No abstract

Some features of the cationic polymerization of hexamethylcyclotrisioxane (D3 ) and octamethylcyclotetrasiloxane (D4) in methylene chloride in the presence of a strong protonic acid (CF3S0 H were compared. The polymerizations of both these monomers show many similarities, which are taken as evidence for analogies in their mechanisms. In particular, in both cases the reaction is affected by the addition of a small amount of water. The role of water is interpreted in terms of its direct participation in some elementary reactions of the process. The addition of water affects also the state of hydrogen bonding association in the system. Some experimental results together with comments concerning the mechanism of ring formation in the polymerization system are included. ) IntroductionA few years ago we published some kinetic results on the polymerization of hexamethylcyclotrisiloxane (4) initiated with triflic acid, which was used as a model system for the study of the mechanism of the cationic polymerization of cyclosiloxane We chose this system since, due to the strain in the six-membered D3ring, the polymerization is relatively fast and can be studied free of major interference by other processes involving polymer and macrocyclic products. However, octamethylcyclotetrasiloxane (D4) is the more commonly used monomer and, having an unstrained ring, it is also representative of larger ring monomers of the same cyclosiloxane homologous series. The polymerization of D4, induced by a protonic acid, was paid much attention during the last 40 years'-16). The interest in this reaction continues to be considerable and some interesting features of this polymerization were recently disclosed by Lebrun, Sauvet and S i g~a l t '~. '~) . This paper presents some data indicating similarities in the cationic polymerization of D4 and D3. A comparison of the polymerizations of these monomers under the same or similar conditions seemed to be of value, since some mechanistic features may be observed in the more complexe system involving D4. Results and discussionPreviously'), it has been shown by us that small amounts of water, initially introduced into the polymerization system of D3 initiated with triflic acid, have a dramatic influence on the kinetics of the process. Without additional water the polymerization 0025-1 16X/86/%03.00

1H NMR and 13C NMR have been used to study the end groups and tacticity in PMMA macromonomers and oligomers. These macromonomers are terminated almost exclusively in one vinylidene group per chain. The end group signals from the macromonomers are identified in both the 13C and 1H NMR spectra. The spectra of the purified oligomers (n = 1‐4) were used to aid in assignment. The macromonomers are predominantly syndiotactic, and the tacticity measured is consistent with Bernoullian statistics. The tetramer is a mixture of r and m isomers in a 4:1 ratio. It is shown that T1 experiments can provide a useful method of distinguishing resonances due to low molecular weight impurities from those due to stereochemical or isomeric effects in macromonomers. The absence of internal double bonds was confirmed by isomerizing the vinylidene group of several oligomers and of the macromonomer, and verifying the absence of the isomerized signals in the NMR spectra of the original materials.