be added to the polyester family, such as polyesteramides,obtained by reaction between diacids, diols and diamines; polyestercarbonates,prepared by reaction of ester-containing monomers; or block copolyesters,prepared by reactions of polyester macromonomers Polycarbonates, sometimesconsidered as a special class of polyesters, are described in Chapter 3

Polyesters are now one of the economically most important classes of mers, with an overall world production between 25 and 30 million tons in 2000,consisting mostly of PET This production is rapidly increasing and is expected

poly-to continue poly-to do so during the next decade, driven by packaging applications,due to a very favorable image of environmentally friendly and recyclable poly-mers in western countries, and by textile applications, due to a strong demand inthe far-east area to satisfy the needs of an increasing population

The synthesis and properties of polyesters have been treated in numerousencyclopedia and general review articles.1 – 9 This chapter, therefore, does notdiscuss these aspects extensively but focuses more on recent academic andindustrial research developments in polyesters and on the practical aspects ofpolyester synthesis

INTRODUCTION 19 2.1.1 History

It has been known since the early nineteenth century that heating carboxylicpolyacids and glycerol resulted in resinous compounds, the exact composition

of which remained unknown In the 1910–1920s, the General Electric Companyled extensive research on the chemistry of phthalic anhydride–glycerol reactionand developed the technology of alkyd resins (glyptal resins), which are essen-tially polyesters of phthalic anhydride, glycerol, and monocarboxylic unsaturatedfatty acids These resins, readily soluble in aromatic or aliphatic hydrocarbons,are still widely used in coatings, varnishes, and paints However, the modernhistory of polyesters began in the 1930s when Carothers10 – 12proved the macro-molecular theory of Staudinger from experimental studies on reactions betweenaliphatic dibasic acids and diols and established the relationships between degree

of polymerization, conversion, functionality, and gel point, that is, the base tionships of step-growth polymerization These polyesters had low melting points,were sensitive to hydrolysis, and, therefore, were not suitable for commercialapplications They could not compete with aliphatic polyamides (nylons), alsodiscovered in the 1930s by Carothers at DuPont Company

rela-To increase polyester melting point and to approach the thermomechanicalproperties obtained with nylons, it was necessary to stiffen the polyester chain

by using rigid aromatic monomers instead of flexible aliphatic ones This wasrealized in the early 1940s in the laboratories of the Calico Printers Association

in the United Kingdom when Whinfield synthesized high-melting-point forming polyesters from terephthalic acid and aliphatic diols.13,14After World War

fiber-II the patent rights on these aliphatic–aromatic polyesters were shared betweenI.C.I and DuPont, and several members of this family became — and are stilltoday — major commercial polymers Poly(ethylene terephthalate) (PET) is nowone of the most produced polymers, primarily for textile and packaging applica-tions Poly(butylene terephthalate) (PBT) finds uses as solid-state molding resin.Poly(trimethylene terephthalate) (PTT), though described in Whinfield’s originalpatent, is a newcomer in the commercial polyester family and has found its firstapplications in the textile industry

At the end of the 1930s, when Carothers and Flory were studying aliphaticpolyesters, a new type of thermosetting resin appeared,15 based on unsaturatedpolyesters Unsaturated polyesters are synthesized by reacting mixtures of satu-rated and unsaturated dibasic acids or anhydrides with aliphatic 1,2-diols Thethermosetting resin is obtained by dissolving this linear polyester in an unsatu-rated monomer, such as styrene, capable of undergoing free-radical copolymer-ization with the unsaturations in polyester chains The liquid resin is transformedinto a rigid, insoluble, and infusible crosslinked polymer network after radicalpolymerization in the presence of heat or catalysts Unsaturated polyester (UP)resins found their first applications in combination with glass fibers for protectiveradar domes during World War II,8but the technology was widely commercially

available only in the mid-1950s UPs are now one of the most important matrixresin for glass-fiber-reinforced composite materials.

