Ring-chain equilibration reaction

The synthesis of cyclic polydimethylsiloxane was first achieved through ring-chain equilibration of siloxane oligomers in the presence of potassium silanolate, as shown in Fig. 51 [163-165]. Cyclics recovered from ring-chain equilibration reactions have been fractionated by preparative GPC, yielding... 

An understanding of the thermodynamics of ring-chain equilibration reactions is important in Inorganic Chemistry where such reactions abound (sse, for example. Allcock (65), Gee (66), Gimblett (67), and Moedritzer (65)). Two such reactions are found (i) in polyphosphazene chemistry (69) where a polymerisation-depolymerisation equilibrium can be established (70) and (ii) in silicon-sulphur chemistry, where the analogue of the poly(dimethylsiloxane) system has been studied (71) and shown to contain cyclics such as the dimer ( (013)2 SiS)2 the trimer ((CH3)2SiS)3, as well as small amounts of chain molecules of limited chain length. 

The remarkable ease with which poly(dimethylsiloxane) enters into ring-chain equilibration reactions prompted the investigations described in this section, as well as those outlined in Section IIL(C). Both investigations were concerned with the question as to whether it is possible to study the statistical conformations of chain polymers by the equilibrium cyclic concentration method, if the polymer itself cannot undei o ring-chain equilibration reactions. The polymers chosen for study were n-paraffinic and polystyrene chains. In each case, dimethylsiloxane linkages were incorporated into the molecules and ring[Pg.56]

Finally, it is noted that some studies of ring-chain equilibration reactions in solid PET samples have been carried out (i 7,130). Heating PET chip containing about 2.2% w/w cyclics up to the nonamer resulted in a decrease in cyclic content to about 0.7% w/w, and an increase in molecular weight by a factor of about two. Similar decreases in the concentrations of cyclic oligomers have been found to result when melt equilibrated samples of nylon. 6 are heated in the >lid state, and these will be discussed in detail in the next section. 

Wichterle and his co-workers (167-169) established that ring-chain equilibration reactions can be carried out in nylon-6 at temperatures well below the melting point of the polymer [see Section III.(E)2 for similar reactions in poly(ethylene tereph-thalate)]. Such ring-chain equilibration reactions have been carried out recently (155) by heating samples of nylon-6 or e-caprolactam at 459 K in three clearly defined experiments. The individual cyclic oligomer concentrations were determined for each solid sample of nylon-6. The results obtained are plotted in Fig. 18 and 19, where they are compared with cyclic concentrations in a polymer prepared by rii -chain equilibration in the melt at 525 K (see Fig. 17). 

PREPARATIVE TECHNIQUES Ring-chain equilibration reactions. ... 

Various reactions, both of polymerization and of polymer degradation, can produce cyclic polymer molecules. A well-known process is the ring-chain equilibration reaction, which may be used to produce cyclic siloxanes and o er cyclic polymers. The linear chain reacts intramolecularly and yields a cyclic and a linear chain. In the initial stages, the molar fraction of cyclics increases at the expense of the linear chains. After some time, equilibrium conditions are achieved and the molar fraction of cyclics remains constant. In some cases, all the sites in the macromolecular backbone are equivalent and no peculiar bond exists which is preferentially attacked. This case is referred to as thermodynamically controlled cyclization. 

INTRODUCTION The molar cyclization constants from ring-chain equilibration reactions of poly(phenylmethylsiloxane) (PPMS) in both the bulk state and in solution were investigated in detail by Beevers and Semlyen. Based upon these studies Clarson and Semiyen have described scaling up such reactions to successfully isolate cyclic poly(phenylmethylsiloxanes), that is,... 

PROPERTIES OF SPECIAL INTEREST Viscous fluids having good thermal stability. PREPARATIVE TECHNIQUES Ring-chain equilibration reactions. ... 

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