ORGANIC AND PHYSICAL CHEMISTRY OF POLYMERS phần 7 docx

Arbitrarily, this chapter is divided into three parts devoted respectively to thesynthesis of the following basic structures/architectures: • End-functionalized polymers [i.e., α- and α,

DEGRADATION REACTIONS OF POLYMERS 371

If Ch represents the chromophoric group, the following reactions can occur, wherethe exponents 1, 2, and 3 denote the singlet, doublet, and triplet states and theasterisk denote an excited state

In the case of ethylene/carbon monoxide copolymer, the degradation occursgenerally with the triplet state which exhibits the maximum lifetime and corre-sponds to a free-radical carried by the carbonyl group This radical evolves to giveeither

9.4.2.4. As for any organic molecule, the sensitivity to thermal degradation

of a polymer is in close relationship with the energy of its bonds Polymers are,however, less stable thermally than its homologous simple molecule This is due

to the disordered motion of the chains above the glass transition temperature andthe energy associated with it, which can be concentrated on a particular bond ofthe macromolecular backbone

to a chain reaction.

Hydrocarbon chains can also undergo a homolytic rupture of their carbon–

carbon bonds that are particularly weakened by head-to-head sterically hindered

+

Two situations can occur, depending upon the temperature applied with respect

to the polymer ceiling temperature (see Section 8.2.1) When this temperature isbelow its ceiling temperature, the free radicals generated undergo a rearrange-ment with stabilization of the species formed until disappearing by combination ordisproportionation:

AA

AA

,etc

DEGRADATION REACTIONS OF POLYMERS 373

With chains containing hetero-elements in their backbone, each polymer is a ticular case For example, in the case of cellulose, the following degradation isobserved above 180◦C:

OH

CH2OH OH

OH

CH2OH OH

O OH

CH2OH OH O

+

O

OH

CH2OH OH O

Cell

O

OH

OH HO

CH2O

OH

CH2O OH HO

O

Cell

O OH

O Cell

From the knowledge of structural parameters that determine the polymer mostability, the structure of the ideal thermostable polymer can be designed asfollows:

ther-• The interatomic chemical bonds should be strong

• The chains should be rigid and generate strong molecular interactions in order

to exhibit little mobility

degradation by depolymerization m

t

Mn

t

degradation by random chain breaking m

t t

Mn

Figure 9.2 Variations of both molar mass and mass of polymer samples with time during

degradation (1) depolymerization; (2) random chain breaking.

For example, the structure of the polyimide that is shown below meets thesecriteria; it is approximately thermostable up to 450◦C:

NO

ON

O

O

O

n

9.4.2.5. Polymers are sensitive to mechanical degradation that occur through

homolytic scissions, similarly to those caused by thermal degradation The tication of certain polymers in the molten state, in particular of natural rubberbefore addition of stabilizing additives, may cause a drastic reduction of their molarmasses; in the case of polypropene, thermal and mechanical degradations both com-bine their effects to degrade it The mechanical energy can also be provided by anultrasonic generator

mas-9.4.2.6. When several sources of energy are combined to cause degradation, their

synergism can be sometimes spectacular For instance, degradations by

photo-oxidation and thermal photo-oxidation are particularly effective —in particular, in the

degradation of polyethylene films

It is frequent that additives that are incorporated in polymers to either generate orimprove a given property impart an accelerating effect on the degradation process.The corresponding mechanisms are often complex but are not different from thebasic phenomena described above

STABILIZATION OF POLYMERS 375

9.5 STABILIZATION OF POLYMERS

The side effects due to degradation can be alleviated through a precise knowledge

of the mechanism involved For instance, the autocatalytic thermal rination of poly(vinyl chloride) and poly(vinylidene chloride) suggests that basicadditives can well stabilize them and prevent their degradation by neutralizing theHCl gradually formed

dehydrochlo-Bases utilized for this purpose can be either organic molecules (N,Ndiphenylurea, dihydropyridin, polyols, etc.) or salts or metallic oxides such as3PbO·PbSO4·H2O as well as barium, cadmium, calcium, zinc, lead carboxylates,

-or thiolates

Instead of preventing the dehydrochlorination process, which generates coloredconjugated sequences, it may be more appropriate to use additives that react with thechromophoric polyene formed and reduce the length of the conjugated sequences

