Subject cyclic oligomers formation

The ring-opening polymerization of D4 is controlled by entropy, because thermodynamically all bonds in the monomer and polymer are approximately the same (21). The molar cyclization equilibrium constants of dimethylsiloxane rings have been predicted by the Jacobson-Stockmayer theory (85). The ring—chain equilibrium for siloxane polymers has been studied in detail and is the subject of several reviews (82,83,86—89). The equilibrium constant of the formation of each cyclic is approximatdy equal to the equilibrium concentration of this cyclic, Kn [(SiRjO) J. Thus the total concentration of cyclic oligomers in the equihbrium is independent of the initial monomer concentration. As a consequence, the amount of linear polymer decreases until the critical dilution point is reached, at which point only cyclic products are formed. 

The hydrolysis of (EtO)4Si (and the subsequent polycondensation of Si-OH containing molecules) has been the subject of considerable investigation because of its importance in the fabrication of glasses and colloidal silica via the sol-gel process see Sol-Gel Synthesis of Solids). Hydrolysis of the Si-0 bond in alkoxysilanes is also very widely used to attach silicon compounds to surfaces and in coupling reactions. The fundamental reaction types involved in the formation of polymeric silica materials (via, for example, cyclic oligomers similar to those shown in Figures 4 and 5) from monomeric (RO)4Si compounds are shown in Scheme 43. 

The initiation reaction yields an imide moiety, which constitutes a growth center for propagation reaction. Addition of certain imides such as acyl lactams as coinitiators essentially eliminates the initiation reaction and makes possible the polymerization at relatively low reaction temperatures. Mechanistic and kinetic aspects of the anionic polymerization of lactams have been treated quite extensively [16b]. The discussed subjects relate to the various equilibria governing the polymerization process. They comprise equilibria allied to monomer conversion, to the formation of cyclic oligomers, and to the effect of initiator concentrations. 

To avoid oligomer formation, Roberts and Rainier utilized an internal rather than a terminal olefin as a precursor to the cychzation reaction (Scheme 3.57) [62]. To this goal, internal olefin 303 was synthesized and subjected to enol ether formation but with the titanium efhylidene rather than the methylidene reagent for the acyclic enol ether forming reaction. Surprisingly, this relatively minor modification resulted in the conversion of 303 into cyclic enol ether 300 [63]. No acyclic enol ether was observed. The authors argued that the relatively moderate yield was due to the instability of the products and was not necessarily a result of an inefficient reaction. 

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