The Polymerization of Dienes

Three kinds of polymer segments are formed in the polymerization of dienes 1-4 cis-, 1-4 trans-, and 1-2 segments (or 3-4 in polymerization of isoprene or other monosubstituted dienes). The latter may form isotactic or syndiotactic diads when the proportion of the 1-2 form is sufficiently high, e.g. a syndiotactic, highly 1-2 polybutadiene was described recently by Ashitaka et al. 123), although the so far examined 1-2 polybutadienes produced by homogeneous anionic polymerization were found to be atactic (unpubl. results of Bywater, Worsfold). 

The commonly observed, and technically troublesome, formation of gel in the polymerization of dienes testifies to the occurrence of cross-linking processes during their polymerization. According to Eq. (13), gel should begin to form when p reaches the critical value pc = l/y , where is the weight average degree of polymerization of the primary molecules. Thus... 

As to the first route, we started in 1969 (1) in investigating unconventional transition metal complexes of the 5 and 4f block elements of periodic table, e.g., actinides and lanthanides as catalysts for the polymerization of dienes (butadiene and isoprene) with an extremely high cis content. Even a small increase of cistacticity in the vicinity of 100% has an important effect on crystallization and consequently on elastomer processability and properties (2). The f-block elements have unique electronic and stereochemical characteristics and give the possibility of a participation of the f-electrons in the metal ligand bond. 

This field is still active today numerous recent patents [46, 63-89] and several reports [90-92] have detailed the efficiency of such types of systems for the hydrogenation of unsaturated polymers resulting from the polymerization of dienes (butadiene, isoprene, 1,3-cyclohexadiene) or the co-polymerization of dienes and styrenes (block co-polymers of butadiene and styrene SB, SBS, SBSB polymers). 

Raising the temperature of a radical chain reaction causes an increase in the overall rate of polymerization since the main effect is an increase in the rate of decomposition of the initiator and hence the number of primary radicals generated per unit time. At the same time the degree of polymerization falls since, according to Eq. 3.3, the rate of the termination reaction depends on the concentration of radicals (see Example 3-2). Higher temperatures also favor side reactions such as chain transfer and branching, and in the polymerization of dienes the reaction temperature can affect the relative proportions of the different types of CRUs in the chains. 

Lithium Diethylamide. This compound has been used as an initiator for the polymerization of diene by Vinogrador and Basayeva (1). In order to compare this initiator with lithium morpholinide (a lithium-nitrogen initiator with a built-in polar modifier), we have prepared lithium diethylamide according to the procedure described by vinogrador and Basayeva (1) and utilized it as an initiator for THF-modified butadiene polymerizations. 

The mechanism of coordination polymerization of 1,3-butadiene and, in general, that of conjugated dienes follows the same pathway discussed for alkene polymerization that is, monomer insertion into the transition metal-carbon bond of the growing polymer chain occurs. One important difference, however, was recognized very early.47,378,379 In the polymerization of dienes the growing chain end is tt-allyl complexed to the transition metal ... 

An analysis of the ionic factors for the polymerization of dienes to cis and trans structures is possible in the same way as for isotactic mono-enes. The mechanism which controls the steric structure of poly 1,4 dienes is parallel to that we have already seen for the mono-olefins. Roha (2) listed the catalysts which polymerize dienes according to the polymer structures produced. It was shown that the highly anionic as well as the highly cationic catalyst systems with increasing ionic separation produced trans-poly-1,4-dienes. This is analogous to the production of syndiotactic polyolefins. 

The mechanism for the polymerization of dienes to the ds 1,4 structure is also parallel. Sterns and Foreman (112) presented a cyclic 6-membered transition state to produce the cis structure. Orr (113) concluded that the configuration of the monomer itself has no role to play in determining the amount of cis or trans isomer produced from isoprene and butadiene. There is no requirement for the prealignment of the diene monomer by coordination between a diene molecule and the gegen ion. The diene must only assume the ds transition state during reaction. 

Natta (67) has studied the effect of two isomeric forms of titanium trichloride on the polymerization of dienes and of isotactic polypropylene. He has found that the alpha isomer, which gave the higher isotactic polypropylene, gave greater amounts of trans 1.4-polydiene than the beta, which gave cis 1.4-diene. 

Although the polymerization of diene monomers is most familiar for 1,3-dienes, as in the production of rubbers, the polymerization of 1,6-dienes to yield polymers containing six-membered rings ( cyclopolymerization ) has been well established for many years43. Gibson et al.441 have used cyclopolymerization of 1,6-diynes to prepare polymers which are effectively substituted polyacetylenes, the archetype being the polymerization of 1,6-heptadiyne ... 