A hypsochromic effect is observed in this case, which decreases the absorption inthe visible range

When the degradation of a polymer gives rise to free radicals, the addition ofcompounds that can trap and neutralize these radicals is the logical solution Forexample, polyolefins are stabilized by addition of substituted phenols or polyphe-nols, which act as antioxidants:

O+H

OH+

•

•

The phenoxy radical formed are too stable to propagate the reaction of degradation.Carbon black is also an excellent antioxidant that is commonly used when its col-oration is not a drawback for the application contemplated It is systematically used

to stabilize polyalkadienes as it contributes to the reinforcement of their mechanicalproperties in addition to its capacity to stabilize against heat and oxidation.There are two categories of products that can protect against photodegradation:

• The first corresponds to UV radiation absorbers that possess a molecular ture enabling them to absorb sunlight up to λ = 360 nm These compoundsare thus used as screen that absorb photons and dissipate thermally the cor-responding energy; derivatives of benzophenone are often used in this case.When the application contemplated permits, carbon black can also be utilized

struc-as a “total screen.”

• The second category is that of free radicals traps For example, hinderedamines are excellent light-stabilizing compounds: their oxidation generatesstable free radicals called nitroxide that are able to efficiently trap reactivefree radicals Very recently, such nitroxide radicals were used to prevent a

polymer from degrading by a free radical mechanism as shown below:

Tetramethylpiperidyloxyl (TEMPO)

