Handbook of Polymer Synthesis Second Edition Episode 5 potx

The average degree of polymerization is equal to the ratio of converted moles of monomer starting concentration [M ]0 over the number of moles of initiator [I ] reacted: The anionic poym

Polymeric Dienes

Walter Kaminsky and B Hinrichs

University of Hamburg, Hamburg, Germany

Homopolymers of conjugated dienes such as 1,3-butadiene, isoprene, chloroprene, andother alkylsubstituted 1,3-butadienes, as well as copolymerzs with styrene andacrylonitrile, are of great economical importance [1–3] The conjugated dienes canpolymerize via 1,4 or 1,2 linkage of monomeric units In addition to this, 3,4 linkageoccurs with butadienes bearing substituents in the 2-position In the case of 1,4 linkage thepolymer chain can exist as cis or trans type:

ð13Þ

1,2 Linkage yields a tertiary carbon atom, thereby making it possible to formisotactic, syndiotactic, and atactic polybutadiene (3), in analogy to polypropene The rare3,4 linkage also gives isotactic, syndiotactic, or atactic configuration This applies only tohigh stereoselectivities Further isomeric structures are formed when next to head-to-taillinkages; head-to-head and tail-to-tail linkages also occur The polymerization of dienescan be initiated ionically by coordination catalysts or by radicals [4–10]

II POLYBUTADIENE

Polybutadiene belongs to the most important rubbers for technical purposes In 1999 morethat 2 million tons were produced worldwide, that is about 20% of all synthetic rubbers[11,12] The cis type made by 1,4-addition is economically the most importantpolybutadiene [13,14] Trans- as well as isotactic, syndiotactic, or atactic 1,2-polybutadienecan also be synthesized in good purity with suitable catalysts For anionic polymerizationwith butyllithium or the coordinative process with Ziegler catalysts, 1,3-butadiene must becarefully purified from reactive contaminants such as acetylene, aldehydes, or hydrogensulfide

A Anionic Polymerization

Metal alkyls, preferably of alkali metals, are used as initiators The polarization of thecatalyst exerts a strong influence on the stereospecifity (Table 1) [15,16] Lithium alkylsgive a polymer with the greatest trans-1,4-portion The stereospecifity is also influenced

by catalyst concentrations, temperatures, and associative behavior [17–34] In moreconcentrated solutions, alkyllithium, especially butyllithium, which is the preferredinitiator, forms hexameric associates that are dissociated in several steps to finally givemonomers [35–53] Only monomeric butyllithium is suited for the insertion Isobutyl-lithium shows an association grade of 4 in cyclohexane [36] Branched alkyl groups gavehigher activities than those with n-alkyl groups As postulated by the kinetic model forvery weak initiator concentration, the reaction order is 1 and less than 1 for higherconcentrations [54–62] This results in a series of reactions:

With hydrocarbons as solvents, the rate of the starting reaction is up to a factor of

100 smaller than that of the propagation step This difference is caused by the absence

of a double bond in conjugation to lithium in butyllithium In contrast to this, the use ofether accelerates the starting reaction such that propagation becomes the rate-determiningstep [63–67]

In the absence of chain transfer reagents, the molecular weight increases steadilywith increasing conversion of monomer In this way one gets living polymers with verynarrow molecular weight distribution when the starting reaction is fast or lithium octenyl

is used as a starter (Poisson distribution) The average degree of polymerization is equal

to the ratio of converted moles of monomer (starting concentration [M ]0) over the number

of moles of initiator [I ] reacted:

The anionic poymerization of 1,3-butadiene is normally carried out in solvents[102–109] Aliphatic, cycloaliphatic, aromatic hydrocarbons, or ethers as solvents could beused Working in ethers requires low temperatures because of the high reactivity and lowstability of the lithium alkyl in this solvent Using n-hexane as solvent, a butadieneconcentration of 25 wt% and a polymerization temperature of 100 to 200
C is preferred.Low-molecular-weight polybutadiene oils result when the polymerization iscatalyzed by a mixed system of butyllithium, 1,2-bis(dimethylamino)ethane, and potas-sium t-butanolate [110–112] With 1,4-dilithium-1,1-4,4-tetraphenylbutane it is possible

to get bifunctional living polymers (seeding technique) [113–118]

B Coordination Catalysts

A large number of complex metal catalysts have been employed in the polymerization

of conjugated dienes [119–139] Table 3shows a selection of catalyst systems that have

been used for the polymerization of butadiene Some systems yield polymers with a highpercentage of cis-1,4 linkage, while others favor the formulation of trans-1,4 or trans-1,2linkages As in the case of Ziegler–Natta catalysis of propene, the active centers aretransition metal-carbon bonds They normally form a 3-alloyl bond [140]:

