Cyclic cyclosiloxanes

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. 

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. 

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... 

The sequential addition method also allows the synthesis of many different block copolymers in which the two monomers have different functional groups, such as epoxide with lactone, lactide or cyclic anhydride, cyclic ether with 2-methyl-2-oxazoline, imine or episul-Hde, lactone with lactide or cyclic carbonate, cycloalkene with acetylene, and ferrocenophane with cyclosiloxane [Aida et al., 1985 Barakat et al., 2001 Dreyfuss and Dreyfuss, 1989 Farren et al., 1989 Inoue and Aida, 1989 Keul et al., 1988 Kobayashi et al., 1990a,b,c Massey et al., 1998 Yasuda et al., 1984]. 

There is an interesting analogy between cyclosiloxanes and the cyclic metasilicates [Si 03 ] " n = 3-6), e.g. (Me2SiO)3 (10.42) and [SisOg] " (10.43) the methyl groups in the dimethylsiloxanes are replaced by the formally iso-electronic in the silicates. Cyclic metasilicates occur naturally in certain minerals. For example, the trimer (n = 3) is found in CasSisOg (a-wollastonite), whereas the hexamer (n = 6) is a constituent of Be3Al2Si60ig (beryl). 

Cyclic compounds having the formula [SiH2- O].. have the. generic name cyclosiloxanes and are named similarly to the cyclosilazanes. Example ... 

The unique hormonal activity of siloxanes is limited to methylaryl-substituted, linear di- and trisiloxanes and cyclic tri- and tetrasiloxanes (67, 75, 76, 81). In general, at least one arylsilicon grouping is necessary for activity. The substitution of an alkyl group for a phenyl group either decreases the activity markedly or eliminates it entirely. Cyclosiloxanes... 

Thermomechanical studies have indicated that the glass transition temperature of synthesized copo-lymers is decreased as the volume of cyclosiloxane ring in the chain is increased. The results of X-ray structural analysis indicate that the copolymers represent amorphous systems, and increase of cyclic fragment volume leads to an insignificant increase of the interchain distance. 

The very straightforward results concerning the mechanism of propagation and cyclization in the polymerization of cyclosiloxanes were obtained by studying the radiation-induced cationic polymerization of 6-, 8-, and 10-membered cyclic siloxanes (D3, D4, Ds) [252,253]. 

The consumption of cyclosiloxane during polymerization and copolymerization was monitored by GC. The conversion of D3 was nearly 100 % (Fig. 1). In contrast, the ring-opening polymerization of d/ yielded a conversion of about 60 % only (Fig. 2). During the copolymerization with D3 the cyclic vinylsiloxanes showed a similar behavior as observed during homopolymerization. 

We also used HPLC in connection with MALDI-MS. Our aim was to separate cyclic siloxanes, even larger ones not distinguishable by SFC, from linear ones quantitatively. Hexane and ethyl acetate were mixed in a ratio of 90 to 10 by volume and used as mobile phase in HPLC separations of mixtures of the cyclic siloxane sample with the diol sample. Cyclosiloxanes elute first, followed by the diols. Under these conditions only a small overlapping area is to be seen (Fig. 4a). The sample was fractionated and MALDI-MS spectra were taken from the fractions. 

Fig. 4b shows a MALDI-MS spectrum containing only linear diols which eluted at 100-150 s out of the column. Fig. 4c a cyclosiloxane fraction. Thus MALDI-MS may assist HPLC in developing separation methods it enables characterizing cyclic and linear components in a qualitative manner. The short time required and its easy handling with MALDI-MS is noteworthy in comparison with SFC-MS techniques. 

Cyclic compounds having the formula (SiH2X) are called cyclosilazanes, cyclosilthianes, cyclosiloxanes, and cyclosil-melhylenes for compounds with X = NH, S, O, and CH2. The oxa-aza convention can be used for complex systems. For example, Me3Si-CHPh-SiMe(CH=CH2)-0-SiMe3 may be named 2,2,4,6,6-pentamethyl-5-phenyl-4-vinyl-3-oxa-2,4,6-trisilaheptane. 

Cationic polymerization of D2 at room temperature initiated by EtsSiH + PhsC B(C6Fs)4 proceeds fast and chemoselectively with exclusive siloxane bond cleavage and reformation. Taking into account the similarity of cationic polymerization of D2 to that of D4, this reaction is a good model for study of the role of cyclic trisilyloxonium ions in the cationic polymerization of cyclosiloxanes. 

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]. 

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