Cyclosiloxanes, polymerization

Rings Ha, b and c were found to polymerize in bulk with DMSO as an activator and various catalytic amounts of the anionic initiator, potassium poly(dimethylsiloxane)diolate (for polymerization of Ila and b, see (22)). The latter is well known as an efficient initiator of cyclosiloxane polymerization. Thus... 

Cyclosilazanes are found to be reluctant to polymerize by the ring-opening process, probably for thermodynamic reasons. On the other hand, six- and eight-membered silazoxane rings are able to undergo anionic polymerization under similar conditions to those which have been widely used for cyclosiloxane polymerization provided there is no more than two silazane units in the cyclic monomer. They can also copolymerize with cyclosiloxanes however, the chain length of the linear polymer formed is substantially decreased with increasing proportion of silazane units. 

Cyclopropylmethyl bromide, physical properties of, 4 350t Cyclosilicates, 22 453t Cyclosiloxanes, polymerization of, 14 259 Cyclotetramethylenetetranitramine, 10 735-736 Cyclothiazide, 5 168 Cyclotrimethylenetrinitr amine, 10 735 17 160... 

The molecular weight of the polymer at equilibrium during cyclosiloxane polymerizations is controlled by including a hexaalkyldisiloxane such as MM or an oligomeric species such as MDM or MD2M. The equilibrium is represented as follows ... 

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 reactivity of the metal silanolate catalyst is dependent on the nature of the metal counter-ion, the larger metal ions giving rise to more active catalysts. For example, in the metal silanolate series the order of catalyst activity23 is Liquaternary ammonium and quaternary phosphonium silanolates having the same order of activity as caesium silanolate. Lithium and sodium silanolates are not very powerful catalysts for cyclosiloxane polymerization unless used in conjunction with an activating solvent such as tetrahydrofuran (THF) or dimethyl sulphoxide (DMSO). 

Monomers which can be polymerized with aromatic radical anions include styrenes, dienes, epoxides, and cyclosiloxanes. Aromatic radical anions... 

The position of the equiUbrium depends on a number of factors, such as concentration of siloxane units and the nature of substituents on the sihcon, but is independent of the starting siloxane composition and the polymerization conditions (81,82). For a hulk polymerization of dimethyl siloxane, the equihbrium concentration of cycHc oligomers is approximately 18 wt % (83). The equiHbrium mixture of cyclosiloxanes is composed of a continuous population to at least but D, D, and make over 95 wt % of the total cycHc fraction (84). 

Anionic Polymerization of Cyclic Siloxanes. The anionic polymerization of cyclosiloxanes can be performed in the presence of a wide variety of strong bases such as hydroxides, alcoholates, or silanolates of alkaH metals (59,68). Commercially, the most important catalyst is potassium silanolate. The activity of the alkaH metal hydroxides increases in the foUowing sequence LiOH < NaOH < KOH < CsOH, which is also the order in which the degree of ionization of thein hydroxides increases (90). Another important class of catalysts is tetraalkyl ammonium, phosphonium hydroxides, and silanolates (91—93). These catalysts undergo thermal degradation when the polymer is heated above the temperature requited (typically >150°C) to decompose the catalyst, giving volatile products and the neutral, thermally stable polymer. 

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). 

The kinetics of this process is strongly affected by an association phenomenon. It has been known that the active center is the silanolate ion pair, which is in equUibrium with dormant ion pair complexes (99,100). The polymerization of cyclosiloxanes in the presence of potassium silanolate shows the kinetic order 0.5 with respect to the initiator, which suggests the principal role of dimer complexes (101). 

Cationic polymerization of cyclosiloxanes is well known but used much less frequently than anionic reactions. The most widely used catalysts include sulfuric acid and its derivatives, alkyl and aryl sulfonic acids and trifluoroacetic acid1 2,1221. Due to their ease of removal, in industrial applications acid catalysts are generally employed on supports such as bentonite clay or Fuller s earth. 

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. 

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... 

Chojnowski, J. Ring-Opening Polymerization of Cyclosiloxanes. In Gelest Catalog Arkles, B., Larson, G., Eds. Gelest Inc. Morrisville, PA, 2004, pp 389-105. 

Boileau and coworkers11 have used a novel trimethylsilylmethyl lithium initiator MesSiCTDLi (1), in combination with a cryptand [211], for the ring-opening polymerization of cyclosiloxanes. Initiation of hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4) polymerization has been followed by H, 7Li, 13C and 29Si NMR. 

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