ð10Þ

Table 2 Influence of polar compounds on the microstructure (1,2 content)

1,2 Structure (wt%) for polymerization temperature

The propagation reaction proceeds via insertion into these carbon–transition metal bondsafter the diene has been coordinated as a p-complex:

compounds are soluble in hydrocarbons, is used It is essential for a high cis content ofthe products that the catalyst contains iodine Those of TiCl4 and R3Al only leadpredominantly to the formation of trans-1,4-polybutadiene Aromatic hydrocarbons(benzene, toluene) are used as solvents The polymerization is a first-order reaction withrespect to the 1,3-butadiene concentration [150,151] As TiCl4gives living polymers, themolecular weight increases almost linearly with the conversion of monomer [152] Athigher degrees of conversion, the molecular weight can be controlled by varying thecatalyst concentration or composition The molecular weight distribution Mw/Mnrangesfrom 2 to 4 with a cis content between 90 and 94% Regulation of molecular weights can

be achieved by the addition of 1,5-cyclo-octadiene [153]

Supported Ziegler catalysts are also used [154–156] High cis contents up to 98%can be obtained with cobalt salts [cobalt octanoate, cobalt naphthenate, tris(2,4-penta-dionato) cobalt] in combination with alumoxanes which are synthesized in situ by hydro-lysis of chlorodiethylaluminum or ethylaluminum sesquichloride Only 0.005 to 0.02 mmol

of cobalt salt is needed for the polymerization of 1 mol of 1,3-butadiene [157–159] At 5
Cthe molecular weight varies from 350 000 to 750 000 depending on the alkylaluminumchloride, while at 75
C the variation is between 20 000 and 200,000 The polymerizationrates are fast over a considerable range of chloride content The cis-1,4-structure increaseswith chloride content The molecular weight increases with the chloride level [160].Nickel compounds can also be employed as catalysts [161–170] A three-componentsystem consisting of nickel naphthenate, triethyl-aluminum, and boron trifluoridediethyletherate is used technically The activities are similar to those of cobalt systems.The molar Al/B ratio is on the order of 0.7 to 1.4 Polymerization temperatures range from

5 to 40
C On a laboratory scale the synthesis of cis-1,4-polybutadiene withallylchloronickel giving 89% cis, 7.7% trans, and 3.4% 1,2-structures is particularlysimple [8] In nickel compounds with Lewis acids as cocatalysts, complexes with 2,6,10-dodecatriene ligands are more active than those with 1,5-cyclooctadiene (Table 4)[171].The influence of the ligand on cis or trans insertion is particularly obvious for 3-allylnickel systems

ð1720Þ

Alkanolates or carboxylates of lanthanides and actinides, especially uranium, areparticularly well suited for the production of cis-1,4-polybutadiene [172–187] Of thelanthanides, compounds of cerium, praseodymium, and neodymium are combined withtrialkylaluminum and a halogen containing Lewis acid [188,189] The polymerization canalso be carried out in aliphatic solvents at 20–90
C [190].

The microstructures are influenced primarily by the nature of the alkylaluminumcompound With triethylaluminum the portion of trans-1,4 double bonds reaches arelatively high level of 10%, while tris(2-methylpropyl)aluminum and bis(2-methylpropyl)aluminum hydride yield cis-1,4 contents as high as 99% [190] Similarly, high cis-1,4portions are obtained in the polymerization of 1,3-butadiene with 3-allyluraniumcomplexes The osmometric measured mole mass ranges from 50 to 150 000, the molecularmass distribution between 3 and 7 The extremely high temperature-induced crystallizationrate of uranium polybutadiene in comparison with titanium or cobalt polybutadienecorresponds to a greater tendency toward expansion-induced crystallization A technicalapplication, however, is in conflict with the costly removal of weakly radioactive catalystresidues from the products [132]

Me5CpTiF3) are significantly more active than the corresponding chlorinated ones

At higher polymerization temperatures a corresponding behavior can be observed,however with increasing polymerization temperature also the activity of the complexesincrease The activities of the 1,3-dimethylcyclopentadienyl titanium trihalides are thehighest and reach about 700 kg Br/mol Ti*h It makes no difference if one of the fluorides issubstituted by another ligand like perfluoroacetic or perfluorobenzoic acid (Me5CpTi-

F2(OCOCF3), Me5CpTiF2(OCOC6F5))

The activity reaches a maximum value for all catalysts after a short induction period

of 5 to 10 min After this, the activity decreases to a value being constant for a longerperiod of time of up to about 1 h

