Cyclosiloxanes addition polymerization

With the purpose of increasing the range of available block copolymers, comonomers other than methacrylates and acrylates can also be involved in sequential polymerization, provided that they are susceptible to anionic polymerization. Dienes, styrene derivatives, vinylpyridines , oxiranes and cyclosiloxanes are examples of such comonomers. The order of the sequential addition is, however, of critical importance for the synthesis to be successful. Indeed, the pX a of the conjugated acid of the living chain-end of the first block must be at least equal to or even larger than that of the second monomer. Translated to a nucleophilicity scale, this pK effect results in the following order of reactivity dienes styrenes > vinylpyridines > methacrylates and acrylates > oxiranes > siloxanes. 

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. 

Monomers that can be polymerized with aromatic radical anions include styrenes, dienes, epoxides, thiiranes, and cyclosiloxanes. For epoxides and cyclosiloxanes, the mechanism of initiation involves nucleophilic addition of the radical anion to these monomers as shown in Scheme 7.6, in contrast to the electron transfer mechanism occurring for hydrocarbon monomers (Scheme 7.5) [45]. 

AB and ABA block copolymers have been prepared by the sequential addition of hexaphenyltricyclosiloxane and octaphenylcyclotetrasiloxane on a PDMS with one and two lithium silanolate chain-ends, respectively [153-157]. Some redistribution reactions occurred due to the harsh conditions required for the ROP of the perphenyl cyclic monomers. Finally, some multiblock copolymers have been prepared by the anionic equilibrium polymerization of cyclosiloxanes, which leads to a random distribution of different units-that is, diethyl, diphenyl, methylphenyl siloxy units [157], dimethyl, diphenyl siloxy units [158], and dimethyl, methylvinyl-siloxy units [159]. 

As this subject has not previously been reviewed, the decision was taken to present here the details of different studies on core-shell particles, where PDMS chains are produced by the ROP of cyclosiloxanes (see Table 3.8). Among numerous factors which affect the final particle structure can be included the nature of the seed latex, the sequence of addition (some studies were carried out in batches), the content of the crosslinker, and the ratio between each phase. It can be concluded from these complex studies that, when batch processes proceed with a simultaneous polymerization, it is most likely that homogeneous composite particles will be formed. 

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