N−O•

~~~~~~pol• + TEMPO• ~~~~~~pol-TEMPO

All the additives used should exhibit a very high compatibility with the mer to stabilize in order to prevent their migration to the surface and theirsubsequent elimination; the durability of their effect depends on this factor

poly-LITERATURE

E Mar´echal, Chemical Modification of Synthetic Polymers, in Comprehensive Polymer

Science, G C Eastmond, A Ledwith, S Russo, and P Sigwalt Pergamon Press., Oxford,

p 1, 1989

J C Arthur, Jr., Chemical Modification of Cellulose and its Derivatives in Comprehensive

Polymer Science, G C Eastmond, A Ledwith, S Russo, and P Sigwalt (Eds.), Pergamon

Press, Oxford, p 49, 1989

M Laz´ar, T Bleha, and J Rychl´y, Chemical Reactions of Natural and Synthetic Polymers,

Ellis Horwood Ltd., Chichester, 1989

N A Plat´e, A D Limanovich, and O V Noah, Macromolecular Reactions, Wiley,

Chichester, 1995

macromolecular synthesis The whole set of these methods is also called

macro-molecular engineering , and in many aspects this domain of polymer chemistry is

close to that of polymerization reactions and/or that of polymer chemical cation (see Chapters 7, 8, and 9)

modifi-The variety of potentially accessible macromolecular structures/architectures isendless, and only the most representative ones are described here Recent develop-ments in the field of “living” and/or “controlled” polymerizations further expandthe possibilities of macromolecular engineering

Arbitrarily, this chapter is divided into three parts devoted respectively to thesynthesis of the following basic structures/architectures:

• End-functionalized polymers [i.e., α- and (α,ω-di) functionalized polymers],including macromonomers

• Block and graft copolymers

• Polymers with complex topology

Organic and Physical Chemistry of Polymers, by Yves Gnanou and Michel Fontanille

Copyright  2008 John Wiley & Sons, Inc.

377

10.2 END-FUNCTIONALIZATION OF POLYMER CHAINS (SYNTHESIS

OF REACTIVE PRECURSORS)

Remark It is important to point out the difference between functionalized

polymers, which are polymers carrying functional groups, and functional polymers, which are those exhibiting a specific property and used for a par-

ticular application or a given function Functionalized polymers may also befunctional polymers

When an accurate control of the structure targeted is not necessary, it is relativelyeasy to obtain functionalized chain ends Widely used in industry, two methods offacile and straightforward functionalization are briefly described below

The simplest method to obtain α,ω-difunctionalized polymers is to resort tostep-growth polymerization and control the degree of polymerization of the formed

polycondensates by means of the stoichiometry (r ) of the initial reactants:

a < b

The Carothers relation (Section 7.2.1) for nonstoichiometric conditions gives

X n= 1+ r

r(1− 2p) + 1 where r = a/b is the stoichiometric imbalance and p is the extent of the reaction Alternatively, end-functionalization of growing chains can also be obtained via

transfer in chain addition polymerization For instance, dihydroxy polybutadiene

telechelics (i.e carrying one hydroxyl group at each chain-end) are industrially

produced by free radical polymerization of butadiene initiated by hydrogen peroxidewhich simultaneously gives rise to transfer reactions:

2 HO

H2O2

H2O2HO

of polymerization also called telomers For instance, the free radical polymerization

of vinyl monomers in halogenated solvents producesα,ω-halogenated oligomers:

END-FUNCTIONALIZATION OF POLYMER CHAINS (SYNTHESIS OF REACTIVE PRECURSORS) 379

A

ClA

Experimental conditions are generally selected in such a way that n and n
arelimited to few units Some of these telomers can subsequently serve as precursorsfor polycondensation reactions

To obtain better defined structures than the latter ones, it is advisable to rely on

“living” and/or “controlled” polymerizations for the production of functionalizedchains: the end-standing functions can be introduced at the initiation step uponselection of an appropriate initiator or at the end of the polymerization through adeactivating molecule carrying the desired functional group

10.2.1 Functionalization Through Initiator

Due to the high reactivity of the propagating active centers, it is often sary to protect the functional group carried by the initiator For instance, hydroxylfunctions in initiators for anionic polymerization require protection, and acetal func-tions are generally used to this end Such acetal functional groups are introduced

neces-at the ends of polystyrene chains by means of an acetal-containing alkyllithiuminitiator that triggers “living” anionic polymerization; upon coupling, such “living”carbanionic polystyrene with dimethyldichlorosilane, α,ω-bisacetal chains can beindeed generated:

O

α,ω-Dihydroxyl chains, as well as other end functions (–C–I), can be subsequentlyobtained by chemical modification of these acetal functional groups.

