Cyclosiloxanes formation

The anionic polymerization of cyclosiloxanes was examined In benzene and toluene with lithium cryptates as counterions. Only one type of active species Is observed In the case of LI + [211] thus, the kinetics of the propagation and of the by-product cyclosiloxanes formation can be studied In detail for the first time. The reactivity of cryptated sllanolate Ion pairs toward the ring opening of D3 Is greatly enhanced compared to that of other systems. 

The mechanism of anionic polymerization of cyclosiloxanes has been the subject of several studies (96,97). The first kinetic analysis in this area was carried out in the early 1950s (98). In the general scheme of this process, the propagation/depropagation step involves the nucleophilic attack of the silanolate anion on the sUicon, which results in the cleavage of the siloxane bond and formation of the new silanolate active center (eq. 17). 

Thus, Andrianov et al. (26) attempted to catalyze polymerization of a number of alkyl and alkyl/aryl cyclosilazanes using catalytic amounts of KOH or other strong bases at temperatures of up to 300°C. In general, the reactions proceed with evolution of NHj, hydrocarbons and the formation of intractable, crosslinked, brittle products even at low temperatures. Contrary to what is observed with cyclotri-siloxanes, no evidence was found for the formation of linear poly-silazanes. Copolymerization of mixtures of cyclosilazanes and cyclosiloxanes gave somewhat more tractable polymers with less evolution of hydrocarbons or ammonia, however very little was done to characterize the resulting materials. 

The formation of the linear cyclic polymers is dependent upon the reaction conditions. Hydrolysis with water alone gives rise to 50-80 per cent linear polydimethyl siloxane a, w-diols and 50-20 per cent polydimethyl-cyclosiloxanes. 

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

Scheme 10.21 Born-Haber cycle for formation of Li complexes of cyclosiloxanes (D ).

The formation of n-complexes of silenes, as well as of germenes or stannenes is of central importance to the reactivity of the unsaturated systems. I will only offer one example out of many to illustrate the facts. As we found out, our stable silaneimine (Bu2Si=NSirBu3 takes up carbon monoxide under formation of supersilyl isocyanide as well as cyclosiloxanes (Scheme 7). 

A large effective cation size should suppress the cation-siloxane coordination, favor the Iree ion pair 3 in reaction 10, and enhance charge separation. The anticipated effects would be values of n approaching 1 in the rate equation 9, greatly enhanced rates of polymerization, and suppressed formation of cyclosiloxanes. Evidence that these effects are achieved is indicated by the effects seen with R4N countercations (35), the lithium cryp-tates (25, 27), and the crown ether-potassium silanolate complexes (39, 40). Additional evidence for the influence of the countercation on the equilibria is seen in deviations of the amounts of oligomer produced in equilibrated poly(dimethylsiloxane) from the normal distribution caused by specific interactions between the potassium silanolate chain ends (37, 38). More de-... 

Me, Ph, CjH FeCp) A series of fluorinated alcohol derivatives were also prepared.The addition of (Me2SiO)3 followed by Me3SiCl or further addition of cyclosiloxane gives slloxane or poly (slloxane) substituents. The addition of CO2 to the lithiated intermediate gives a carboxylate function which can be converted to the free acid or esterifled with p-nitrobenzylbromide. The addition of styrene to [NP(CH2Li)Ph] causes anonic polymerization of styrene and thus the formation of poly(methylphenylphosphazene)-graft-polystyrene copolymers. 

Summary PDMS-6-PEO short-chain diblock copolymers were prepared via anionic ring-opening polymerization of cyclosiloxanes. Applying this method, various well-defined block copolymers with different compositions were synthesized and their phase behavior was investigated. The polymers predominantly showed lamellar phases in aqueous solutions. At small surfactant concentrations, vesicles were formed, as observed via cryogenic TEM. The aggregates of the diblock copolymers were used for the formation of lamellar thin films, applying the evaporation-induced self-assembly approach. 

Most of the compounds generated from the pyrolysis of poly(dimethyl-siloxane-co-methylphenylsiloxane) are cyclosiloxanes with methyl or phenyl substituents. They are generated by a similar mechanism as shown for the formation of hexamethyl-cyclotrisiloxane during the pyrolysis of poly(dimethyl siloxane). 

In the above cyclosiloxane synthesis sufficient solvent is added during catalytic equilibration to favor cyclic formation. After equilibrium is established, the catalyst is neutralized or removed, and the oligomeric products isolated by distillation or crystallization. 

---