L-oxa-2,5-disilacyclopentanes

The discussion continues regarding the role of silanone and cyclodisiloxanes as reactive intermediates in the formation of Si-O-Si bond.25 In studies of the reaction of dimethyldichlorosilane, phenylmethyldichlorosilane, or diphenyldichlorosilane with dimethyl sulfoxide in the presence of 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane, Weber and co-workers obtained products of the insertion of diorganosiloxy unit into the cyclic siloxane, accompanied... 

Using the same method, cyclic block copolymers of PS and PDMS were prepared [280,281 ]. The cyclization reaction takes place in very dilute solution but using dichlorodimethylsilane as the coupling agent. Prior to the addition of D3 the difunctional living PS chain was end-capped with 2,2,5,5-tetramethyl-l -oxa-2,5-disilacyclopentane. In this way, the initiation reaction of D3 is much faster, leading to relatively narrow molecular weight distributions (Scheme 109). 

Chojnowski and Mazurek16 have studied the reaction of phenyldimethyl silanolates with cyclosiloxanes under conditions where siloxane bond redistribution reactions involving the polymer and the catalyst are completely suppressed. For the reaction of sodium phenyldimethyl silanolate (I) with 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane (II) they find that the rate of disappearance of I in an excess of II follows first-order kinetics. The observed first-order rate constant k varies with the initial... 

The heats of polymerization of two cyclic siloxanes have been measured the six-membered ring, hexamethyltrisiloxane, and the five-membered paraffin-siloxane , 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane (shown in Table 8.) For the corresponding cyclopentane the heat of polymerization is —5 2 kcal/mole, whilst for the paraffin-siloxane it is —10 0 kcal/mole. Thus, the effect of replacement of —(CHgla— by —Si(Mc2)—0—Si(Mc2)— in the cyclopentane molecule appears to either increase the ring strain in the monomer, or to stabilize the polymer, to the extent of some... 

Ziatinov and co-workers [9] synthesised poly(sila alkylene siloxanes) of 2,2,5,5 tetramethyl-l-oxa-2,5 -disilacyclopentane and carried out thermal stability measurement by TGA. 2,2,5,5- Tetramethyl-l-oxa-2,5-disilacyclopentane was shown to be quite stable at temperatures up to 315 °C in nitrogen. At higher temperatures, however, the polymer undergoes catastrophic decomposition and by 515 °C virtually no polymer is left. On the other hand, in air decomposition begins at 190 °C only 20% of the initial sample weight remaining at 350 °C and 15% at 800 °C. 

ROP of 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane is also catalyzed by a mthenimn duster in the presence of hydro-silane (eqns [36] and [37]). The preinitiation step involves activation of hydrosilane by a mthenimn complex followed by a nudeophilic attack of the monomer on the so-aaivated silane. 

The most simple cycle is the 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane (1 in Scheme 3.3). This and equivalent monomers (structures 2,3 and 4 in Scheme 3.3) have been polymerized using different catalytic means. The team of Loy and Rahimian [187, 188] carried out the polymerization of all these monomers using a basic catalyst (i.e. tetrabutylammonium hydroxide TBAH) and Bronsted adds, namely formic and triflic adds or cationic photoinitiators. Monomer 3, a soUd, was either polymerized in THF or copolymerized with 1. The polymerization was very fast (<30 min), and resulted in a material that could resist temperatures of up to 500 °C. The authors were confident that these dicydic monomers, produced via crossUnked materials, would in time replace the conventional sol-gel technique, as the polymerization is straightforward, solvent-free, has less than 5wt% shrinkage, and generates no porosity in the final material. 

The ROP ofcycHc siloxanes and carbosiloxanes catalyzed by NHCs has also been documented. Hedrick, Waymouth and colleagues showed that these organocata-lysts would mediate the ROP of 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane (TMOSC) and hexamethylcyclotrisiloxane (D3) to produce the corresponding ring-opened polymers with a controlled molecular weight and narrow polydispersity (Scheme 14.10) [32]. 

Scheme 14.10 ROP of 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane (TMOSq, hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4).

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