By a similar process, macromonomers —that are chains carrying a polymerizablegroup at one of their ends —can be synthesized The homopolymerization of suchmacromonomers affords “comb-like” polymers and their statistical copolymeriza-tion with simple monomers graft copolymers whose branches have all roughly thesame size The preparation of α-norbornenylpolystyrene is illustrated below:

H

n

(i) benzene (ii)TMEDA (iii) MeOH

LiCl

OH

10.2.2 Functionalization by Deactivation of ‘‘Living’’ Chain Ends

This method is generally preferred to the preceding one because it allows one tosynthesize end-functionalized chains with a variety of functional groups One ofthe possible pathways is to react a “living” chain with a bifunctional deactivatingmolecule used in large excess to minimize the coupling of two growing chains.For instance:

However, the reaction of growing active centers with a deactivating molecule, ing as precursor of the functional group to be introduced, is the most commonlyused method The preparation ofω-hydroxy polybutadiene by deactivation of “liv-ing” polybutadiene carbanions by ethylene oxide is an illustration of this strategy:

END-FUNCTIONALIZATION OF POLYMER CHAINS (SYNTHESIS OF REACTIVE PRECURSORS) 381

“living” cationic polymerization of vinyl ethers:

I

I2(or ZnCl2)

2n

O—R

OI

10.2.3 Hetero-functionalization

Hetero-telechelic polymers carry a different functional group at each of their end(X at one, Y at the other); these functional groups can possibly be antagonist andreact with each other and bring about step-growth polymerization Such polymerscan be represented as

It is necessary to carry out each functionalization at different times of the synthesis

to obtain such hetero-difunctionalized polymers —that is, for example, at the

initiation step and then at the end of the polymerization—for the synthesis ofα-amino,ω-hydroxy polycaprolactone:

(i) NaN3 (ii) HCO–O – , NH +

4 / Pd (10%)

CO–O (CH2)5

n

Br–(CH2)12–O–AlEt2 Br–(CH2)12–O–[CO–(CH2)5–O]n–H

H2N–(CH2)12–O–[CO–(CH2)5–O]n–HWhatever the nature of the end functional groups, theseα- or α,ω-difunctionalizedpolymers are difficult to characterize by conventional methods of analysis due tothe very low concentration of the said functional groups The use of techniques ofhigh sensitivity and precision such as mass spectrometry (MALDI-TOF) is helpfulwith respect to the identification of these functional groups

10.3 BLOCK AND GRAFT COPOLYMERS

As shown in the chapter devoted to copolymerization, the distribution of monomerunits in statistical copolymers is closely dictated by the concentration of the copoly-merizing comonomers in the reaction medium and their reactivity ratios Ini-tially set by the experimenter, the relative concentrations of comonomers can

be subject to an uneven variation throughout polymerization, depending upon themonomers’reactivity ratios, with the latter being primarily determined by the type ofchain polymerization used To obtain statistical copolymer chains of fairly constantcomposition, one generally resorts to continuous processes in which the composi-tion of the reaction medium remains roughly constant throughout polymerization.Being detailed in Chapter 8, the synthesis of statistical copolymers will not bedeveloped further here

10.3.1 Synthesis of Block Copolymers

Owing to their propensity to self-organize in mesophases, block copolymers exhibitspecific features and properties that are exploited at industrial level The morphology

of these mesophases depends primarily, among other parameters, on the nature of thecomonomers and the relative length of the blocks, on the dispersion of molar masses,

on the possible presence of residual homopolymers, and on the overall architecture

of the copolymer (including more than two blocks, star block copolymers, etc.) It

is thus essential to precisely control these structural parameters, and “living” and/or

“controlled” polymerizations are particularly suitable for their synthesis

There are three main synthetic strategies for the preparation of block copolymers:

• Sequential “living” polymerization of comonomers

• Polymerization of monomer B initiated by a macroinitiator polyA*

• Coupling of two polymer precursors through a covalent bond

BLOCK AND GRAFT COPOLYMERS 383

10.3.1.1 Sequential ‘‘Living’’ and/or ‘‘Controlled’’ Polymerization of Two Monomers The principle is schematically shown hereafter:

A* + n1M1

n2M2

A (M1)*n1

A (M1)n

1(M2)*n2

This method is industrially utilized to produce styrene –butadiene diblock mers (SB) by anionic polymerization The corresponding triblock copolymer (SBS),which is a thermoplastic elastomer, is obtained by coupling the precedent “living”diblock copolymer using dimethyldichlorosilane:

Remark It has to be stressed that in the syntheses of block copolymers by

anionic means, the first monomer (A) to be polymerized should not exhibit

an electro-affinity higher than that of the second monomer (B) (see Section8.6)

