Cyclosiloxane monomers

Tris(dimethylsilyl)methane HC(SiMejH)3 was prepared according to the literature procedure. [12] 2,4,6-Trivinyl-2,4,6-trimethyl-cyclotrisiloxane was synthesized nsing Takiguchi method. [13] The cyclosiloxane monomers were kept melted over CaH for several days in an ampoule equipped with a teflon stopcock, and distilled (separately) under vacuum into the polymerization apparatus. All solvents were carefnUy dried according to literature procedures [14] and distilled prior to then-use. Other reagents were used as received. 

List II. Typical Cyclosiloxane Monomers Used in Ring-Opening Polymerization ... 

The most promising approach for preparing block copolymers of polystyrene (A) and polydimethylsiloxane (B) involves polymerization of cyclosiloxane monomers with living a,polystyrene anions (6). The original approach, however, gives materials contaminated with the... 

This later led to the development of the most popular silicone polymers, the polysiloxanes, which, at high molecular weight, exhibit rubbery properties. Such elastomers are generally obtained from a cyclosiloxane monomer by a ring-opening reaction, as shown in Equation (A). Because of the unusual ease of rotation around the silicone-oxygen bond, the polymer chain is much more flexible than carbon-based polymers, so that a semi-liquid material is obtained, even at molecular weights of several hundred thousand. Fine... 

ROP of functional cyclosiloxanes is a convenient route to all-siloxane copolymers. " " This may be realized in several ways. Sequential ring-opening copolymerization involves polymerization of one cyclosiloxane monomer to the desired chain length, then the second monomer is introduced, which adds to the growing chains. Diblock and triblock copolymers in the case of bifunctional initiator are produced provided the process is kinetically controlled. Otherwise, a copolymer with random or partly ordered sequencing is obtained. The kinetic control of the copolymerization process imposes the use of cyclotrisiloxanes as monomers. Thus, sequential copolymerization of two or more cyclotrisiloxanes is a good method for the precise synthesis of all-siloxane block copolymers. " ... 

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

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. 

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]. 

It is synthesised from the corresponding D3 cyclosiloxane and exhibits a Tg of -68 °C. Such a polymer can easily be crosslinked by various processes of silicone chemistry (e. g., peroxides, SiH or Si-vinyl derivatives). Fluorosilicones can be used for their resistance to swelling in non polar medium (e.g., Skydrol fluids) even at 100 °C but their main application is to replace elastomers of fluorinated monomers at low temperatures. 

The reactions of vinylcyclosiloxanes and vinylcyclosilazanes with styrene in the presence of RuHCl(CO)(PCy3)2 opened a new route to functionalized monomers for the ring-opening polymerization of cyclosiloxanes and cyclo-silazanes (Eq. 29) [56]. 

Studied in ref. [26] is the effect of HFC reaction conditions on the configuration sequences in POCS-4. Since the mesomorphous state in PMCS-4 is formed in stereoregular trans-tactic polymers only [27, 32] and spatial configuration of initial monomers is not always fully preserved in poly-mers, the effect of HFC conditions on transformation of =Si-CI and =Si-OH centers in initial corn-pounds has been studied. The detected fact of cyclosiloxanes partial inversion at Cl atoms substitution at silicon was expected, as reported before [33, 34], More detailed description of reflex correlation was carried out in ref. [35], Symbols and mark projections of units and bonds to the pla-ne perpendicular to the cycle plane. 

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. 

The lack of reliable information on kinetic parameters for the copolymerization of important cyclosiloxanes with ring sizes exceeding three is a serious deficiency. Industrial and routine laboratory syntheses rely almost entirely on such monomers, and in many cases, the processes are copolymerizations. An understanding of these constants would give valuable information on copolymer microstructure and its control. Unambiguous studies... 

Sequential polymerization consists of first polymerizing a given cyclosiloxane to the desired chain length whereupon the comonomer cyclosiloxane is then introduced into the reaction medium and proceeds to grow from the living chain end. The reaction is terminated at the desired conversion129,130. The active reaction centre must only react with the monomer and not with the linear chains already formed. Under these conditions pure block copolymers will be produced. 

Another way to reach flexibility of polymere with main-chain mesogens is the use of not quite rigid mesogens. This method was recently developed by employing cyclosiloxanes as mesogens monomers the cyclosiloxanes with eight or... 

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]. 

---