The substitution pattern influences the induction period The most active pounds show the shortest induction period, whereas the less active ones need a clearlylonger period

com-Table 4 Polymerization of 1,3-butadiene

Cocatalyst

Molar ratio,HX/Ni

Reactiontime(h)

Yield(%)

cis-1,4(%)

trans-1,4(%)

1,2(%)

The activity increases linear with increasing butadiene concentrations in the startingphase of the polymerization The kinetic order of the butadiene concentration is 1 Atconstant Al:Ti ratio the polymerization rate is given by

where cbis the concentration of butadiene The activity increases with an increasing Al:Tiratio, reaches a maximum at an Al:Ti ratio of about 700 and decreases slowly withincreasing Al:Ti ratios

High molecular weights are obtained for the polybutadienes produced with thesecatalysts

The di- and trimethylcyclopentadienyl titanium trichlorides give the highest lar weights while the fluorinated compounds have significantly lower molecular weights,even if their activity is higher, as shown for the Me4CpTiF3and Me5CpTiF3complexes(Table 6)

molecu-The glass transition temperatures range of 90.1 and 96.9
C The polybutadienesproduced with the most active catalysts have the highest content of cis-1,4 units and thelowest glass transition temperature

For all catalysts, the cis-1,4 structure units of the polybutadiene range between

a content of 74 and 85.8%, the trans-1,4 between 0.5 and 4.2%, and the 1,2-units between13.7 and 22.6% (Table 7).The most active systems generate the polymer with the highestcontent of cis-1,4 and the lowest content of trans-1,4 and 1,2-units The fluorinatedcompounds show a similar behavior A mechanism for the formation of these micro-structures is published by Porri [192]

There is no dependence of the microstructure on the polymerization time(between 10 and 120 min the cis content is 81.8 0.3% for MeCpCl3) and on the Al:Tiratio (between Al:Ti ¼ 500 and Al:Ti ¼ 10 000 the cis content is about 80.7 1.2 forMeCpTiF3)

D trans-1,4-Polybutadiene

Butadiene can be polymerized with Ti/Al catalyst systems A sharp change in structure

of polybutadiene can be seen by varying the mole ratio of TiCl4to R3Al At Ti/Al ratios of

Table 5 Activities of titanium complexes for the polymerization of 1,3-butadiene in 100 mltoluene, 10 g 1,3-butadiene, 0.29 g MAO, [Ti] ¼ 5  105mol/l, Al/Ti ¼ 1000, T ¼ 30
C, poly-merization time ¼ 20 min

a

Activity: kg BR/ mol Ti*h.

0.5 to 1–5, the cis content of the 1,4-polybutadiene increases to about 70% at a ratio of 1,and then falls off so that trans-1,4-polybutadiene is obtained at Ti/Al ratios of 1.5 to 3.Under these conditions it is a good catalyst for preparing trans-1,4-polybutadiene.Also heterogeneous catalysts consisting of TiCl4 immobilized on MgCl2 have beenreported [193].

Other catalysts contain the transition metals vanadium, chromium, cobalt, andnickel as their main components [194–202] The polymerization activity is usually far lowerthan in the synthesis of cis polymers (see Table 2) Addition of a donor such astetrahydrofuran, which directs the bonds into a trans-position to the catalyst of titaniumtetraiodide and triethylaluminum, results in the formation of a polybutadiene with 80%trans-1,4-double bonds [197]

Table 6 Molecular weights of the polybutadienes produced with fluorinated and chlorinatedcatalysts

Catalyst

X ¼ ClMolar mass M[g/mol  106]

X ¼ FMolar mass M[g/mol  106]

Another possibility is anionic polymerization with alkyllithium in combination withbarium compounds such as barium 2,4-pentanedionate [192–194, 203–205] Also,cobalt(II) chloride in combination with diethylaluminum chloride and triethylamine isused, yielding a polymer with 91% trans-1,4 and 9% 1,2 structures.

E 1,2-Polybutadiene

The synthesis of crystalline, syndiotactic 1,2-polybutadiene is also successful withcompounds of titanium, cobalt, vanadium, and chromium [194,206–210] Alcoholates[e.g., cobalt(II) 2-ethylhexanoate or titanium(III) butanolate] with triethylamine ascocatalyst, are especially well suited for this purpose They are capable of producingpolymers with up to 98% 1,2 structure Amorphous 1,2-polybutadiene is produced withmolybdenum(V) chloride and diethylmethoxyaluminum [211] Addition of esters ofcarboxylic acids raises the vinyl content of the products [212] The influence of thecoordination at the center atom is remarkable Trisallylchromium polymerizes 1,3-butadiene to 1,2-polybutadiene, while bisallylchromchloride gives 1,4-polybutadiene