Triblock copolymers can be obtained in two steps from bifunctional initiators,

as for the preparation of telechelic homopolymers Such initiators are useful in the

preparation of polystyrene-block -polydiene-block -polystyrene, which turns out to

be excellent thermoplastic elastomers

Such triblock copolymers can be derived by anionic polymerization using dilithium

initiators For the central polydiene block to exhibit a high content in 1,4-cis units,

it is essential that the polymerization be carried out in hydrocarbon solvents (apolarmedium) and in absence of any polar additive Such dilithium initiators are gener-ated from the reaction of butyllithium with an adequate precursor as shown below:

Bu

in the growth of the two blocks In this case the cross-over from polyA* to polyB*

is obtained, after isolating poly A*, which is subsequently used as initiator for thepolymerization of B This method is useful whenever the two comonomers A and

B are prone to polymerize by two different mechanisms; the active centers thatare responsible for the polymerization of monomer (A) have to be transformedinto reactive species that are appropriate for the initiation of the polymerization

of monomer (B) For instance, the transformation of polystyryl carbanionic activecenters into cationic sites was exploited to polymerize tetrahydrofuran and obtain

PS-block -PTHF diblock copolymers as shown below:

CH3OH

OH)O

+

−

+

Another well-known example pertains to polysiloxane-block -polyamide diblock

copolymers obtained by anionic copolymerization of ε-caprolactam from a dimethylsiloxane (PDMS) macroinitiator; the latter is prepared by hydrosilylation

poly-of the unsaturated moiety carried by an acyllactam, using aα-hydrogenated PDMS:

BLOCK AND GRAFT COPOLYMERS 385

H2PtCl6N

The polyamide (PA-6) block is grown by polymerization of ε-caprolactam in thepresence of NaAlH2Et2 as catalyst through the so-called “activated monomer”mechanism:

10.3.1.3 Covalent Coupling of Two Polymeric Precursors This method

requires selective, fast and complete coupling reaction to give satisfactory results.Due to their incompatibility, polymer chains of different nature tend to min-imize their contacts and therefore the collisions between their antagonist reac-tive sites hardly occur An example of this method is given by the synthesis ofstyrene-dimethylsiloxane block copolymers:

CH3

CH3H

+ +

The synthesis of SBS triblock copolymer obtained by coupling of two “living” SBblock copolymers by means of Cl2Si(CH3)2 is another well-known example

10.3.2 Synthesis of Graft Copolymers

Like block copolymers, graft copolymers also form mesophases when their bone and grafts are incompatible; they are found in the same domains of applica-tions as block copolymers, but they are generally easier to synthesize and are thuswidely used

back-Three general methods could be followed for the preparation of graft copolymers:

• Initiation of polymerization from a main chain carrying appropriate reactivesites (grafting from),

• Grafting of preformed chains onto the main chain (grafting onto),

• (Co)polymerization of macromonomers

10.3.2.1 ‘‘Grafting from’’ Method This method consists of taking advantage

of the presence of reactive sites on a polyA backbone to initiate the polymerization

of a monomer B and generate grafts polyB:

The example described below refers to nitroxide-mediated free radical tions Initially, a methacrylic monomer carrying an alkoxyamine group is copoly-merized at moderate temperature with a comonomer (H2C=CHA) to form the back-bone In a second step the grafts are grown by thermal activation ofalkoxyamine groups, which produce, in addition to stable nitroxyl free radicals,reactive free radicals capable of initiating the polymerization of H2C=CHB (seeSection 8.5.11 for the mechanism)

PolyBB

PhO

BLOCK AND GRAFT COPOLYMERS 387

Active centers can also be generated on the backbone by chemical modification ofits monomeric units:

OO

X

CH2 CH nY

X and Y are antagonist functions whose mutual reaction brings about “grafting.”The deactivation of “living” polystyrene chains prepared by anionic means on aPMMA backbone is a well-known example of the “grafting onto” method:

PS CH2 CH, Li− +

PMMA

+ CH3OLi

CHO

CH2

PSPMMA

The proportion of grafts introduced in such graft copolymers is determined by the[PS−,Li+]/[MMA] ratio, and the reaction is fast and total Many other examples

of graft copolymers have been described following the same principle

10.3.2.3 Copolymerization of Macromonomers “Comb-type” polymers

can be obtained by homopolymerization of a macromonomer (see Section 10.2.1)

In the latter case, all monomeric units of such a comb-type polymer carry a graft;

in such structures the proportion of main-chain-forming monomers is very low.Moreover, main-chain crowding due to the grafts becomes even more rigid since

it is long Graft copolymers with varying graft density can also be obtained bycopolymerization of a macromonomer with a conventional comonomer

