Hexamethyl cyclotrisiloxane

Anionic polymerization in suitable systems allows the preparation of polymers with controlled molecular weight, narrow molecular weight distributions and functional termination. The functional termination of a living anionic polymerization with a polymerizable group has been used frequently in the preparation of macromonomers (4). Our research has encompassed the anionic homo and block copolymerizations of D- or hexamethyl cyclotrisiloxane with organolithiums to prepare well defined polymers. As early as 1962 PSX macromonomers were reported in the literature by Greber (5) but the copolymerization of these macromonomers did not become accepted technique until their value was demonstrated by Milkovich and... 

The anionic ring-opening copolymerization of hexamethyl-cyclotrisiloxane (Dj) with 2,4,6-trivinyl-2,4,6-trimethyl-cyclotrisiloxane (Vj) was performed under dry Nj atmosphere in a glass ampoule equipped with a teflon stopcock. All other reactions, except the preparation of Pt(0)-[poly(vinylmethyl-co-dimethyl)siloxane]-carbosilane complexes, were performed using standard Schlenk s or syringe techniques under an atmosphere of argon. 

Vinyltrimethylsilane (97%), trimethylchlorosilane (98%), chlorodimethylsilane (97%), bromoform (96%), 5-bromo-l-pentene (95%), nBuLi (2.5M solution in hexanes), MeLi (1.6M solution in diethyl ether), n-decane (puriss. p.a., standard for GC, > 99.8%) and hexachloroplatinic(lV) acid hydrate were purchased from Aldrich. Platinum divinyltetramethyldisiloxane complex (Karstedt s catalyst, 3% solution in xylenes), hexamethyl-cyclotrisiloxane (95%), vinylmethyldichlorosilane (97%) and 1,1,3,3-tetramethyldisiloxane (97%) were bought from ABCR. Bromine (puriss) was bought from Eluka. Triethylamine (pure for analysis) and zinc oxide (pure) was purchased from Chempur. Solvents (tetrahydrofurane, diethyl ether, methylene chloride, pentane, ethyl acetate) were supplied by POCh (Polish Chemical Reagents). 

Ethyl-phenyl-diarsenylen XIII/8, 157 Hexamethyl-cyclotrisiloxan XIII/5, 331... 

The structure of the active propagation center in the cationic ring-opening polymerization (ROP) of cyclosiloxanes is still controversial. Trisilyloxonium ions generated from hexamethyl-cyclotrisiloxane, D3, and octamethylcyclotetrasiloxane, D4, were observed at low temperature by Olah et al. [I] and their participation as intermediates in cationic ROP of cyclosiloxanes was postulated. However, the main objection against this mechanistic concept is a relatively low rate of polymerization when an initiator able to form persistent tertiary oxonium ions is used [2]. 

Almost all fimctional silicone fluids of today s industrial production are either of a cyclic nature, containing the appropriate residues, or are linear oils bearing reactive functionalities at both ends or in the chain. The chemical nature of silicone synthesis done by equilibration and condensation is prohibitive for formation of asymmetrical silicones, in contrast to organic molecules like oleic acid or even ethanol. Currently there is only one way of preparing monofunctional silicone fluids, which is through kinetic anionic ring opening polymerization of the cyclic silicone monomer hexamethyl-cyclotrisiloxane (D3). 

Most of the compounds generated from the pyrolysis of poly(dimethyl-siloxane-co-methylphenylsiloxane) are cyclosiloxanes with methyl or phenyl substituents. They are generated by a similar mechanism as shown for the formation of hexamethyl-cyclotrisiloxane during the pyrolysis of poly(dimethyl siloxane). 

Scheme 3.6 Sequential anionic polymerization of dimethylsila[l]ferrocenophane and hexamethyl-cyclotrisiloxane.

Although styrene is the most common monomer used for the preparation of macromonomers anionically, other monomers capable of anionic polymerization have been used. Several papers have appeared in the literature concerning the anionic ring opening polymerization of hexamethyl cyclotrisiloxane (D3) followed by suitable termination for the preparation of macromonomers. One of the methods that widely used functionalization reactions, capable of producing monodisperse macromonomers with controlled functionality, is illustrated in the following Scheme 36. 

By the same token, kryptated potassium hydroxide initiates polymerization of hexamethyl-cyclotrisiloxane in benzene but not in polar solvent. About 10 4 M solution of kryptated KOH converts in about five minutes 60% of 5 10 2 M siloxane into its polymer. Other examples are given in Ref. 116. 

In contrast to the polymerization of D4, the anionic polymerization of hexamethyl cyclotrisiloxane (D3) with lithium as counterion is a living polymerization which produces polydimethylsiloxanes with well-defined structures. Useful initiators include lithium silanolates or the product from the reaction of 3 mol of butyllithium with D3 in a hydrocarbon solvent as shown in Scheme 7.19. It is noteworthy that no polymerization occurs in the absence of a Lewis base promoter such as THF, glymes, DMSO, or HMPA. 

Figure 2/-2. Dependence of the yield u on temperature for hexamethyl cyclotrisiloxane polymerized in the crystalline state. Tm is the melting temperature of the monomer. (After E. J. Lawton, W. T. Grubb, and J. S. Balwit.)...

The propagation reaction in the acid-initiated polymerization of hexamethyl-cyclotrisiloxane was described" as concurrent monomer addition, coupling of linear chain fragments, and cyclic oligomer formation by cleavage of siloxane bonds. The cationic polymerization of cyclic siloxanes was reviewed. ... 

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