ð2223Þ

1 Polymerization Processes

Polybutadiene can be produced in nonaqueous media or by a radical mechanism in anaqueous emulsion The field of homopolymerizations is dominated by the processes innonaqueous media, as described Emulsion polymerization is characterized by gooddissipation of the reaction heat The monomer concentration is on the order of 50 wt%.The reaction is initiated by free radicals, which are preferably formed from organichydroperoxides such as p-menthane hydroperoxide [213,214] Sodium formaldehydesulfoxylate and iron(II) complexes are employed as reducing agents At reactiontemperatures below 5
C the polymerization is discontinued at a degree of conversionbetween 50 and 60%, to avoid cross-linking The product features low stereospecifity(14% cis-1,4, 69% trans-1,4, and 17% 1,2 structures) At higher temperatures degradation

of the polybutadiene lowers the molecular weight [215,216]

The homopolymerization of isoprene

ð24Þ

can take place with a cis-1,4, trans-1,4, 1,2, or 3,4 connection.

In addition, the 3,4- and 1,2-polyisoprenes can both exist in three forms: isotactic,syndiotactic, and atactic Thus there are eight possible structures if we disregard head-to-head possibilities The part of the structure elements in the polymer depends on thecatalysts In general, the polymerization activity is lower compared to polybutadiene

Of the various structures of polyisoprene, only cis- and trans-1,4-polyisoprene and atactic3,4-polyisoprene are important (Table 8) [217–219]

A cis-1,4-Polyisoprene

Natural rubber (hevea) is 98% cis-1,4-polyisoprene with 2% 3,4-structure It can besynthesized by anionic polymerization with alkyllithium compounds or with Ziegler–Natta catalysts [220–225] The polymerization is carried out in solvents Impurities such asacetylenes, carbonyl compounds, hydrogen sulfide, and water have to be removed[217,226–228]

1 Anionic Polymerization

cis-Polyisoprene can be obtained with butyllithium under certain condition The ciscontent depends on the initiator and monomer concentrations as well as on thetemperature [23,49] In aliphatic solvents up to 97% 1,4-cis polymer could be obtained(Table 9) The strong influence of the initiator concentration is explained by a two stepmechanism [229]

Table 8 Homopolymerization of isoprene; microstructure of polyisoprenes

First, dissociation of the lithium alkyl association [Structure (29)] takes place,followed by activation by complexing of the monomer lithium alkyl with the cis-isoprene[Structure (31)] For insertion in a second step, a dimer alkyllithium is necessary [Structures(32) and (33)]

The living polymerization shows no breaking-off or transfer reactions and thereforegives polymers with a narrow molecular weight distribution [230] The molecular weight

Table 9 Dependence of polyisoprene microstructure on butyllithium concentration

can be calculated as follows:

Mcal¼½isoprene

The polymer is highly linear without branching For the synthesis of polyisopreneswith an extremely narrow molecular weight distribution (Mw/Mn¼1.05) a vacuum orseeding technique could be used [231, 232] In the second case the polymerization is startedwith separately prepared polyisoprene of low molecular weight Polar solvents such asethers and amines have an influence on the microstructure [233–236] The initiation stepincreases in relation to the propagation step [237]

The anionic polymerization leads to polymers with an active lithium end group.This can be used for further reactions By treatment with chlorsilanes such as 1,2-bis(dichloromethylsilyl)ethane, a four-star polymer results; with 1,2-bis(trichlorosilyl)ethane, a six-star polymer Aromatic divinyl compounds used for the same purpose havebeen described [238–240]

2 Coordinative Catalysts

Titanium tetrachloride in combination with aluminum trialkyl (ratio 1:1) gives optimumactivity in isoprene polymerization The Ziegler system TiX4/R3Al (X ¼ halides) yieldseither cis-1,4, trans-1,4 or 3,4-polyisoprene, while the unmodified lithium systems producepredominantly cis-1,4-polyisoprene (Table 10) Using TiCl4and R3Al cis-1,4-polyisoprene

is obtained at Ti/Al ratios of 0.5 to 1.5 [160] At lower Ti/Al ratios, oligomers are formed

At ratios of 1.3 to 1.6, mixed cis/trans polymers are obtained; at 1.6 to 2, polyisoprenes Ratios above 2 give resinous materials that are cyclized trans-polymers Theother titanium halides were found to be equivalent to TiCl4in these reactions Catalystefficiency is increased by complexing the R3Al with ethers and tertiary amines

trans-1,4-It is important to mix the catalyst components and alter the heterogeneous systembefore adding the monomer [241, 242] Titanium (II) seems to be inactive Therefore thecatalyst could be stabilized by addinng electron donors such as ethers and esters [243–246].Instead of alkylaluminum, alane etherates such as HAlCl2O(C2H5) are used [247–251].The best results in obtaining high yields of cis-1,4-polyisoprene are given by rareearth catalysts [252–257] Similar to the polymerization of butadiene, three componentcatalysts (transition metal compound, Lewis acids, and alkyl aluminum) are used It isnecessary to have an excess of 4 to 10 times of the aluminum component Most attractive

