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 CpTiCl 3 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 1 H NMR ( x S = 0.93−0.1) and 13 C NMR, respectively. The chemical shift assignments of the monomer tetrads (SSSS, SSEE, EESE, EEbE, EEEE, SEES, EEES; S = styrene, E = ethylene, b = 1-butene) were also determined by 13 C 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 13 C 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 °C, and the sPS homosequences are observed in the crystalline “δ-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 13 C 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.