Copolymers, block styrene-diene microstructure

Copolymerization Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyllithium initiators produces a tapered block copolymer structure because of the large differences in monomer reactivity ratios for styrene (rs < 0.1) and dienes (rd > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkali metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstructure in diene polymerizations (57), the addition of small amounts of an alkali metal alkoxide such as potassium amyloxide ([ROK]/ [Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstructure (58,59). 

Random Styrene-Diene Copolymers. Random copolymers of butadiene (SBR) or isoprene (SIR) with styrene can be prepared by addition of small amounts of ethers, amines, or alkali metal alkoxides with alkylhthium initiators. Random copolymers are characterized as having only small amounts of block styrene content. The amoimt of block styrene can be determined by ozonoly-sis or, more simply, by integration of the nmr region corresponding to block polystyrene segments (S = 6.5-6.94 ppm) (180). Monomers reactivity ratios of tb = 0.86 and rs = 0.91 have been reported for copolymerization of butadiene and styrene in the presence of 1 equiv of TMEDA ([TMEDAMRLi] = 1) (181). However, the random SBR produced in the presence of TMEDA will incorporate the butadiene predominantly as 1,2 imits. At 66°C, 50% 1,2-butadiene microstructure will be obtained for copolymerization in the presence of lequiv of TMEDA (134). In the presence of Lewis bases, the amounts of 1,2-polybutadiene enchainment decreases with increasing temperature. 

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