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Mechanism of formation of cyclic species in cationic ring‐opening polymerization of cyclodimethylsiloxanes
Author(s) -
Sigwalt P.,
Masure M.,
Moreau M.,
Bischoff R.
Publication year - 1993
Publication title -
makromolekulare chemie. macromolecular symposia
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.257
H-Index - 76
eISSN - 1521-3900
pISSN - 0258-0322
DOI - 10.1002/masy.19930730115
Subject(s) - cationic polymerization , mechanism (biology) , chemistry , polymerization , ring (chemistry) , ring opening polymerization , polymer chemistry , polymer , organic chemistry , philosophy , epistemology
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 ((CH 3 ) 2 SiO units) in the monomer. The case of initiation by trifluoromethanesulfonic (triflic) acid in methylene chloride solution at 20°C has been particularly studied. With D 4 , D 5 , D 6 and D 7 , for which the polymerization rate increases with the size of the ring, all types of cycles D x are formed in relative amount decreasing with their size ([D 7 ] < [D 6 ] < [D 5 ] < [D 4 ]). 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 D 3 is completely different. The high polymer (HP) and the cyclics (D 3x multiples of D 3 like D 6 , D 9 , …) 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 D 3 , and a desactivating one for D 4 , D 5 and D 6 . “Copolymerization” experiments between D 3 (or D 4 ) 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 D 4 by triflic acid is propagated by activated triflic esters, that of D 3 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 D 3x cyclics by intramolecular heterocondensation.