Cyclosiloxanes

Cyclosiloxanes are inorganic heterocylic rings that contain alternate silicon and oxygen atoms in their cyclic structures [10]. While silicon carries two substituents and is therefore tetracoordinate, oxygen has none and is dicoordinate. In this regard the cyclosiloxanes are similar to cyclophos-phazenes. However, unlike cyclophosphazenes, cyclosiloxanes do not contain a valence unsaturated skeleton. 

Some cyclosiloxanes can also be prepared by condensation reactions. For example, the compound [ SiPh2 SiMe2 203] can be prepared by the condensation of Me2SiCl2 with tetraphenyldisiloxane-l,3-diol [10, 13] (see Eq. 6.7). 

The X-ray crystal structures of a number of cyclosiloxanes have been determined [13]. The structural data for a few selected cyclotrisiloxanes and cyclotetrasiloxanes are summarized in Table 6.1. The stmctural features of cyclotrisiloxanes can be summarized as follows. 

Most of the six-membered rings are planar while the eight-membered rings are nonplanar. The Si-O bond distances depend on the substituents on silicon. The shortest distances are found for chlorocyclosiloxanes (see Table 6.1, entries 1 and 8). 

The commercial importance of polysiloxanes (silicone polymers) was discussed in Sec. 2-12f. The higher-MW polysiloxanes are synthesized by anionic or cationic polymerizations of cyclic siloxanes [Bostick, 1969 Kendrick et al., 1989 Noll, 1968 Saam, 1989 Wright, 1984]. The most commonly encountered polymerizations are those of the cyclic trimers and tetra- 

The polymer is commonoly referred to as poly(dimethylsiloxane), but the IUPAC name is either poly(oxydimethylsilanediyl) or catena-poly[(diphenylsilicon)-p-oxo] depending on whether one uses the nomenclature rules for organic or inorganic polymers. The prefix catena refers to a linear polymer, not branched or crosslinked. 

The anionic polymerization of cyclic siloxanes can be initiated by alkali metal hydroxides, alkyls, and alkoxides, silanolates such as potassium trimethylsilanoate, (CH3)3SiOK, and other bases. Both initiation 

An interesting aspect of this polymerization is that AH is nearly zero and, amazingly, AS is positive by 6.7 J mol-1 K 1 [Lee and Johannson, 1966, 1976]. The driving force in this polymerization is the increase in entropy (disorder) on polymerization. A positive value of 

AS is very rare for a polymerization process. The only other reported instances of positive AS values are those for the polymerizations of the cyclic octamers of sulfur and selenium and cyclic carbonate oligomers (Sec. 7-5c) [Brunelle et al., 1989]. All other polymerizations involve a decrease in entropy because of the decreased disorder for a polymer relative to its monomer. The positive AS values for the cyclic siloxane, S, and Se probably result from the high degree of flexibility of the linear polymer chains due to the large-sized atoms that they contain. This flexibility leads to greater degrees of freedom in the linear polymer compared to the cyclic monomer. 

---

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. 

Asathana S., Majoros I., and Kennedy J.P., TPEs Star-block comprising multiple polystyrene-b-PIB arms radiating from a crossUnked polydivinylbengene core. Rubber Chem. TechnoL, 71, 949, 1998. Shim J.S. and Kennedy J.P., Novel thermoplastic elastomers. II. Properties of star-block copolymers of PST-b-PlB arms emanating from cyclosiloxane cores, J. Polym. Set, Part A, Polym. Chem., 37, 815, 1999. 

The second approach involves the reaction of enefunctional or multifunctional alkylsilane or cyclosiloxane ... 

Yang, W.-H., Chen, I-L., and Wu, D.-H., Chemically bonded phenylsilicone stationary phases for the liquid chromatographic separation of polycyclic aromatic hydrocarbons and cyclosiloxanes, /. Chromatogr. A, 722, 97, 1996. 

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. 

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. 

The cyclosiloxane (MT)4 consisting of 4 stereoisomers shows 6 well separated signals in the T-region (about -65 ppm) [2], The relaxation data may be assigned to individual isomers. Thus, independent arguments confirm the shift assignment. 

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 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. 

Hydrolysis with 50-85 per cent sulphuric acid yields mostly high Molecular weight linear polymers having only small amount of 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... 

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

Backer, M. W. Pernisz, U. C. Photoexcitation and Photoemission Spectra of Phenyl-Substituted Cyclosiloxanes. In Synthesis and Properties of Silicones and Silicone-Modified Materials Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Van Dyke, M. E., Eds. ACS Symposium Series 838 American Chemical Society Washington, DC, 2003 pp 105-116. 

The Si—O—Si group in oligosilsesquioxanes is more stable toward sulfuric acid than in cyclosiloxanes. 01igo(hydrosilsesquioxanes) are susceptible to attack by... 

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]. 

---