10.4 POLYMERS WITH COMPLEX TOPOLOGY

Interest in complex structures/architectures arises from the observation that themacroscopic properties of a material is largely affected by the chains’architectureand, in particular, by the presence and the location of branching points It is notthe aim of this section to give a comprehensive account of the synthesis of allpossible architectures/topologies, but rather to describe the most important oneswith respect to their applicability and their preparation by traditional methods ofmacromolecular synthesis

10.4.1 Macrocycles

Mono- and polycyclic structures of macromolecular size are characterized by theabsence of any chain end Because the entropy of such architectures is lower thanthat of their linear counterparts, they exhibit specific conformational properties insolution as well as in the molten state

The method generally utilized to obtain macrocyclic polymers is to react difunctionalized prepolymers with a bivalent coupling reagent; it is a bimolecularcoupling which has to be carried out in highly dilute solutions so as to favor theformation of macrocyclics at the expense of linear polycondensates Its principle

α,ω-is schematically represented below:

POLYMERS WITH COMPLEX TOPOLOGY 389

For instance, macrocyclic polybutadienes could be derived from living dicarbanionic polybutadienes, which were reacted with 1,3-bis-(1-phenyl-ethenyl)benzene As shown below, the coupling agent used here is also a precursor to abifunctional initiator and the addition of ethylene oxide after cyclization permitsthe difunctionalization of the macrocyclics formed:

α,ω-CH CH CH2 ] [ CH2

CH CH CH2 ] [ CH2

n

CH CH CH2

CH2Li

CH2

CH2OH

CH2

CH2OH +

Whatever the experimental conditions utilized, a significant fraction of linear mers resulting from intermolecular additions is always formed

poly-Monomolecular coupling affords higher yields in macrocyclics The principle

of such syntheses rests on the reaction between the chain ends of linear polymerscarrying antagonist reactive sites As shown in the example represented below,

a vinyl ether is first polymerized cationically under “living” conditions, with theactive centers formed being unable to react with the styrene-like double bond of theinitial molecule In a second step, upon addition of a strong Lewis acid, carbocationsare created at the growing ends that can react with the styrenic unsaturation Due

to the high dilution of the medium, an instantaneous intramolecular cyclizationoccurs

addition to a dilute SnCl4 solution cyclization

*RO

RO

ORO

RO

OO

10.4.2.1 ‘‘Convergent’’ Method This method involves the coupling of

mono-functional linear chains to a core agent fitted with antagonist functions The number

of the latter determines that of branches in the resulting stars For example, four-armpolystyrene stars were obtained by reaction of polystyryllithium chains with atetravalent molecule such as SiCl4:

++

SiCl4

POLYMERS WITH COMPLEX TOPOLOGY 391

It is recommended to use a slight excess of monofunctional linear chains to obtainthe expected four-arm stars in a quantitative yield

The convergent method is particularly appropriate for anionic polymerization asplurifunctional electrophilic deactivators are available in large number

10.4.2.2 ‘‘Divergent’’ Method The divergent route involves the growth of

the star arms -or branches- from a core-initiator whose functionality determinesthe resulting number of branches Many examples can be found in the literature,all based on “living “and/or “controlled” polymerizations

O

ZC

Br

O C

O C Z C

Br

O O

O

O C Z C

O C

Z C Br

O

O C

Z C Br

10.4.2.3 Combination of ‘‘Convergent’’ and ‘‘Divergent’’ Methods.

Contrary to the stars described previously, those resulting from the combination

of convergent and divergent methods are characterized by a large fluctuation oftheir functionality Their core is obtained by copolymerization of a monofunctionallinear chain with a diunsaturated monomer

This method was used to prepare stars with branches of two different types

(mik-toarm copolymers) For instance, the reaction of “living” carbanionic polystyrene

chains (PS−, K+) with p-divinylbenzene (DVB= tetravalent molecule) gives rise

to the formation of stars whose polyDVB core is fitted with a large number of banionic sites; the number of branches depends roughly on the [DVB]/[PS−, K+]ratio, but it cannot be controlled with full precision It is possible to take advantage

car-of the carbanionic species present at the core to initiate the polymerization car-of adifferent monomer—for instance, ethylene oxide:

PS

PEO

PEO

PEOPEO

PEO

,

_

K+,

_

K+,

Dendrimers comprise a central core, a precise number of monomer units linked one

to another by branching points, and an exact number of outer functional groups.Dendrimers are thus isometric objects with supposedly no fluctuation of their size

