Controlled synthesis of vinylmethylsiloxane-dimethylsiloxane gradient, block and alternate copolymers by anionic ROP of cyclotrisiloxanes

Three methods for the controlled synthesis of vinylmethylsiloxane-dimethylsiloxane copolymers have been explored. These methods were based on the anionic ring-opening polymerisation of 2-vinyl-2,4,4,6,6-pentamethylcyclotrisiloxane (VD2) and on the copolymerisation of this monomer with hexamethylcyclotrisiloxane (D-3) All of them permit control of the molecular weight of the copolymer, the structure of the chain ends and the distribution of vinyl groups along the chain. Polymerisation of VD, was performed using n-butyllithium as an initiator in THF. Studies on the sequencing of the siloxane units in the polymer were performed using Si-29 NMR spectroscopy and first order Markov chain statistics. These methods were complemented by determination of the sequencing at the chain end bearing a fragment of initiator. The structure of the copolymer obtained at -30 degreesC is highly regular. Almost 90% of the monomer is added to the chain as a result of the attack of the silanolate centre on the vinyl-substituted silicon in the monomer. The copolymerisation of VD2 with D-3 leads to copolymer with gradient distribution of the vinyl groups along the chain. The conversion of both monomers was followed by gas chromatography and the results were analysed by a simulation method and by the classical methods of Mayo-Lewis and of Kelen-Tudos. These methods gave similar values for the reactivity ratio r(D3) = 0.22, 0.21 and 0.22, r(VD2) = 7.8, 7.7 and 8.3 from the M-L, K-T and simulation methods, respectively. The sequential copolymerisation of D-3 with VD2 gave the diblock AB copolymers. The block formed in the second step may be contaminated with mers; coming from the comonomer. (C) 2002 Elsevier Science Ltd. All rights reserved.