Although they were the first step-growth polymers fully characterized, it isinteresting to note that the first commercial applications of aliphatic polyestersappeared only in the late 1950s and the 1960s, with the development ofpolyurethane foams and elastomers (see Chapter 5)

The 1970s were a period of intensive research on block copolymers, especially

in step-growth polymerization A number of polyester block copolymerswere reported in the literature, but only one found commercial success: thepolyesterether block copolymers marketed by DuPont under the tradenameHytrel and exhibiting the characteristics of thermoplastic elastomers.16 Duringthis period and later in the 1980s, much attention was also focused on high-performance wholly aromatic polyesters Although the first publications on thesepolyesters appeared at the end of the 1950s, the commercial introduction ofthe first one, the amorphous poly(bisphenol-A isophthalate-terephthalate) (UnionCarbide’s Ardel), took place only in the mid-1970s.6 In spite of relativelyhigh cost, liquid crystalline thermotropic polyesters, such as poly(6-hydroxy-2-

naphthoic acid-co-4-hydroxybenzoic acid) (Ticona’s Vectra), described at the end

of the 1970s17, now find a number of applications in high-technology markets

In the 1990s, environmental concerns began to be gaining ground The tility of the ester linkage, able to undergo hydrolysis, alcoholysis, and acidolysis

versa-in some conditions, makes polyesters the polymers of choice to fulfill the versa-ing demand for recyclable and/or biodegradable polymers This has resulted in

increas-a renewed interest in increas-aliphincreas-atic polyesters, such increas-as poly(lincreas-actones), poly(lincreas-actides),

or copolyesters containing aliphatic moieties PET production is also stronglydriven by the demand of recyclable polymers

Polyester chemistry is the same as studied by Carothers long ago, but polyestersynthesis is still a very active field New polymers have been very recently or will

be soon commercially introduced: PTT for fiber applications; poly(ethylene thalate) (PEN) for packaging and fiber applications; and poly(lactic acid) (PLA),

naph-a biopolymer synthesized from renewnaph-able resources (corn syrup) introduced

by Dow-Cargill for large-scale applications in textile industry and solid-statemolding resins Polyesters with unusual hyperbranched architecture also recentlyappeared and are claimed to find applications as crosslinkers, surfactants, orprocessing additives

2.1.2 Applications

Polyesters are one of the most versatile classes of polymers ever produced,covering a wide range of properties and applications Polyesters are present infibers, engineering thermoplastics, and high-performance polymers as well as inthermosetting resins and elastomers Table 2.1 lists the chemical structure, abbre-viations, and uses of some commercially important thermoplastic polyesters

2.1.2.1 Thermoplastic Polyesters

In terms of volume and economic importance, thermoplastic polyesters are inated by PET, which has experienced a tremendous development in fibers and

dom-INTRODUCTION 21

molding resins due to a strong demand for textile applications and in food aging and bottle markets for glass replacement PET has a very favorable envi-ronmentally friendly image, and recycling is a supplementary force driving PETconsumption The 2000 world PET production can be estimated to be ca 26 milliontons and, according to the forecasts of PET industry, is expected to reach a total

pack-of 55 million tons by 2010.18 By comparison with the enormous volume of PETproduced, the production of other polyesters appears quite modest (Table 2.2)

2.1.2.1.1 Poly(ethylene terephthalate)

PET FIBERS Fibers are the largest end use for PET The world production of PETfibers surpassed that of nylon fibers in the beginning of the 1970s and has nowreached a level close to that of cotton, with about 18 million tons in 2000.19,20

PET fiber production is expected to grow during the next few years at a yearlyrate close to 7%, reaching 25 million tons by 2005.19 The growth of PET fiberproduction may be assigned to the combination of many factors: low productioncosts, excellent properties, ability to be blended with natural fibers such as cotton,growing population and increasing wealth in the far-east area, and, depending ontheir relative price, substitution for cotton or other man-made fibers such asacrylic and polyamides

Polyester fibers are available in a wide variety of fiber products: staple forblending, filament yarns, high-strength industrial yarns, and nonwoven fabrics.They are present in all segments of the textile market: garments, carpets, nonwo-ven, and upholstery Industrial applications include ropes, filters, conveyor belts,and tire cords The emergence of new high-technology polyester fibers such assilklike very fine PET microfibers is also worth mention

PET SOLID-STATE RESIN PET resin, the second largest application of PET, enced a double-digit yearly growth rate during the last decade21 and is expected

experi-to continue experi-to do so during the next one The 2000 world production of PETresin was close to 6.5 million tons, driven by food packaging applications, mainlyreheat stretch blow-molded bottles (87% of total resin) for carbonated and non-carbonated soft drinks and water.22,23 PET bottles are also used for a number ofother beverages and foods, including syrups, pickled food, and salad dressing.PET containers are also used in toiletries and cosmetics The excellent price-to-performance ratio of PET recently pushed the development of new, sophisticatedapplications: Multilayer and coated high-barrier PET containers are reaching thebeer bottle market in Europe and the United States This market is expected

to be a great opportunity of growth in PET demand during the next decade.23

Apart blow-molding applications, PET solid-state resin applications are extremelydiverse, ranging from sheet thermoforming applications to glass-fiber-reinforcedresin for injection-molding applications

