Synthesis and characterization of syndiotactic styrene-ethylene block copolymers

Six samples of styrene-butadiene block copolymers (sPS-B 1-6) containing a range of styrene molar fractions (x(S) = 0.9-0.16) were prepared using MAO-activated CpTiCl3 and converted to the corresponding styrene-1-butene-ethylene terpolymers (sPS-PE, 1h-6h) through the selective hydrogenation of the unsaturated butadiene segments with p-toluenesulfonhydrazide. The monomer compositions and chemical microstructures of the sPS-PE copolymers were determined by H-1 NMR (x(S) = 0.93-0.1) and C-13 NMR, respectively. The chemical shift assignments of the monomer tetrads (SSSS, SSEE, EESE, EEbE, EEEE, SEES, EEES; S = styrene, E = ethylene, b = l-butene) were also determined by C-13 NMR as well as the average monomer block lengths (n(S) = 72-4; n(E) = 2-51). The block structure of the sPS-PE copolymers was further confirmed by DSC (differential scanning calorimetry), WAXD (wide-angle X-ray diffraction), and solid-state CP-MAS C-13 NMR (cross polarization magic angle spinning) analysis. The sPS-PE samples with high styrene content (x(S) = 0.93-0.43) exhibit melting points in the range of 242-200 degreesC, and the sPS homosequences are observed in the crystalline delta -clathrate form. At low styrene content and short average sPS block lengths (x(S) = 0.1; n(S) = 4) the styrene domains are largely amorphous. Crystalline blocks of polyethylene were identified in the samples with long average block lengths (n(E) = 21-51) by means of X-ray powder diffraction and solid-state CP-MAS C-13 NMR. In general, the initial crystallinity of the polyethylene blocks is lower than would be expected on the basis of the chemical composition of samples 1h-6h but can be increased with thermal treatment, which produces the polyethylene orthorhombic crystalline form.