Polydimethylsiloxane cyclization

More quantitative chemical evidence for random coil configuration comes from cyclization equilibria in chain molecules (49). According to the random coil model there must be a very definite relationship among the concentrations of x-mer rings in an equilibrated system, since the cyclization equilibrium constant Kx should depend on configurational entropy and therefore on equilibrium chain and ring dimensions. Values of /Af deduced from experimental values on Kx for polydimethylsiloxane, both in bulk and in concentrated solution, agree very well with unperturbed dimensions deduced from dilute solution measurements(49). 

Fig. 53 Synthesis of cyclic polydimethylsiloxane for cyclization of linear a,

A cyclic polydimethylsiloxane was also prepared by the end-to-end reaction, i.e., a,ft)-dianion-functionalized polymers, which are then cyclized simply by reaction with a difunctional electrophile to give a cyclic polymer. The cyclic polydimethylsiloxane was synthesized from a commercially available o. tw-dihydroxy-polydimethylsiloxane (M = 2.460 g/mol). The Unear precursor was deprotonated using sodium hydride in dilute THF (< 10 2 M) and then end-coupled using a dichlorosilane coupUng agent (Fig. 53). The uncy-clized anionic linear precursors are then removed by a macroporous anion exchange resin. The successful cyclization and purification is monitored by IR and 29Si NMR, GPC, and MALDI-TOF mass [176],... 

The manufacture of pure octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5), which arc either marketed as such or are used as raw materials in the production of polydimethylsiloxanes by the polymerization process, is carried out by the so-called cyclization process. The hydrolysis or methanolysis product is heated in a suspension of potassium hydroxide and an inert liquid (e.g. mineral oil). This method is chosen to hinder polymerization of the siloxanes to highly viscous liquids. The potassium hydroxide catalyzes an equilibrium reaction in which the Si-O-Si bonds are cleaved and newly made (equilibration). Since in this process the, by comparison with the linear siloxanes, more volatile octamethylcyclotetrasiloxane and decamethylcyclopenta-siloxane are continuously distilled off from the siloxane mixture, the equilibrium is shifted in a direction favoring the desired cyclic siloxane thereby enabling all of the siloxane to be so converted. 

A number of results were found that may have otherwise gone unnoticed in briefer studies (1. The cyclization mechanism of degradation which occurs for polydimethylsiloxane is... 

It is interesting that it was reported that no evidence of cyclization of the diols was observed during polymerization. The marked influence on the heterolytic stability by the fluorohydro-carbon alkyl spacers is amply pointed out by the following experiment, A sample of polymer Ild for (x=6) formed no appreciable amount of volatile reversion products when heated at 270 C under high vacuum in the presence of potassium hydroxide. Under these conditions, both polydimethylsiloxane and polymethyl (3,3,3-triflu-oropropyl )siloxane generate and lose volatile reversion products almost quantitatively. 

Wright, P. V., Cyclization Equilibrium Constants and the Conformations of Polydimethylsiloxane Chains. J. Polym. Sci., Polym. Phys. Ed. 1973,11,51-64. 

The synthesis of amine terminated polysiloxane (PSX) has been reported elsewhere (4). Aminopropyl terminated polydimethylsiloxanes with s of 1070 and 2670 g/mole, as determined by potentiometric titration, were used. The synthesis of high molecular weight, fully cyclized, soluble polyimides endcapped with phthalic anhydride (PA) has also been reported (12). 

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