PET FILM Due to a slow crystallization rate in the unoriented state but rapidcrystallization upon orientation, PET is extremely well suited for the biaxial filmorientation process In this process, the amorphous film obtained after extrusion

TABLE 2.2 World Production and Prices of Some Commercially Important Polyesters

aEstimated 2000 world production unless otherwise specified.

bFrom www.plasticsnews.com for pure resin unless otherwise specified.

cRefs 18–20.

dRef 443.

eRef 24.

fPEN film price, ref 444.

gData not available.

2.1.2.1.2 Other Aromatic–Aliphatic Thermoplastic Polyesters

PBT is mainly used as glass-fiber-reinforced engineering thermoplastic, althoughPBT fibers can also be made PBT crystallizes much more rapidly than PET and

PEN presents many advantages over PET in terms of thermomechanical mances However, the cost of dimethyl 2,6-naphthalene dicarboxylate monomerlimits its present use to specific applications, for example, high performance long-playing magnetic tapes, photographic film for advanced photo system (APS),high-strength fibers, and aerospace-related products Taking advantage of out-standing barrier properties, PEN and PEN/PET copolymers are also expected

perfor-to find new applications in the pasteurizable, hot-fill, and returnable/refillablecontainer and bottle markets.24

Poly(1,4-cyclohexylenedimethylene terephthalate) (PCT) finds some nicheapplications in the packaging market as amorphous, clear, extrusion, or injection-molding material for films and bottles

PTT was already mentioned in Whinfield and Dickson’s 1941 original patent13

that described PET and PBT However, no economically viable route to propanediol monomer was available until the 1990s when Shell developed aroute based on ethylene oxide hydroformylation and Degussa a route based onacrolein hydration/hydrogenation.25 Shell and DuPont launched respectively theirCorterra and Sorona PTT polymers in 2000 DuPont uses the acrolein route butplans to produce 1,3-propanediol from cornstarch by a biotechnological process

1,3-in 2003.26 PTT fibers present a combination of properties, including stretchrecovery, resilience, softness of touch, dyeability, and easy care which, makethem very attractive for the fabric, carpet, and apparel markets PTT is alsointended to compete for film and engineering thermoplastic applications withPET, PBT, and polyamides.27 According to Shell Chemicals, the PTT demand isexpected to exceed 1 million tons by 2010.28

2.1.2.1.3 Wholly Aromatic Polyesters

This polyester class comprises amorphous high-Tg copolyesters, known as phous polyarylates, and semicrystalline polyesters that often exhibit anisotropicliquid crystalline (LC) melts Liquid crystalline polyesters are often termed aspolyester LCPs

amor-Copolyesters of bisphenol-A and iso- and terephthalic acid are amorphousengineering thermoplastics with excellent heat and ultraviolet (UV) light resis-tance and general properties that compare well with other engineering resins.Their amorphous morphology imparts on them properties of transparency anddimensional stability with somewhat low solvent resistance Amorphous pol-yarylates are light-yellow to amber transparent resins that find a number of nicheapplications for injection-molded parts in automotive, electronics, safety andbuilding equipment — for example, headlight housings, fire helmets, face shields,and transparent exterior parts such as solar energy components and glazing

Polyarylates can be blended with a wide range of commercially available moplastics, including polyamides, polycarbonates, polyetherketones, polyesters,and poly(phenylene sulfide), thus broadening their application domain.