Shen et al.120,121) found that the compounds of lanthanoid metals (from La to Lu) were active for the stereospecific polymerization of butadiene in the presence of alkylaluminum. Recently, Ouyangetal.122) reported that a NdCl3/C2H5OH/Al(C2Hs)3 catalyst exhibited a living character for the polymerization of diene and ethylene at temperatures below —30 °C. Diblock or triblock copolymers of diene and ethylene were obtained upon further addition of a diene monomer to a living polydiene or polyethylene. 

The stereochemistry of the polymerization of dienes is most conveniently discussed in two sections (a) polymerization in hydrocarbon solvents and (b) polymerization in the presence of amines, ethers and other electron donors. 

In addition to the polymerization of dienes the versatility of NdP-based catalysts is exceptional regarding the number of different non-diene monomers which can be polymerized with these catalysts. Acetylene is polymerized by the binary catalyst system NdP/AlEt3 [253,254]. Lactides are polymerized by the ternary system NdP/AlEt3/H20 [255,256]. NdP/TIBA systems are applied in the copolymerization of carbon dioxide and epichlorhy-drine [257] as well as for the block copolymerization of IP and epichloro-hydrin [258]. The ternary catalyst system NdP/MgBu2/TMEDA allows for the homopolymerization of polar monomers such as acrylonitrile [259] and methylmethacrylate [260]. The quaternary system NdP/MgBu2/AlEt3/HMPTA is used for the polymerization of styrene [261]. 

In discussions about the nature of the active species in the polymerization of dienes by Ziegler/Natta catalyst systems allyl species have already been suggested in the 1960s [273-278]. This discussion has continued through the past decades [139,279-283]. Today, it is widely accepted that Nd-allyl-groups are the key element in the insertion of dienes into the Nd carbon bond. 

Like carboxylates acetylacetonates are bidentate ligands for the Nd center. In the mid 1970s Monakov et al. started investigations on the use of Nd acetylacetonates for the polymerization of dienes [323,324]. Nd-acetylacetonate and Nd-benzoylacetonate were again mentioned in 1980 by Shen et al. [92], During the time of Nd-BR commercialization the influence of acetylace-tone on Nd-based catalyst systems was intensely studied by JSR. The increase of the solubility of Nd-salts in hydrocarbon solvents by acetylacetone was claimed in 1983 [325,326]. From this time onwards JSR filed numerous patents in which acetylacetone containing Nd catalyst systems were described [327-343]. 

Nd-boranate-based catalysts were also used in the polymerization of dienes. Nd(BH4)3 (THF)3/TEA yields poly(butadiene) with a frans/cz s-ratio 50/50. If Nd(BH4)3 (THF)3 is combined with a stoichiometric amount of MgBu2 catalyst activities are increased, control of molar masses is improved and poly(diene)s with a trans- 1,4-content of up to 99% are obtained. [348, 349]. 

In the polymerization of dienes with Ziegler/Natta catalyst systems it is a well-established fact that the presence of halide donors is essential in order to achieve high catalytic activities and high cis-1,4-contents [360,361]. The halide free catalyst system NdO/TIBA is a good example for a catalyst with a poor performance and a high trans- 1,4-specificity [362,363]. For various binary and ternary catalyst systems the qualitative impact of chlorides on the stereochemistry of BR is demonstrated in a series of fundamental experiments the results of which are summarized in (Table 5) [364],... 

Table 5 clearly demonstrates that cis-l,4-BR is only obtained if chloride is present in the catalyst system. In the absence of chloride BR with a high transit-content is obtained. These features were recently confirmed by Evans et al. who used well-defined Nd carboxylates for the polymerization of dienes [365]. According to Evans et al. halide atoms are transferred from the halide donor to Nd [366]. The role of halides for the achievement of high cis-1,4-contents was also demonstrated by Kwag et al. On the basis of density... 

The activation energies available for binary and ternary catalyst systems are in a range from 20 to 70 kj mol x, which according to Odian is characteristic for polymerizations mediated by Ziegler/Natta-catalysts [456]. It is interesting to note that the polymerization of dienes catalyzed by NdCl3-based catalyst systems show lower activation energies than Nd-carboxylate-based catalyst systems. The influence of halide donors on the temperature depen-... 

