Siloxanes, polymerization

An example of the use of silica gel to separate the products from a phenyl siloxane polymerization employing a high efficiency column... 

Shirakawa polyacetylene, 444 Siloxanes, polymerization, 239 Size exclusion chromatography, 262-263 Solubility, specialty polymers, 256 Spacers, flexible polymer backbones, 97 Specialty polymers, polar/ionic groups, 256 Stability, polymers, 256 Storage moduli, vs. temperature behavior, 270... 

Siloxane polymerization differs mechanistically from the formation of hydrocarbon polymers in that it is essentially an acid-base process, as might be expected from the strong alternation of electronegativites along the het-eroatomic chain, and the radical initiators that catalyze the homocatenation of alkenes do not work for siloxanes. Long, unbranched polysiloxane chains are favored by higher condensation reaction temperatures and basic catalysts such as alkali metal hydroxides. Acidic condensation catalysts tend to produce polymers of lower molar mass, or cyclic oligomers. 

Although regarded as a mature and established field, siloxane polymerization has some distinguishing features that are well described in the literature but are often overlooked. These features, as well as some of the more-recent observations and current thinking in this field, are reviewed briefly in this chapter. Two topics not usually included in other reviews, copolymerization and condensation polymerization, are also discussed. 

Ml he history OF LINEAR POLYSILOXANES dates back at least 116 years (i), and research activity in this area steadily accelerated during this period as synthetic methods improved and as the fundamental nature of polymers became clear. The industrial prominence of poly(dialkylsiloxane)s was a particularly strong impetus to the development of this field (2). Thus, a large body of literature has accumulated, which has been extensively reviewed. The reviews by Wright (3), Sigwalt (4), and Kendrick et al. (5) are excellent and current, whereas that by Voronkov et al. (6) covers the earlier literature. Siloxane polymerization has now become suflSciently commonplace, so that it is sometimes discussed in general textbooks on polymer chemistry (7). 

This chapter will not duplicate the already existing reviews either in their comprehensiveness or special emphasis. Instead the distinguishing features of siloxane polymerization will be highlighted briefly, and perspectives will be given on copolymerization and condensation polymerization. 

The present refinement in linear siloxane polymerization is a monumental achievement resulting from the astute observations and ingenuity of many chemists over the past 120 years. The workers cited in this chapter are only some of the more recent contributors. Still, much work is yet to be done, and the critical reader should be left with many questions. For example, the equilibria 2 and 3 are traditionally the basis for explaining the distribution of molecular sizes and byproducts, but they exclude any role for the reactive chain ends. Yet, the accumulating evidence of the critical role of the counterions at the reactive ends in the mechanism of the process suggests that the equilibria may have to be rewritten to include the reactive ends. Definitive experiments are needed to settle the point. 

Keywords phosphazene base, synthesis, siloxane polymerization... 

Through steric hindrance and conjugative effects, these ionic phosphonium salts are very stable to hydrolysis. This, coupled with the lipophilic nature of the cation, results in a very soft, loosely bound ion pair, making materials of this type suitable for use as catalysts in anionic polymerization [8 - 13]. Phosphazene bases have been found to be suitable catalysts for the anionic polymerization of cyclic siloxanes, with very fast polymerization rates observed. In many cases, both thermodynamic and kinetic equilibrium can be achieved in minutes, several orders of magnitude faster than that seen with traditional catalysts used in cyclosiloxane polymerization. Exploiting catalysts of this type on an industrial scale for siloxane polymerization processes has been prevented because of the cost and availability of the pho hazene bases. This p r describes a facile route to materials of this type and their applicability to siloxane synthesis [14]. 

The discussion following will deal mainly with the important equilibrations and polymerizations of cyclic siloxanes. For further information concerning these methods and for excellent discussions of other types of siloxane polymerizations, the reader is referred to Voronkov (O, Noll O), and Meals (10). In addition, Noll (7, Eaborn (11), and Arkles and Peterson ( 1 2) offer reviews of the general chemistry of silicon compounds. 

Rates of anionic polymerization are influenced by the number of siloxane units present in the monomer rings. Some characteristics are given in Tables 6 and 7 (10). Due to ring strain in the three unit rings, all of the cyclotri- siloxanes polymerize faster than the cyclotetrasiloxanes. In the dimethyl-siloxanes, D3 reportedly pol)rmerizes approximately 50 times faster than D4 ( ). 

The title block copolymers with characteristics ranging from thermoplastic elastomers to polyethylene-like thermoplastics are obtained from ring opening polymerization of hexamethylcyclotrisiloxane with living a,(D-dilithiopoly-styrene. Chain scissions and oligomerizations which usually complicate siloxane polymerization are avoided, and molecular parameters regulating physical and mechanical properties are conveniently controlled to provide a unique family of thermoplastic materials. 

Scheme 7.18 Intermolecular and intramolecular exchange reactions in siloxane polymerizations.

This relationship is valid for a matched screw geometry. The apex leakage term V12 is determined by the apex width cubed see Eq. 10.103. Thus, increases in the apex angle will cause very strong increases in the apex leakage flow. Figures 10.53 and 10.54 compare output predictions made with Eq. 10.105 to the experimental results obtained by Nichols with dimethyl-siloxane polymeric fluids [19]. 

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