Table 10 Ziegler catalysts for isoprene polymerization: influence of the Ti/Al ratio on themicrostructure

is neodymium salt With increasing temperature and the Al/Nd ratio, the molecular weight

of the polymer decreases The cis-1,4 content is higher than 95% and the 3,4 part is lessthan 5% (seeTable 8)

B trans-1,4-Polyisoprene

The natural products gutta-percha and balata consist of trans-1,4-polyisoprene With theaid of vanadium trichloride and triethylalumium, trans-1,4-polyisoprene can be producedwith 98% trans-1,4 enchainments [133,258] The optimal Al/V ratio is the range of 5 to 7.The activity can be increased by the addition of small amounts of ether, heterogenerization

on supports (kaolin, TiO2), or blending with titanium(III) chloride or titanium alcoholates[259–261] Further catalysts featuring lower activity, however, are allylnickel iodide,trisallylchormium on silica, or complexes of neodymium [262–265]

Pure trans-1,4-polyisoprenes as well as trans-1,4-polybutadienes can be synthesized

by polymerization in inclusion compounds [266–269] As typical hosts for this dienes, theinclusion compounds or clathrates of urea, thiourea, or perhydrotriphenylene [PHTP;

Eq (36)] are used [270,271] The host forms the frame of the crystal and the guest is placed

in the cavities existing in the lattice Polymerization is generally started by subjecting theinclusion compound to irradiation with a-, g-, or x-rays and proceeds by a radicalmechanism [272,273] Also, free radical initiators such as di-tert-butylperoxide could beused [274] Inclusion in urea yields crystalline trans-1,4 polymers, whereas trans-1,4-polyisoprene obtained in PHTP is amorphous There is no trace of 1,4-cis units or of 1,2,3,4, and cyclic units The reason for the amorphous product is the presence of a substantialnumber of head-to-head and tail-to-tail junctions in addition to head-to-jail junctions[275, 276]

D Metallocene-catalysts

Also, half sandwich titanium compounds can polymerize isoprene (Table 11)

As of steric effects the unsubstituted cyclopentadienyl compound is more active thanthe substituted ones The fluorinated compounds are much more active (up to a factor of30) than the chlorinated ones The glass transition temperature of the polyisoprenes isabout 52
C.

The polymerization of 2-chloro-1,3-butadiene(chloroprene), which is made from acetylene

or 1,3-butadiene [284–287] is strongly exothermic (75 kJ/mol) It can be initiated radically,anionically, cationically, or with Ziegler catalysts [288] Only the free-radical process,which is usually run as an emulsion polymerization, is of technical importance [289–294].Compared with polybutadiene and polyisoprene, polychloroprene features improved gaso-line and aging resistance, low-temperature flexibility, and is less combustable [295–297]

The properties of polychloroprene are influenced by polymerization conditions aswell as by the nature of the additives In the radical polymerization the monomer is builtinto the polymer in trans-1,4, cis-1,4, 1,2, and 3,4 structures [Structures (37)–(40)] [298]

In addition to head-to-tail-enchainments, also head-to-head and tail-to-tail enchainmentsoccur, with a probability of 10 to 15% (Table 12)[297, 299] Polymers with a high trans(>98%) or cis content (>95%) are both possible [300, 301] The glass transitiontemperature for the trans-polychloroprene is  45
C; that for the cis polymer is  20
C at

a degree of crystallinity of about 12%

Table 11 Homopolymerization of isoprene Polymerization conditions: 50 ml toluene, 50 mlisoprene [Ti] ¼ 5  105mol/l, Al/Ti ¼ 200, Tp ¼ 30
C, tp¼5–24 h

Products with a low degree of crystallinity that can be decreased by comonomerssuch as 2,3-dichloro-1,3-butadiene are suitable for applications as rubbers, whereas morecrystalline polymers which are produced at lower polymerization temperatures are used inapplications as components of adhesives Due to the increased reactivity of the chlorineatoms in 1,2 structures, they tend to trigger aging reactions [302].