Another positive aspect of Nd catalysis is the high selectivity of Nd catalysts towards the polymerization of dienes. This feature is exploited in two ways. First, homopolymerization of BD in the presence of styrene as a solvent and second, selective homopolymerization of IP in the crude C5 cracking fraction. 

As in Nd-catalyzed solution processes in gas-phase polymerization of BD regulation of molar mass is a serious problem as there are no agents for the control of molar mass readily available. Vinyl chloride and toluene are no viable options. Vinyl chloride is ruled out due to ecological reasons and toluene is not applicable due to low transfer efficiencies and the required low concentrations if applied in a gas-phase process. For the control of molar mass and MMD in the polymerization of dienes a combination of different methods is recommended [457,458] (1) temperature of polymerization, (2) partial pressure of BD, (3) concentration of cocatalyst (or molar ratio of Al/MNd)> (4) type of cocatalyst, (5) residence time of the rare earth catalyst in the polymerization reactor. 

Hsieh and co-workers focused on the catalyst system NdCl3/EtOH/TEA. They formulated several Nd allyl species which are supposedly present during the polymerization of dienes [139]. 

Also mathematical models for the polymerization of dienes with catalyst systems of the type NdX3 TBP/TIBA were developed [615,616]. 

Scheme 26 Proposed mechanism for the polymerization of dienes by the catalyst system Nd(N(SiMe3)2)3/[HNMe2Ph]+ [B(C6F5)4] /AlR3 [320], copyright Soc Chem Ind. Reproduced with permission. Permission is granted by John Wiley Sons Ltd. on behalf of the SCI...

Most reaction models which describe the mechanism of diene polymerization by Nd catalysts have been adopted from models developed for the polymerization of ethylene and propylene by the use of Ti- and Ni-based catalysts systems. A monometallic insertion mechanism which accounts for many features of the polymerization of a-olefins has been put forward by Cossee and Arlman in 1964 [624-626]. Respective bimetallic mechanisms date back to Patat, Sinn, Natta and Mazzanti [627,628]. The most important and generally accepted mechanisms for the polymerization of dienes by Nd-based catalysts are discussed in the following. 

We have not discussed the polymerization of dienes very much so far. Part of the reason for this stems from the added complexity caused by the second double bond. Depending upon the polymerization conditions chosen, polymers with mixtures of repeat units can form. If only one of the double bonds participates in the polymerization, this is referred to as 1,2-polymerization. If both double bonds participate, the polymerization is called 1,4. Common monomers that fall in this category include 1,3-butadiene and isoprene ... 

The structure and chemical properties of metal-allyl compounds (ir-allylic, dynamic and a-allylic) which can be considered as models of a living polymer chain in butadiene polymerization have been studied. The polymerization of dienes proceeds only in dynamic allylic systems through the metal-ligand ir-bond in a-isomers. 

The polymerization of dienes with 7r-allylic complexes is of great interest since the complexes may be considered as model compounds for a propagating polymer chain. 

The polymerization of dienes by sodium metal has been known since the beginning of this century [184]. Ziegler et al. [185] postulated dianion formation from butadiene... 

The polydispersity increases with reaction time. At temperatures higher than 60 °C, the polydispersity and the molecular weight of products obtained by buradiene polymerization in dioxane16) strongly depend on temperature. The optimum characteristics (best yield, lowest polydispersity) are obtained between 50° and 60 °C. The polymerization of dienes, alkyl acrylates and styrene in the presence of 4,4 -azobis-(4-cyano-n-pentanol) has also been described 22 25). 

Dolgoplosk et al. [301] have reported kinetic data on the polymerization of dienes using 7r-allylic (7r-allyl and TT-crotyl) chromium compounds in solution and deposited on a silica support, and on the copolymerization of butadiene and isoprene using the supported TT-allyl catalyst and a Cr03/Si02 Al2 03 catalyst. Full details of the kinetics. 

Table 6. Various transfer constants for the polymerization of dienes and vinylacetate initiated by thermally decomposed hydrogen peroxide...

Polymerizations involving CCT of this interesting class of monomers has been described in only one reference.391 The polymerization of dienes in the presence of CCT catalysts provides oligomers, 101, with a terminal pair of conjugated double bonds, lOld. 

Conjugated Dienes. The assessment of reactivity in the polymerization of dienes has also been fraught with difficulty. Once again the active carboanions are relatively unstable and the identification and quantification of the actual intermediates are still the subject of much work, and make the interpretation of kinetic data extremely difficult. In the case of butadiene in polar... 

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