ther-Liquid crystalline polyesters are copolyesters of rigid para-oriented monomerssuch as 4-hydroxybenzoic acid, hydroquinone, terephthalic acid, or 2,6-hydroxynaphthoic acid They exhibit an LC mesophase in the molten state,usually a nematic one: Due to high stiffness, polymer chains tend to line upparallel to each other, instead of forming random coils and isotropic melts.This unique structure is retained in the semicrystalline solid forming materialstermed as “self-reinforced” thermoplastic resins, by analogy with fiber-reinforcedpolymers Polyester LCPs offer a combination of properties that makes themextremely well suited for precision molding of small parts: extremely high meltfluidity, high heat resistance and dimensional stability, exceptional tensile strengthand modulus in the flow direction, and low-temperature expansion coefficient.Their major market is electronics in applications such as miniature connectorswith thin walls and surface-mount interconnection devices where LCP flowproperties allow high yield molding.29,30 Other applications comprise precisionparts for audiovisual equipment and under-hood parts for automotive industry.Owing to their outstanding barrier properties to oxygen, carbon dioxide, and watervapor, applications as barrier layer in multilayer packaging materials are alsounder consideration.30 – 33Following analysts, LCP demand is growing at a yearlyrate higher than 20% from a 2000 production estimated to be ca 12,000 tons.34,35 2.1.2.1.4 Polyester Thermoplastic Elastomers

Thermoplastic elastomers (TPEs) are polymers that present the typical mechanicalproperties of rubber and the processing ease of thermoplastics Polyester TPEsare multiblock copolymers containing blocks of amorphous low-Tg polyethercovalently bonded to blocks of semicrystalline PBT polyester At service tem-perature, the material exhibits biphasic morphology with microphase separationbetween a low-Tg polyether-rich phase and a semicrystalline PBT-rich phase.The semicrystalline microphase acts as physical crosslinks for the amorphouslow-Tg regions imparting the material properties of an elastomer At processingtemperature, the PBT phase melts, allowing the polyester TPE to be processed

by conventional methods such as extrusion or injection-molding Polyesterethersare structurally related to linear polyurethanes (Chapter 5).16,36A typical member

of this class of polyesters is DuPont’s Hytrel, a

poly(poly(oxytetramethylene)-block -poly(butylene terephthalate)) obtained by replacing some of 1,4-butanediol

by dihydroxy-poly(oxytetramethylene) polyether in PBT synthesis Depending onthe polyether–PBT ratio, polyester TPE properties range from that of rubberypolymers to that of rigid engineering thermoplastics They replace thermoplas-tics when improved impact resistance, flexural and compressive properties, andspring characteristics are required: industrial and automotive hydraulic tubing,hoses, gaskets and bellows; autoclavable medical tubing; and jacketing for elec-trical and fiber-optics cable Because of their high water vapor permeability,some polyoxyethylene polyether-based grades are used as waterproof breathable

BIORESORBABLE POLYESTERS Bioresorbable implants, for instance orthopedic ations or sutures, are devices designed to slowly degrade in the body afterimplantation so that a second surgical intervention is not required for implantremoval after healing Polyesters and copolyesters of lactic acid and glycolicacid (Table 2.3) have been used as bioresorbable sutures since the 1960s Thesepolyesters exhibit a set of properties that makes them the ideal bioresorbablepolymers: They have good mechanical and processing properties, can be ster-ilized, give no toxic or inflammatory response, and are slowly and completelyhydrolyzed into natural metabolic by-products that are eliminated by the body.Since then, other polymers have been accepted for use as bioresorbable poly-mers: poly(dioxanone) (PDO), poly(trimethylene carbonate) (PTC), and poly(ε-caprolactone) (PCL) and their copolymers.38 Copolymer composition can beadjusted to fulfill a large range of properties and degradation times Polyestersproduced by microorganisms, such as poly(hydroxyalkanoic acid) (PHA), havealso been considered for use in biomedical devices.39,40 However, syntheticbiodegradable polymers are generally preferred for such applications since theyoffer a more reliable source of raw material with respect to antigenicity con-cerns.38 Bioresorbable polymers can be melt processed by conventional methodssuch as injection-molding, compression-molding, and extrusion Sterilization isachieved by irradiation or by exposure to ethylene oxide They are used in woundclosure (sutures, staples), osteosynthetic materials (orthopedic fixation devices:pins, screws, rods, bone plates), cardiovascular surgery (stents, grafts), and intesti-nal surgery (anastomosis rings).41They also find interesting applications as matrixmaterials for implanted drug release devices or drug-containing microspheres ormicrocapsules.41,42

fix-ENVIRONMENTALLY DEGRADABLE POLYESTERS The use of environmentally able/compostable harmless plastics — ecofriendly plastics — that disappear after

biodegrad-a few weeks or months in soil is biodegrad-an elegbiodegrad-ant wbiodegrad-ay of debiodegrad-aling with solid wbiodegrad-aste posal — a crucial problem for large modern cities Environmentally degradablepolymers are defined as polymers that are hydro-biodegradable and completelyconverted by microorganisms to carbon dioxide (or methane in anaerobic con-ditions), water, and biomass To be attractive, such polymers should presentthe thermomechanical properties of common plastics, typically between those oflow-density polyethylene and polypropylene, and should be melt processable by