Chloroprene is mainly produced by emulsion polymerization [303–308]

A Sulfur Modified Chloroprene

Chloroprene is very reactive and can be polymerized with elemental sulfur The resultingblock copolymer consists of chloroprene and sulfur segments of various lengths Thesulfur can either be dissolved in the liquid monomer or added as a dispersion The sulfurbridges are easily cleaved by iodoform or other additives, thus permitting a variation ofmolecular weights over a wide range [297]

Next to isoprene, pentadienes and 2,3-dimenthyl-1,3-butadiene are produced as tadienes on a large scale Poly-2,3-dimethyl-1,3-butadiene was one of the first syntheticalrubbers [309, 310] Terminally substituted 1,3-butadienes give 1,4 monomeric units each ofwhich contains one or two asymmetric carbon atoms [R ¼ H or alklyl group; R0¼alkylgroup; (41) and (42)] Therefore, monomers of this type can lead to different stereoregular1,4-polymers: cis-1,4 iso- or syndiotactic, trans-1,4 iso- or syndiotactic [311,312]

alkylbu-A Poly(2,3-dimethyl-1,3-butadiene)

2,3-Dimethyl-1,3-butadiene is produced by dehydration of pinacol, which in turn is made

by reductive coupling of acetone, followed by purification via sulfur dioxide adducts [313]

It can be polymerized radically (emulsion polymerization), anionically, cationically, or

by coordinative catalysts [314–319] Due to the sterical hindrance of two methyl groups,

Table 12 Microstructure of polychloroprene as related to polymerization temperature

trans-1,4

Temp.(
C) Head/tail (%)

Head/head,tail/tail (%)

1,2 enchainment is hindered in comparison to 1,3-butadiene [320] dimethyl-1,3-butadiene)].

[cis-1,4-poly(2,3-ð43Þ

By analogy with the polymerization of isoprene with Ziegler catalysts, the structure of the polymer is determined by the aluminum/titanium ratio [321–323] At Al/Tiratios smaller than 1 the portion of trans-1,4 structures goes up to 75%, while theformation of a cis-1,4 polymer requires a Al/Ti ratio of at least 1 In either case some 10%

micro-of 1,2 structures are formed in the reaction [324] The polymerization is carried out inbenzene, toluene, or hexane as solvents The trans-1,4 polymer has a higher melting point

of 260
C, compared to 190
C for the cis-1,4 polymer Since this is connected with a highdegree of crystallinity, these polymers do not possess any rubber elasticity

Catalysts on the basis of complexes of cobalt or iron salts [e.g., cobalt(II) chloride/pyridine, cobalt(II) acetate/AlR2Cl] yield mixed structures with more than 20% 1,2double bonds and rubber elastic-like polymers [325,326] Rare earth catalysts havealso been described [327] A crystalline cis-1,4 polymer with a melting point of 198
Cand a molecular weight of 100 000 is obtained with aluminum alkyls/neodymium com-pounds at a molar ratio of 31:1 The yield is in the range of 30% Cobalt(II) acetate

in combination with diethylaluminum chloride or rhodium salts also yields a cis-1,4polymer [328,329]

Large amounts of trans-1,4-poly(2,3-dimethyl-1,3-butadiene) can be prepared byinclusion polymerization [330–332] Urea or thiourea are used as templates Trans-1,4polymer (99
C) is also obtained with p-allylnickel chlorides in combination withtetrachlor-1,4-benzoquinone Anionic polymerization by butyllithium allows good control

of the products microstructure over a wide range [97]

B Poly(alkyl-1,3-butadienes)

Polymers of some of the higher 2-alkyl-1,3-butadienes give vulcanizates with tensilestrength and elasticity comparable to that of natural rubber Poly(2-ethylbutadiene)and poly(2-phenylbutadiene) are most important 2-Ethyl-1,3-butadiene can be polymer-ized in the same way as isoprene [333,334] The polymer has a glass transition temperature

of 76
C [335] A polymer rich in trans-1,4 structures is obtained by catalysis withvanadium(III) chloride/triisobuthylaluminum In contrast to trans-1,4-polyisoprene, theproduct can be used as rubber, due to its reduced tendency to crystallization [336, 337].Additional alkyl-substituted polybutadienes are listed in Table 13 Parallel to anincrease in the alkyl substituents’ volume and electron donor properties, there is a decrease

in selectivity and activity for cis-1,4 insertions, although the vulcanizing properties of theproducts are improved [338–345]

Cationic polymerization of substituted alkyl-1,3-butadienes is accompanied by aconsiderable loss of double bonds (up to 80%) due to the formation of cyclic products [346].Tin(IV) chloride in trichloracetic acid, tungsten(VI) chloride, and boron trifluorideetherate have been tested as cationic catalysts [347, 348] In addition to polymerization,isomerizations are observed with these catalysts

Table 13 Polymerization of 2-alkyl-1,3-butadienes with various: influence on the microstructure.

