Homogeneous diene polymerization

The principles apphed in the previous section to essentially polar monomers can be extended to the stereoregnlar polymerization of dienes by alkali metals and metal alkyls. We have already seen that the cis-trans isomerism presents a variety of possible structures for the polydiene to adopt and complicates the preparation of a sample containing only one form rather than a mixture. Thus polyisoprene may contain units in the 1,2 or 3,4, or cis-1,4 or trans-1,4 configuration without even considering the tacticity of the 1,2 or 3,4 monomer sequences in the chain. 

Most work has centered on the preparation of a particular form of geometric isomer because the type and distribution of eaeh isomeric form in the ehain has a profound inlluenee on the mechanieal and physieal properties of the sample. The original diseovery that metallie lithium in a hydroearbon solvent eatalyzed 

Lithium and hthimn-alkyl catalysts produce highly specific stereostructures, but when replaced by Na or K, this elfect diminishes. 

Stereospeeifie polymerization takes place in the bnik state or in hydrocarbon solvraits, but the addition of a polar solvrait leads to drastie changes. 

To explain these features, the following mechanism has been put forward. Initiation produees a Schlenk adduct VII. 

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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). 

Non-metallocene complexes, such as aryloxide 31 and amide 138, have also been utilized as catalyst systems for the polymerization of a-olefins. Moreover, the homogeneous olefin polymerization catalysts have been extended to metals other than those in Group 4, as described in Sect. 7. Complexes such as mono(cyclopentadienyl)mono(diene) are in isoelectronic relationship with Group 4 metallocenes and they have been found to initiate the living polymerization of ethylene. These studies will being further progress to the chemistry of homogeneous polymerization catalysts. 

Butadiene) Group 4 metal complexes and (allyl) complex systems derived thereof have also been suggested as reactive intermediates at various homogeneous Group 4 metal complex-catalyzed conjugated diene polymerization reactions.151... 

In homogeneous coordination polymerization, the L are ligands such as cycloocta-diene. The pathway of termination by hydrogenolysis is shown dashed. 

Radical polymerization is initiated by a free radical, which subsequently adds to a vinyl or diene monomer to produce a propagating radical. To obtain information about the structure and concentrations of initiating and propagating radicals in radical polymerizations, use of ESR spectroscopy has called the interest of physical or polymer chemists. However, ESR measurements on these radicals in solution poly merizati on were found to be difficult, except for the case where polymers precipitated, because otherwise the concentrations of the radicals were too low. Thus, these measurements had to be limited to polymerization systems in highly viscous solutions or in the solid state, where the disappearance of free radicals by bimolecular reactions is suppressed. Bresler et al. -i7) succeeded for the first time in obtaining ESR spectra of free radicals which were produced in homogeneous bulk polymerization of methyl methacrylate (MMA), methyl acrylate (MA) and vinyl acetate (VAc) at conversions of 50-60% (in the gel state). 

The absence of any fortuitous chain termination step is an established feature of homogeneous anionic polymerization of diene and styrene in hydrocarbon solvent with organolithiums. Similarly, in the absence of adventitious chain terminating agents, the chain ends formed with Ba/Mg/Al catalysts are stable to an appreciable extent. 

The first unequivocal proof of feasibility of homogeneous polymerization free of termination and chain-transfer was reported in 1956, and the concept of living polymers and its ramifications were fully developed in those publications 2). Although the initial work dealt with anionic polymerization of styrene and the dienes, the underlying ideas were soon applied by other workers to a great variety of polymeric systems, as shown by the brief review of some of those developed recently. 

Nickel is frequently used in industrial homogeneous catalysis. Many carbon-carbon bond-formation reactions can be carried out with high selectivity when catalyzed by organonickel complexes. Such reactions include linear and cyclic oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions [1], Many of the complexes that are active catalysts for oligomerization and isomerization reactions are supposed also to be active as hydrogenation catalysts. 

For polar monomers, heterogeneity is seldom a requirement for isoselective polymerization with traditional Ziegler-Natta initiators syndiotactic polymers are obtained only with the soluble initiators. Styrene and 1,3-dienes are intermediate in behavior between the polar and nonpolar monomer. These monomers undergo isoselective polymerization with both homogeneous and heterogeneous traditional Ziegler-Natta initiators. 

Kinetics in Non-Polar Media. Polymerization of vinyl monomers in non-polar solvents, i.e., hydrocarbon media, has been almost entirely restricted to the organolithium systems (7), since the latter yield homogeneous solutions. In addition, there has been a particularly strong interest in the polymerization of the 1,3-dienes, e.g., isoprene and butadiene, because these systems lead to high 1,4 chain structures, which yield rubbery polymers. In the case of isoprene, especially, it is possible to actually obtain a polymer with more than 90% of the eis-1,4 chain structure (7, 8, 9), closely resembling the microstructure of the natural rubber molecule. 

In a typical ABS mass polymerization process, styrene and acrylonitrile are copolymerized in the presence of a diene-based rubber. Initially, the rubber is dissolved in the monomers and a continuous homogeneous phase prevails. 

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