INTRODUCTION 29

conventional methods Aliphatic polyesters and aliphatic –aromatic copolyestersare ideally suited to fulfill these specifications During the last few years, anumber of companies have put biodegradable polymers on the market Almostall these polymers are polyesters or copolyesters: aliphatic polyesters such aspoly(ε -caprolactone), poly(butylene-co-ethylene succinate), poly(lactic acid) and

microbial poly(hydroxyalkanoic acid), copolyesters of terephthalic acid with ious aliphatic diols and diacids, and polyesteramides (Table 2.3)

var-Price is of course the determining factor for applications including fast foodservice cups, containers and cutlery, paper coatings, disposable films for foodpackaging, ground covers, garden bags, and seed mats The biodegradable plasticsmarket is expected to reach 90,000 tons/year by 2003 with an estimated price

of U.S $3–$4/kg,43 making microbial polyesters by far too expensive for theseapplications.44

Special mention must be made of poly(lactic acid), a resorbable polyester, obtained from renewable resources through fermentation ofcorn starch sugar This polymer can compete with conventional thermoplasticssuch as PET for conventional textile fibers or engineering plastics applications.The first Dow-Cargill PLA manufacturing facility is scheduled to produce up to140,000 tons of NatureWorks PLA per year beginning in 200245 at an estimatedprice close to that of other thermoplastic resins: U.S $1/kg.46 Other plants areplanned to be built in the near future.45

biodegradable/bio-2.1.2.2 Polyester Polyols

The most representative member of this class of polyesters is the mass (M n≈ 1000–3000) hydroxy-terminated aliphatic poly(2,2
-oxydiethyleneadipate) obtained by esterification between adipic acid and diethylene glycol Thisoligomer is used as a macromonomer in the synthesis of polyurethane elastomersand flexible foams by reaction with diisocyanates (see Chapter 5) Hydroxy-terminated poly(ε-caprolactone) and copolyesters of various diols or polyols anddiacids, such aso-phthalic acid or hydroxy acids, broaden the range of propertiesand applications of polyester polyols

low-molar-Polyester polyols account for only ca 10% of the total polyol market, which isdominated by polyether polyols such as hydroxy-terminated polyoxyethylene orpolyoxypropylene Polyester polyols are preferred for applications where bettermechanical properties, wear resistance, and UV stability are required The largestapplication of polyester polyols is flexible specialty polyurethane foam in thefurniture, packaging, and automotive industries Polyester polyols are also usedfor nonfoam applications such as coatings, paints, sealants, and adhesives.47

2.1.2.3 Thermosetting Polyester Resins

2.1.2.3.1 Unsaturated Polyester Resins

Unsaturated polyesters are low-molar-mass polyesters obtained by melt condensation of 1,2-diols with saturated and unsaturated anhydrides or dibasic

poly-acids.7A typical unsaturated polyester composition consists of maleic anhydride,phthalic anhydride and/or isophthalic acid, and 1,2-propanediol For almost allapplications, unsaturated polyesters are diluted with a copolymerizable monomersuch as styrene The resulting syruplike unsaturated polyester resin (UPR) can

be compounded with fillers and/or glass fibers and cured in the presence of radical initiators It yields thermoset articles with a wide range of properties,depending on the composition of the initial unsaturated polyester, the nature andconcentration of crosslinking monomer, the presence of reinforcing fibers, andthe choice of initiator and catalytic systems

free-Cast UPRs are not reinforced with glass fibers but are filled with large amounts(up to 90%) of low-cost fillers Their applications include polyester cementsand mastics, simulated marble and wood, and various encapsulation compounds.UPR-laminating resins are sprayed on glass fiber or laid on glass cloth with

an initiator and used for molding, continuous lamination, filament winding, orpultrusion applications Gel coats are protective layers primarily used in themarine industry and consisting of pigmented, filled UPRs that afford high glosssurfaces with good environmental resistance They are usually sprayed with aperoxide initiator onto the mold before reinforcement with glass fiber and lami-nating resins The largest applications of UPRs are fiberglass-reinforced moldingresins for high-temperature and/or high-pressure cure Semifinished compoundscontaining highly reactive UPR, high-temperature initiators, reinforcing glassfiber, fillers, mold release agents, pigments, and thermoplastics to prevent shrink-age, are required for fast molding cycles in automated industries: the so-calledsheet molding compounds (“SMCs”) and bulk molding compounds (“BMCs”) areready-to-use compounds for compression-molding (SMC and BMC) or injection-molding (BMC) applications

