Structural characterization of novel styrene-butadiene block copolymers containing syndiotactic styrene homosequences

Samples of poly(syndiotactic styrene-co-butadiene) (sPS-B) have been prepared using methylalumoxane (MAO) activated CpTiCl3 under a range of experimental conditions. Copolymerization activities were low (24-36 kg mol(-1) of cat. h(-1)) and nearly constant over the range of polymerization temperatures investigated (25-70 degreesC). Eight sPS-B samples were synthesized at 25 degreesC with styrene molar fractions (x(s)) covering a full range of compositions (x(s) = 0.15-0.92). The copolymer samples were characterized by solution and solid state C-13 NMR spectroscopy, X-ray powder diffraction analysis, and thermal (DSC) analysis. The C-13 NMR analysis of the copolymer microstructure suggests that the butadiene homosequences are primarily cis-1,4 and the styrene segments are syndiotactic. However, some 1,2- and trans-1,4-butadiene units are occasionally formed during the polymerization process. The C-13 NMR resonances diagnostic of the monomer triads comprising styrene units (SSB, SBS, BBS, BSB, and SSS) were assigned and the average styrene block lengths (n(S)) calculated. Copolymer samples in which ns > 9 are crystalline due to the syndiotactic polystyrene segments detected in the delta-toluene clathrate form with melting points in the range 205-230 degreesC. Syndiotactic polystyrene segments with shorter styrene block lengths are amorphous and partially included in the poly(butadiene) rubber domains. TEM images of sample 13 (x(S) = 0.37, n(S) = 8.8) show the presence of small sPS fibers.