MECHANISM OF FORMATION OF CYCLIC SPECIES IN CATIONIC RING-OPENING POLYMERIZATION OF CYCLODIMETHYLSILOXANES

A general feature of the cationic polymerization of all cyclodimethylsiloxanes is the formation of various cyclic products (cyclics) together with that of a linear high polymer. However, the types of cyclics as well as their rate of formation may vary considerably according to the number x of D units ((CH3)2SiO units) in the monomer. The case of initiation by trifluoromethanesulfonic (triflic) acid in methylene chloride solution at 20-degrees-C has been particularly studied. With D4, D5, D6 and D7, for which the polymerization rate increases with the size of the ring, all types of cycles Dx are formed in relative amount decreasing with their size ([D7] < [D6] < [D5] < [D4]). The high polymer final concentration and molecular weight are independent from triflic acid concentration. This may result from a polymerization-depolymerization reaction, involving all the cyclics formed by back-biting reactions occurring with silyl triflates activated by the acid, and leading finally to an equilibrium. The situation with D3 is completely different. The high polymer (HP) and the cyclics (D3x multiples of D3 like D6, D9, ...) are formed simultaneously under kinetic control. The yields of the various cyclics (formed in amount often larger than that of the HP) are proportional to that of the linear HP. The latter is formed from the beginning of the reaction with a molecular weight proportional to HP yield and inversely proportional to the acid concentration. The opposite role of added water on the polymerization is discussed : an activating effect for D3, and a desactivating one for D4, D5 and D6. ''Copolymerization'' experiments between D3 (Or D4) and tetramethyldisiloxane diol confirmed the effect of water and gave new informations about the occurrence - or absence - of condensation reactions in the mechanism of the growth of the polymer chains. A discussion leads to the conclusion that while the cationic polymerization of D4 by triflic acid is propagated by activated triflic esters, that of D3 may also involve the monomer activated by the higher hydrates of the acid and linear oligomeric silanol esters. The latter, formed continuously, may also give the D3x cyclics by intramolecular heterocondensation.