or composition contrary to polymers produced by any conventional chain ization even “living” ones With the samples of highest generations, it is difficult

polymer-to precisely check the quality of the structure formed except for those obtained bythe convergent method (cf following section)

The synthesis of dendrimers is based on condensation reactions and requiresgenerally successive protection-deprotection steps After the growth of each gener-ation, branching points are introduced at the dendrimer ends to multiply by a factorgenerally equal to 2 the number of outer functions from which the next generationwill be grown The molecular objects thus generated exhibit very a peculiar physic-ochemical behavior Owing to their high number of branching points, dendrimerscan be viewed as dense spherical objects whose interpenetration is hindered andviscosity particularly low

Like the case of stars, there are two main methods of synthesis of dendrimers

POLYMERS WITH COMPLEX TOPOLOGY 393

10.4.3.1 ‘‘Divergent’’ Method This method consists of generating two

identical functional groups from an initial one and then repeating the same operationseveral times In the example shown below, four amino groups are first generatedfrom a primary diamine, and the same sequence of reactions is repeated as manytimes as possible, the only limit being the space available to the functional groups

of the generation to be grown:

N

NCN

CNNC

N N N N

N N

N

N N N

N

N N

CN CN

CN CN

CN CN CN

CN CN CN CN

According to the authors who first described their synthesis, defect-freedendrimers of ninth generation could be obtained Actually, the growing num-ber of reactions occurring on each dendrimer at each new generation, as well asthe absence of sufficiently powerful tools of analysis, cast some doubt about theperfection of the objects formed; if isometric objects up to the fourth or the fifthgeneration could be isolated free of defects, dendrimers of higher generations mightcomprise defects that could not be detected by analysis.

The divergent method was also used to produce dendrimers with macromoleculargenerations Such dendritic polymers exhibit the same features as those of regu-lar dendrimers, except that the generations connecting the branching points are ofmacromolecular size The latter are obtained by “living” chain polymerization andare thus subject to a fluctuation—even minimal— of their size In that they differfrom the supposedly perfect regular dendrimers Dendrimers with macromoleculargenerations exhibit properties that are peculiar and completely different from those

of other architectures —in particular, star polymers The size of these dendrimer-likepolymers is not solely controlled by the number of generations, but also by the length

of the branches of each generation The example shown below illustrates the sis of dendrimers of poly(ethylene oxide) Dendrimers made of both polystyrene andpoly(ethylene oxide) generations were also synthesized; they exhibit an amphiphiliccharacter that was exploited for certain applications in aqueous medium

O

O Et

XCH2

3

n

C CH2O CH2CH2O , K Et

O

n

+ _

3

HO

POLYMERS WITH COMPLEX TOPOLOGY 395

10.4.3.2 ‘‘Convergent’’ Method This method consists of (a) the synthesis of

the so-called dendrons —of more or less large size, depending on their generation—

and (b) their coupling to a polyfunctional core An example of this method is shownbelow

The dendron, which hardly exceeds two or three generations, is prepared andisolated defect-free This method is more appropriate than the divergent one toproduce isometric samples Because each dendron is of appreciable size, any flaw

in the coupling reaction can be easily detected and the impurities easily removed

Br

+

OHHO

HO

K2CO3

K2CO318-C-6

18-C-6

OHO

OCBr4

OBr

OH HO

CH3

O

O

O[G2]

attention only recently after the first dendrimers were described, appearing as tutes for the latter They share some common characteristics with dendrimers —forexample, a globular shape and an inaptitude to entangle However, hyperbranchedpolymers and regular dendrimers differ in one major feature, which is the largedistribution in molar mass of the first compared to the perfect isometry of thesecond.

substi-As shown by Flory, hyperbranched polymers can be obtained from condensing monomers carrying one functional group (X) and antagonist functionalgroups (Y) The following scheme illustrates such a hyperbranched polymer result-ing from the self-condensation of the XY2 monomer

self-XZ

Z

ZZ

ZY

YY

Z

ZYZ

ZY

YY

ZY

ZZY

YY

Z

Y

Yself-condensation

Z

ZZ

(

(

(

ZYZ(

(

ZY

Y(

The “dendritic units” are those whose both Y functional groups have reacted with

an X functional group; on the contrary, those whose both Y functional groups areleft unreacted are called “terminal units.” Those whose only one functional group

Y has reacted are called “linear units.” To determine the branched character ofthe resulting condensation polymer, the notion of “degree of branching” (DB) was