The primary UPR end markets are construction, automotive, and marineindustries with applications such as house paneling, tub and shower applications,chemical-resistant storage tanks, pultruded profiles, and fiberglass compositeboat hulls The UPR industry is mature, with a world production close to1.7 million tons (Table 2.2), but must face two important issues: increasinglystrict regulations for styrene emissions and poor recycling potential for polyesterthermosets.48,49

2.1.2.3.2 Alkyd Resins

Alkyd resins are branched polyesters made by reacting dicarboxylic acids oranhydrides, polyols such as glycerol or pentaerythritol, and long-chain unsatu-rated monocarboxylic fatty acids derived from natural oils (e.g., linseed oil, soyaoil, or dehydrated castor oil).50 The unsaturations present in fatty acid chains aresusceptible to undergo oxidative crosslinking by air oxygen in the presence ofcatalytic amounts of siccatives, such as metal salt of organic acids Alkyd resinsare therefore well suited to applications such as air-drying varnishes or archi-tectural paints, typically in solvent-based formulations Their low cost and greatversatility make them the largest type of coatings produced worldwide Because

of regulations for reduction of volatile organic compounds (VOCs) emissions,

INTRODUCTION 31

their use is, however, slowly decreasing.51 To address this issue, waterbornealkyd resin formulations have been developed, but their application is limited bythe tendency of ester groups to hydrolyze in the basic aqueous systems used forsolubilization or dispersion

2.1.3 Issues and Research Trends

The most important trend in industrial and academic research on polyesters isperhaps due to the increasing environmental awareness of end consumers andthe resulting strong demand for biodegradable or recyclable materials made byecofriendly production processes (see Chapter 11) Most thermoplastic polyestersare versatile, fully recyclable polymers, harmless by nature, and this is one of thereasons for the good health and bright future of the polyester industry A number

of biodegradable/compostable copolyester compositions have been developed inthe recent years by the polyester industry to satisfy the demand for biodegradablepackaging materials (Table 2.3) Even the newly introduced PTT, which is other-wise a quite conventional aromatic –aliphatic polyester, is presented as a “green”polymer because one of its monomers (1,3-propanediol) can be produced fromcorn starch, a renewable resource This seems more justified for PLA, which, asmentioned above, is a biodegradable/bioresorbable recyclable polyester exhibitingquite interesting thermomechanical properties that can be produced from renew-able resources Other attempts are the biosynthesis of aliphatic polyesters bytransgenic plants to produce poly(hydroxyalkanoic acid)s (PHAs).44Biosyntheticprocesses based on transgenic potatoes or soybeans are expected to be econom-ically viable in the near future, with production costs as low as U.S $0.5/kgfor PHA.39 On the other hand, many research groups are involved in studies

on the conventional ring-opening polymerization of cyclic esters such as ide, lactide, or ε-caprolactone for the synthesis of biodegradable/bioresorbablepolyesters.52

glycol-Ester groups can undergo a variety of interchange reactions that are used forthe chemical modification of polyesters The chemical recycling of thermoplasticpolyesters into low-molar-mass reactive oligomers by alcoholysis or glycolysis

is described in Chapter 11 (e.g., the synthesis of hydroxy-terminated oligomersfor polyurethane applications) A number of efforts have also been put in thereactive extrusion of polyesters with various reactants, including polymeric ones,for the synthesis of copolyesters, block copolyesters, or modified polyesters.53 – 56

Polyester thermosets, both UPR and alkyd resins, are facing severe regulatoryissues for limiting VOC and toxic compound emissions Although these issuesare not new, only partly satisfying solutions have been proposed until now, andfurther research is clearly needed in this domain

The polymerization of cyclic low-molar-mass polycarbonates, polyarylates,and PBT to high-molar-mass thermoplastics has been extensively studied by theGeneral Electric Company during the last decade.57,58Due to very low viscosity,cyclic oligoesters can be processed like thermosetting resins but retain thermo-plastic properties in the final state, after polymerization in the presence of suitable