Controlled synthesis of siloxane copolymers having an organosulfur group by polymerization of cyclotrisiloxanes with mixed units

The preparation of poly(dimethylsiloxane)s having part of the siloxane units substituted with a model organic thioether was investigated. The sulfur group was generated by the ene-thiol addition to the vinyl function bound to silicon. Two synthetic routes to this copolymer were compared (i) one-step kinetically controlled polymerization of organosulfur-substituted hexamethylcyclotrisiloxane and (ii) two-step synthesis including kinetically controlled polymerization of 1,3,3,5,5-pentamethyl-1-vinylcyclotrisiloxane followed by the thiol addition to the vinyl group in the polymer. A cryptand-lithium silanolate complex generated from [(trimethylsilyl)methyl]lithium was selected as the initiator in this polymerization, while tert-butyl mercaptan was used as the hydrosulfidation reagent. It was shown that synthesis routes lead to a high yield of copolymers having a fairly regular chain structure. The arrangement of siloxane units in the polymer chain was controlled by the propagation step. Polymerization of the vinyl-substituted monomer exhibited a significant degree of regioselectivity as one of the three possible modes of the monomer ring opening contributed over 70% to the propagation. Addition of t-BuSH to the vinyl group in the polymer proceeded without rearrangement of the polymer chain and could be performed without any side reaction of the vinyl group. Model copolymers of regular alternating structure were synthesized by the heterofunctional polycondensation of [2-(tert-butylthio)ethyl]methyldichlorosilane with a series of diols, HO(Me(2)SiO)(n)H, n = 1-4. They were used for comparison with the copolymers obtained by ring-opening polymerization.