POLYMERS WITH COMPLEX TOPOLOGY 397

Y functional groups When the second Y functional group of a linear unit exhibits

an enhanced reactivity compared to that of the first one, the degree of branchingapproaches a value equal to unity In the opposite case —that is, whenever thesecond Y functional group of a linear unit exhibits a lower reactivity—the poly-mers obtained are characterized by degrees of branching lower than 0.5 A largenumber of polymers (polyethers, polyesters, polyurethanes, polycarbonates, etc.)have been prepared by this method from XYn-type monomers with a wide variety

of antagonist functional groups

The easy synthesis and access to such hyperbranched polymers —compared todendrimers which are difficult to prepare —explain the current interest in this kind

of structures

It is also possible to generate hyperbranched polymers from vinyl monomersthat undergo chain polymerizations In addition to a polymerizable unsaturation,these monomers also contain a site that can initiate polymerization afteractivation:

Hyperbranched polymer

*

A

A+

A monomer such as p-(1-chloroethyl)styrene lends itself to this type of reaction

when activated either cationically or by radical means through halogen transfer:

Cl SnCl4

Bu4N +,Cl−

Cl

Bu4N +,Cl−

The number of cationic active sites that are

ready to “self-condensate” increases by one

unit after each addition of monomer molecule

Similarly to dendrimers with macromolecular generations, hyperbranched mers comprising true macromolecular chains between their branching points have alsobeen synthesized They were grown upon repeating generation after generation thereaction ofω-functionalized (or “living”) chains with a polymer backbone carrying

poly-reactive sites Molecular objects with very high molar mass (>100× 106g·mol−1)and low hydrodynamic volume were obtained by this method The example shownbelow refers to the deactivation of polystyryllithium chains (obtained by anionicpolymerization) on short blocks of poly(chloroethylvinylether) (obtained by cationicpolymerization):

( PS , Li )* _ +

O

O(PS) _, Li+ Ph

O

Cl

n

POLYMERS WITH COMPLEX TOPOLOGY 399

(PCEVE)

HO

OMe

m

NH3MeOH +, I

H

OMeO

PS

m + 1 m + 1

OO

“living” polystyryllithium chains Repeated several times, this gives

Under stoichiometric conditions {[PS−,Li+] and [CEVE units]= 1}, the relativecomposition [S]/[CEVE] is determined by the degree of polymerization of PS−,Li+.Such molecular objects thus mainly consist of polystyrene.

LITERATURE

E J Goethals (Ed.), Telechelic Polymers: Synthesis and Applications, CRC Press, Boca

Raton, FL, 1989

Y Gnanou, Macromonomers: synthesis, polymerization and utilization In: The Polymeric

Materials Encyclopedia J C Salamone (Ed.), CRC Press, Boca Raton, FL, 1996.

Y Gnanou, Tailor-made polymers In: The Polymeric Materials Encyclopedia, J C

Salam-one (Ed.), CRC Press, Boca Raton, FL, 1996

A Hult, M Johanson, E Malmstr¨om, Hyperbranched polymers, Adv Polym Sci 143, 1

(1999)

G Hawker, Dendritic and hyperbranched macromolecules: Precisely controlled

macromolec-ular architectures, Adv Polym Sci 147, 113 (1999).

W J Mijs, New Methods for Polymer Synthesis, Plenum Press, New York, 1992.

K Mishra (Ed.), Macromolecular Design: Concepts and Practice, PFI, New York, 1994.

M Lazzari, G Liu, and S Lecommandoux (Ed.), Block Copolymers in Nanoscience, Wiley

VCH, Weinheim, 2006

K Matyjaszewski, Y Gnanou, and L Leibler (Eds.), Macromolecular Engineering ,

Wiley-Interscience, New York, 2007

THERMOMECHANICAL PROPERTIES OF POLYMERS

Thermal properties that directly affect the mechanical characteristics of polymericmaterials are also referred to as thermomechanical properties They are closelyrelated to the morphological structure taken by macromolecular systems but arelittle affected by their dimensionality—at least when the cross-linking density islow Accordingly, only linear and related polymers will be subsequently considered

11.1 GENERAL CHARACTERISTICS

Because thermomechanical properties are specific to a given structural state, it isuseful to point out that solid polymeric materials exist under one of the followingthree physical states, each one being characterized by a specific morphology:

• The crystalline state, which corresponds to an almost perfect ordering ofmacromolecular entities and appears in the form of small-size single crystals,

• The amorphous state, which is a disordered entanglement of polymer chainsand, finally,

• The semicrystalline state, which comprises the two preceding states in varyingproportions measured by the degree of crystallinity

Each one of these states exhibits specific thermomechanical properties and ses, with the semicrystalline state combining the properties of both the amorphousand of crystalline states at the macroscopic level

respon-Organic and Physical Chemistry of Polymers, by Yves Gnanou and Michel Fontanille

Copyright  2008 John Wiley & Sons, Inc.

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