Novel thermoplastic elastomers. I. Synthesis and characterization of star-block copolymers of PSt-b-PIB arms emanating from cyclosiloxane cores

The synthesis and characterization of novel thermoplastic elastomers consisting of multiple polystyrene-b-polyisobutylene (PSt-b-PIB) arms emanating from cyclosiloxane cores is described. The synthesis involved the sequential living cationic block copolymerization of styrene (St) and isobutylene (IB), followed by quantitative allylic end-functionalization of the living PSt-b-PIB+ to produce PSt-b-PIBCH2 CHCH2 prearms, and finally linking by hydrosilation of these prearms with SiH-containing cyclosiloxanes (e.g., 2,4,6,8,10,12-hexamethylcyclohexasiloxane, D). Two types of star-blocks, namely primary and higher-order star-blocks, were prepared: Primary star-blocks containing 3–9 PSt-b-PIB arms were obtained by using various cyclosiloxanes (D to D) and a close to exact stoichiometry between the SiH and allyl groups, [SiH]/[CC] ∼ 1, in the essential absence of moisture ([H2O] ∼ 100 ppm). Higher-order star-blocks consisting of 13–24 PSt-b-PIB arms radiating from complex coupled cyclosiloxanes were prepared by the use of SiH/allyl ratios significantly larger than unity ([SiH]/[CC] = 2–3) in the presence of controlled amounts of moisture ([H2O] ∼ 600 ppm). Reaction conditions (temperature, concentration, stoichiometry, solvent nature, catalyst concentration, etc.) for efficient syntheses have been developed. The products were characterized by 200 and 600 MHz 1H-NMR spectroscopy and triple-detector (RI, UV, LLS) GPC. The microstructure of the condensed cores in the higher-order star-blocks was studied by 2D-NMR (HMQC) spectroscopy, and the number of cyclosiloxane rings in the cores (i.e., the content (wt %) of cores in the star-blocks) was determined.