Catalysts stereospecificity

Stereoregular polymers are made by reactions involving the use of stereo specific catalysts. Stereospecific Catalysts... 

With these catalysts, the cation complexes with the monomer so weakly that a solid surface and low polymerization temperatures are required to achieve sufficient orientation for stereospecificity. Braun, Herner and Kern (217) have shown that lower polymerization temperatures are required (in n-hexane diluent) to obtain isotactic polystyrene as the alkyl metal becomes more electropositive (RNa, —20° C. RK, —60° to —70° C. and RRb, —80° C.). They correlate isotacticity with the polymerization rate as a function of catalyst, temperature or solvent. However, with Alfin catalysts, stereospecific polymerization of styrene is unrelated to rate (226). A helical polymerization mechanism as proposed by Ham (229) and Szwarc (230) is also inadequate for explaining the temperature effects since the probability for adventitious formation of several successive isotactic placements should have been the same at constant temperature in the same solvent for all catalysts. 

See rubber, natural rubber, synthetic catalyst, stereospecific. 

Propylene polymers are used as structural materials in the polyolefin industry. However, they can be used as elastomers by controlling catalyst stereospecificity. 

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