Polymerization of hexamethylcyclotrisiloxane

Poly (dimethyl siloxane) obtained by anionic ring-opening polymerization of hexamethylcyclotrisiloxane (D3) can also be end-capped with vinylbenzyl bromide or other electrophiles, such as p. (chlorodimethylsilyl)styrene80). 

The macromonomer method was used by Fujimoto et al. for the preparation of (PS)(PDMS)(PtBuMA) stars [53], as described in Scheme 20. The lithium salt of the p-(dimethylhydroxy)silyl-a-phenyl styrene was synthesized and used as initiator for the polymerization of hexamethylcyclotrisiloxane (D3). Living PS chains were reacted with the end double bond of the macromonomer, followed by the anionic polymerization of the f-BuMA. 

Polymerization of hexamethylcyclotrisiloxane with 3-butadienyllithium afforded butadienyl-ended polysiloxane macromonomer,20 [27], Polycondensation of a chiral methyl (3-hydroxyisobutyrate at a temperature higher than 150 °C with Ti(0-nBu)4 afforded directly a biodegradable polyester macromonomer, 21 [28]. 

The exterior of carbosilane dendrimers can also serve as initiator sites for polymerization, in which case the dendrimers serve as cores for star polymers. Vasilenko and coworkers95 synthesized a multilithiated carbosilane dendrimer which they then used as an initiator for the ring-opening polymerization of hexamethylcyclotrisiloxane to yield star polymers of narrow weight distribution (Scheme 7). The researchers later extended this work to polymerize styrene, isoprene and ethylene oxide96,97. Star poly(ethylene oxides) were also prepared using hydroxy-terminated carbosilane dendrimers as the core98,99. 

A system where good evidence exists that oligomers are formed directly from monomer is that of the cationic polymerization of hexamethylcyclotrisiloxane (45). Apart from polymer of molecular weight 10s, a mixture of all cyclic siloxanes with ringsize from 6 (monomer) up to 48 have been detected using gaschromatography. 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

The solid state polymerization of hexamethylcyclotrisiloxane has been investigated over the temperature range —196° to 60°C. The rates of polymerization have been related to the presence of ion scavengers, H20, NH3, in the monomer and to the size of the crystals. Using large crystals dried over sodium, G values of polymerization of 11 X 103 were obtained at 50°C. This is five times larger than previously reported values. The reaction is concluded to be surface initiated and to be terminated at a crystal face or at a defect. 

It was shown that in the polymerization of hexamethylcyclotrisiloxane in the presence of CF3SO3H the monomer ist mostly consumed by the addition. However, the condensation has strong impact on the kinetics of this process ° ... 

Very recently the development of the controlled anionic polymerization of hexamethylcyclotrisiloxane (D3) led to the synthesis of a PDMS(PS)2 miktoarm star,97 according to Scheme 37. A benzene THF (50 50 v/v) solution of the living PDMS arm was added... 

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. 

Summary Well-defined amphiphilic block copolymers containing a poly(dimethyl siloxane) (PDMS) and a poly(ethylene oxide) (PEO) block were synthesized by ringopening polymerization of hexamethylcyclotrisiloxane, followed by chain extension with a PEO block of a defined length. These amphiphilic molecules were used as structure-directing agents in a solvent evaporation-driven synthesis approach to self-assembled mesostructured silica films. In addition, a general theoretical approach for an understanding and ultimately a prediction of the phase behaviour of block copolymers is presented. 

The above results, together with the known kinetics of the polymerization of hexamethylcyclotrisiloxane (25,26), the linear increase of molecular weight with monomer conversion (27) in this reaction and the recent proof of the role of tertiary siloxonium ion in this process (72), are in agreement with the simplified mechanism of this reaction represented by equations 5a,b. 

Such a transfer was found earlier to extensively occur in the cationic polymerization of hexamethylcyclotrisiloxane with linear oligomers of formula Me3Si(OSiMe2)nOSiMe3, n = 0, 1,2 (29). The chain transfer constants (k kp) in benzene (30%) were 0.29, 0.18 and 0.18 for n=0, 1 and 2, respectively. The chain transfer constant to the chain end in the cationic polymerization of 1 is expected to be of a comparable value. 

The tethered diphenyldimethylsiloxane oil was prepared by a kinetically controlled anionic ring opening polymerization of hexamethylcyclotrisiloxane (D3) and hexaphenylcyclotrisiloxane (Dj ) in the presence of n-BuLi (Figure 5) (7). Once the lithium salt of D3 and 03 6 was formed in a two step process, it was then quenched with water to give a silanol terminated diphenyldimethylsiloxane product (2). 

Preparative Methods reaction between dimethylchlorosilox-ane and water yields PDMS. The anionic polymerization of hexamethylcyclotrisiloxane yields PDMS. PDMS thimbles are fabricated by curing Sylgard 184 in an oven. 

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