Syndiotactic polymers production

The synthesis of isotactic polymers of higher a-olefins was discovered in 1955, simultaneously with the synthesis of isotactic PP (1,2) syndiotactic polymers of higher a-olefins were first prepared in 1990 (3,4). The first commercial production of isotactic poly(l-butene) [9003-29-6] (PB) and poly(4-methyl-l-pentene) [9016-80-2] (PMP) started in 1965 (5). 

Homopolymerization of butadiene can proceed via 1,2- or 1,4-additions. The 1,4-addition produces the geometrically distinguishable trans or cis stmctures with internal double bonds on the polymer chains, 1,2-Addition, on the other hand, yields either atactic, isotactic, or syndiotactic polymer stmctures with pendent vinyl groups (Eig. 2). Commercial production of these polymers started in 1960 in the United States. Eirestone and Goodyear account for more than 60% of the current production capacity (see Elastomers, synthetic-polybutadiene). 

For syndiotactic polypropylene the symmetric Bernoulli trial, expressed in m and r dyads, is quite adequate for the representation of experimental data, and agrees with the stereochemical control being exerted by the growing chain end (145, 409). In its turn, atactic polypropylene is considered as a mixture of the products of two superposed processes, of the type discussed for isotactic and syndiotactic polymers, and is described by a simplified two-state model (145). 

The initiator formed from VCLt and A1(C2H5)2C1 is one of the most efficient means for syndioselective polymerization of propene, especially in the presence of a Lewis base such as anisole (methoxybenzene) [Doi, 1979a,b Natta et al., 1962 Zambelli et al., 1978, 1980], Other vanadium compounds such as vanadium acetylacetonate and various vanadates [VO(OR)xClp x), where x — 1,2,3] can be used in place of VCI4 but are more limited in their stereoselectivity [Doi et al., 1979]. Trialkylaluminum can also be used as a coinitiator, but only for VCI4. Syndiotacticity increases with decreasing temperature most of these syndioselective polymerizations are carried out below —40°C and usually at —78°C. The initiators must be prepared and used at low temperatures since most of them undergo decomposition at ambient and higher temperatures. There is considerable reduction of V(III) to V(II) with precipitation of ill-defined products that are low in activity and do not produce syndiotactic polymer, when the initiators are prepared at or warmed to temperatures above ambient. 

Soluble catalysts again may exhibit unique selectivities. Hafnium and zirconium fluorenyl metallocenes with methylaluminoxanes give syndiotactic polymers in high yields. The microstructure of the products indicates site stereochemical control with chain migratory insertion resulting in site isomerization with each monomer... 

Polymerization of a-olefins results in stereoisomeric products that have steieoiegularity like that shown in Scheme 63 136). Isotactic and syndiotactic polymers with mmm.. . and rrr.. . arrangements, tespec-... 

The active site in chain-growth polymerizations can be an ion instead of a free-radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. 

Some initiator may also remain unreacted. The side products formed, butyl isopropenyl ketone and methanolatemagnesium chloride, are not inactive under these conditions. They contribute to the formation of oligomers (by incorporation of dormant butyl isopropenyl ketone chain ends) and to the formation of syndiotactic polymer (by coordination of CHaOMgCl with propagating chain ends). 

Perhaps the most striking example of how ion pairing can impact catalysis is found in the mono-cyclopentadienyl (MCP) or constrained geometry catalyst family, 15, discovered by Bercaw and coworkers see Figure 1 for the molecular structure." This system possesses a particularly open active site. In the Exxon extrapolation of the Bercaw Sc catalyst to Ti. Canich reported" the production of crystalline poly-a-olefins for several of the substituted Cp systems. That is. poly-a-olefins with enriched isotacticity were produced. In the Dow extrapolation of the Bercaw Sc catalyst to Ti, syndiotactic polymer was produced." Subsequent efforts have reported slightly enhanced syndiotacticity " and counteranion-dependent isotacticity." The most obvious difference between the Dow and Exxon reports is the solvent. Aside from differences in the Cp ring substitution, Dow utilized Isopar E as solvent whereas Exxon employed toluene. 

Lactic acid is currently produced by fermentation of carbohydrates and is rme of the high potential and versatile biomass-derived platform chemicals, leading to various useful polymer products. PLA is produced by ROP of lactide (derived from lactic acid) and exhibits mechanical properties similar to poly(ethylene terephthal-ate) and polypropylene. Representative examples discussed herein included the synthesis of highly stereo-controlled PLAs, such as isotactic, heterotactic, and syndiotactic PLA materials, rendered by different catalyst/initiator systems. 

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