P. Sigwalt et al., MECHANISM OF FORMATION OF CYCLIC SPECIES IN CATIONIC RING-OPENING POLYMERIZATION OF CYCLODIMETHYLSILOXANES, Die Makromolekulare Chemie. Macromolecular symposia, 73, 1993, pp. 147-166
A general feature of the cationic polymerization of all cyclodimethyls
iloxanes is the formation of various cyclic products (cyclics) togethe
r with that of a linear high polymer. However, the types of cyclics as
well as their rate of formation may vary considerably according to th
e number x of D units ((CH3)2SiO units) in the monomer. The case of in
itiation by trifluoromethanesulfonic (triflic) acid in methylene chlor
ide solution at 20-degrees-C has been particularly studied. With D4, D
5, D6 and D7, for which the polymerization rate increases with the siz
e of the ring, all types of cycles Dx are formed in relative amount de
creasing 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-depolymeriz
ation reaction, involving all the cyclics formed by back-biting reacti
ons occurring with silyl triflates activated by the acid, and leading
finally to an equilibrium. The situation with D3 is completely differe
nt. 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 th
e HP) are proportional to that of the linear HP. The latter is formed
from the beginning of the reaction with a molecular weight proportiona
l to HP yield and inversely proportional to the acid concentration. Th
e 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 tetramethyldis
iloxane diol confirmed the effect of water and gave new informations a
bout the occurrence - or absence - of condensation reactions in the me
chanism of the growth of the polymer chains. A discussion leads to the
conclusion that while the cationic polymerization of D4 by triflic ac
id is propagated by activated triflic esters, that of D3 may also invo
lve the monomer activated by the higher hydrates of the acid and linea
r oligomeric silanol esters. The latter, formed continuously, may also
give the D3x cyclics by intramolecular heterocondensation.