Ring-opening polymerization exchange reactions

Summary Phosfdiazene bases represent a new class of highly active non-ionic catalysts that rapidly polymerize cyclosiloxanes with equilibrium attained in very short reaction times at very low catalyst levels. To date, phosphazene base catalysts have been considered an academic curiosity because of the complicated and hazardous synthetic protocol used to prepare them. A facile synthetic process has been developed, which yields ionic phosphazene bases in three steps with an overall yield of qrproximately 75%. This is achieved through nucleophilic substitution of ionic phosphonitrilic chloride oligomers with secondary amines, followed by anion exchange. These ionic phosphazenes were found to exhibit similar reactivity in the ring-opening polymerization of cyclosiloxanes to that of the non-ionic phosphazene base. 

Montaudo, G., Montaudo, M. S., Puglisi, C., Samperi, F, Spassky, N., LeBorgne, A., and Wisniewski, M., Evidence for Ester-Exchange Reactions and Cyclic Oligomer Formation in the Ring-Opening Polymerization of Lactide with Aluminum Complex Initiators, Macromolecules, 29, 6461, 1996. 

Anionic ring-opening polymerization of POx initiated by partially deprotonated dipropyleneglycol (DPG) in the presence of the phosphonium cation tetrakis[cyclohexyl(methyl) amino jphosphonium as counterion (P Scheme 14) has been reported by Rexin and Miilhaupt. An alcohol/phospho-nium alkoxide mixture (95/5) was utilized to control reactivity, making use of a fast proton exchange reaction between dormant hydroxy compounds and active phosphonium alkoxide ends (Scheme 16). 

Metathesis of alkenes is essentially a class of reactions where an interchange of C atoms between pairs of double bonds takes place. A few representative examples are shown by the reactions listed in Fig. 7.10. The industrial use of metathesis reactions so far has been limited mainly to exchange metathesis (Fig. 7.10, top, backward reaction) as in the SHOP process, and ring-open metathesis polymerization (ROMP). As already mentioned (Section 7.5), Vas-tenamer is a polymer made by Hulls by ROMP from cyclooctene. Similarly, the polymer from norborene by ROMP is manufactured by CdF Chemie and is sold by the trade name of Norsorex . 

Ring opening metathesis polymerization, which has been known since the discovery of the alkene metathesis reaction, has been given the acronym ROMP in recent years. In fact, the ROMP reaction was the first observation made in alkene metathesis chemistry, while the discovery of the exchange reaction in equation (1) actually occurred later. Acychc diene metathesis (ADMET) polymerization (equation 3) has only recently been shown to be a viable method for polymer synthesis, and it has been termed ADMET polymerization. ROMP reactions are driven by the release of ring strain from the monomer, while ADMET polymerization is driven by a shift in the equilibrium caused by the removal of one of the reaction products. 

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