Yield (%)(loss of doublebond) cis-1,4 trans-1,4 3,4 1,22,2-Dimethyl-1,3-butadiene H9C4Li 0 24 98 52 0 482-Ethyl-1,3-butadiene AlR3/TiCl4/BF3 10 5 100 98 0 2 0

Source: Ref [342].

C Phenyl-1,3-butadienes

Poly(2-phenylbutadienes) with a high cis content are also produced with the aluminum/titanium tetrachloride catalysts [349] Phenyl-1,3-butadienes can also beconsidered as vinyl-substituted styrenes, which explains the effects on activities andmicrostructures Poly(2-phenyl butadienes) occur in trans-1,4, cis-1,4, 3,4, and 1,2structures Maximum conversions are achieved with a molar Al/Ti ratio of 1, which leads

triisobuthyl-to the formation of 73% cis-1,4 and 27% 1,2 structures At higher Al/Ti ratios the cis-1,4content goes up to 96% The molecular weights are low, ranging from 2000 to 18,000

In contrast to this, the polymerization of 1-phenyl-1,3-butadiene was found toproduce polymers with high contents of 3,4- but no 1,2-structures

Generally, the molecular weights are low, with a ceiling of 10 000 The more crowdedthe positions of phenyl residue and methyl group, the higher is the 3,4 content At the sametime, there is an increasing tendency towards partial cyclization of the polymers via 3,4structures (Table 14)

Next to chloroprene, numerous other polybutadienes with different substitution patternsand substituents have been synthesized, although they are of no commercial importance(Table 15) [350–358]

Table 14 Microstructure of poly(phenyl-1,3-butadienes).a

Trans-1,4 isotactic, cis-1,4 isotactic and cis-1,4 syndiotactic polypentadienes havebeen prepared The cis-1,4-polypentadienes are of technical interest [361,362].

Table 15 Polymerization of different monomers leading to substituted polybutadienes

B 1,4-Poly(1,3-Pentadiene)

The polymerization of 1,3-pentadiene with cobalt acetylacetonate and chloralumoxane(52) or diethylaluminumchloride

ð52Þ

leads to syndiotactic cis-1,4-poly(1,3-pentadiene) (Table 16)[312] The catalyst is only able

to polymerize the trans-isomer of 1,3-pentadiene (61) The addition of thiophene orpyridine decreases the amorphic part, which contains a high number of 1,2 structures Inthe reaction between AlEt2Cl and Co(acac)2, all the acetylacetonato groups are displacedfrom the cobalt atom with the formation of a Co(I) species The polymerization-activecobalt system (and the nickel system as well) is a cationic system in benzene [363–367]

In systems of this type the mode of presentation of the monomers is probably determined

by the steric interaction between the butenyl group and the incoming monomer and forms

by minimizing the steric interaction the syndiotactic polymer [368]

Neodymium catalysts show a different behavior In this catalytic complex theneodymium is probably in the trivalent state and at least one Nd–Cl bond is present [369].The AlEt3-Ti(OR)4system is also a catalyst in which some alkoxy groups remain bonded

to the titanium Both catalysts give cis-1,4 isotactic polypentadiene The anionic ligandsbonded to the neodymium or the titanium atom of the catalytic species force the newmonomer to react to the isotactic structure

With these catalysts a mixture of cis- and trans-1,3-pentadienes in a wide range could

be polymerized But the polymer obtained from the trans isomer is more clean andcrystalline In the titanium catalyst the Al/Ti ratio plays an important role for themolecular weight With an increasing Al/Ti ratio, the molecular weight of the polymerdecreases [370,371] An optimal values is Al/Ti ¼ 7

With the optically active aluminum triethyltitanium tetramenthoxide system, anoptically active cis-1,4 isotactic polypentadiene was obtained, a fact that can be accountedfor by assuming that at least one menthoxy group is bonded to the titanium atom of thecatalytic complex [370] The melting point of cis-1,4-poly(1,3-pentadiene) is in the range

of 40 to 53
C depending on the cristalline part, and the molecular weight in the range of

20 000 to 400 000

Isotactic trans-1,4-poly(1,3-pentadiene) shows a melting point of 95
C [373] It can

be synthesized with the heterogeneous catalyst triethylaluminum/vanadium trichloride[311] The trans-1,4 units reach nearly 100% Small amounts (10 to 30%) of an amorphicpolymer can be extracted with ether AlR3/a-TiCl3could also be used This system givespolymers consisting of trans-1,4-units; hence the butenyl group has a syn configuration[368] A syn-butenyl group can derive either from coordination of the monomer with onlyone double bond or from coordination with the two double bonds in the cisoidconformation, forming an anti-butenyl group, followed by an anti–syn isomerization.The heterogeneous systems AlEt3/a-TiCl3 and AlEt3/VCl3 are capable of polymerizingboth the trans- and the cis-isomers of 1,3-pentadiene For the cis-isomer the cisoidconformation is unfavored for steric reasons It is therefore likely that for these catalyststhe coordination of the monomer occurs with only one double bond for both isomers ofpentadiene In the presence of CrO3only the trans-isomer of pentadiene is polymerized to

a crystalline polymer with 80% trans-1,4 and 20% 1,2 structures

Table 16 Polymerization of 1,3-pentadiene with various catalysts in benzene.

Microstructure (%)

Catalyst

Al/Metal(mol/mol) Time (h) Yield (%) cis-1,4(1,2) trans-1,2 3,4Bis(pentandithionato)-cobalt/Al(C2H5)2Cl 600 20 78 45 55 0

Al(C2H5)2Cl2(heptane) 600 20 57 (5) 95 0

Al2O(C2H5)2Cl2þthiophene 600 20 82 93 7 0Ti(OC4H9)4/Al(C2H5)3 3 26 2 61 29 10

Source: Ref [312].

Copyright 2005 by Marcel Dekker All Rights Reserved

Cationic polymerization provides, independent of the isomer of the 1,3-pentadiene,high trans-1,4 and trans-1,2 microstructures (Table 17) Studies on the insertionmechanism and the various side reactions have been carried out [373,374].

In principle, 1,4-disubstituted butadienes can give different types of 1,4-stereoregularpolymers: erythro (or threo) trans-1,4 iso- or syndiotactic

With the mode of presentation indicated, the new monomer gives, after insertion, abutenyl group superimposable on the preceding one; hence a diisotactic polymer will beformed

C Poly(methyl-1,3-pentadiene)

2-Methyl-1,3-pentadiene and 4-methyl-1,3-pentadiene are easily polymerized via acationic route [376,377] Even weak acids at low temperatures give high-molecular-weight polymers that are not cyclized and contain a large trans-1,4 portion However,4-methyl-1,3-pentadiene can also give mostly 1,2 structures at low yields The samecompounds that are used for the polymerization of 1,3-pentadiene are employed as acids(Table 18)

In comparison, the cationic polymerization of 3-methyl-1,3-pentadiene yields up

to 70% cyclized poly(3-methyl-1,3-pentdiene) with 1,2- and 1,4-microstructure of theremaining double-bond portion [378]

Also, Ziegler–Natta catalysts can be used with reduced activities Most of thepolymerizates feature low degrees of crystallinity Trans-2-methyl-1,3-pentadienepolymerizes with the homogeneous Ti(OR)4/VCl3/AlR3 catalyst to give amorphoustrans-1,4-poly(2-methyl-1,3-pentadiene), whereas the heterogeneous system consisting

of TiCl4/AlR3 produces a partly crystalline cis-1,4-polymer In this process the4-methyl-1,3-pentadiene is converted almost exclusively to isotactic 1,2-sequences Thetrans-3-methyl-1,3-pentadiene polymerizes with the titanium catalyst to cis-1,4-poly(3-methyl-1,3-pentadiene) with high molecular weights and melting points between 79and 94
C In the anionic polymerization of 2- and 4-methyl-1,3-pentadiene withbutyllithium only the trans-isomers give polymers with 60% cis-1,4 and 40% trans-1,4structures [376] Optically active poly(2-methyl-1,3-pentadiene) with up to 100% trans-1,4-double bonds is obtained by inclusion polymerization in desoxy- and apocholicacid [273]

Using the metallocene-catalysts CpTiCl3/MAO it is possible to polymerize 1,3-pentadiene to a mainly syndiotactic polymer [379]

A Poly(2,4-Hexadiene)

Hexadiene occurs also in several isomers, of which the trans–trans isomer is most reactivewith Ziegler catalysts Results of polymerization reactions with a number of differentcatalysts are compiled in Table 19 [380] Although Ziegler catalysts are normally notcapable of polymerizing olefins with internal double bonds, this is successful in the case

of 2,4-hexadiene, leading to crystalline polymers with high molecular weights [381].Also, cationic polymerization gives high-molecular-weight polymers [382] Anionicpolymerization yields only oligomers [383,384] Exclusively trans-1,4-threo diisotacticmicrostructure is found in crystalline poly(2,4-hexadiene) [385,386] The melting point isnear 87
C

Table 17 Polymerization of 1,3-pentadiene with cationic catalysts in benzene at 20 C.

Microstructure (%)Catalyst mmol Monomer isomer H2O/catalyst (mol/mol) Gel (%) Double bond (%) 1,4 1,2 3,4