SO2-catalyzed anionic copolymerization of oxiranes with cyclic anhydrides
Polimery 2000, No 7-8, 475
SummaryA study was made on the activating effect of SO2 on the reactivity of the oxirane ring to catalyze the amine-initiated copolymerizations of olefin oxides with cyclic anhydrides of carboxylic acids. Ethylene oxide and succinic anhydride were copolymerized anionically in the presence of tri-n-propyl amine (TPA) with SO2 added or not added (Table 2). In the presence of SO2, the induction period was much shorter, the conversion degree much higher and the Mn of the products much smaller. At higher contents (e.g., 10 mol %), SO2 not only acted as a catalyst, but also became incorporated into the resulting polyester chain. The copolymerization reaction with succinic and phthalic anhydrides and the molecular weight of the resulting polyester were studied in relation to oxirane type (Table 3). The structure of the end groups was examined by carrying out the reactions at the succinic anhydride/ethylene oxide/amine (TPA or Py) mole ratios of 1:1:1.5. 1H-NMR (Fig. 3) and MALDI ToF mass spectra showed counter-ions of the ammonium-carboxylic structure (I) to have formed, in which the initiator molecule was bonded to the ethylene oxide monomeric unit rather than to the anhydride monomeric unit (as suggested before, cf. eqn. 1). To study the effect of SO2 on copolymerization, poly(2-vinylpyridine) was used as initiator, whereby the products containing the initiator could be separated from other products (Fig. 5). To confirm the suggested mechanism of amine-initiated copolymerization of oxiranes with cyclic anhydrides of carboxylic acids in the presence of catalytic amounts of SO2 (eqns. 7–9), the isolated zwitter-ion (II) was made to react with succinic anhydride. The resulting product (III) was found (1H-NMR spectra, Fig. 6) to be terminated with an SO2 monomeric unit attached to the anionic backbone end and allowed to see that ammonium-sulfite ions were also formed during the polymerization which were transesterified under the influence of the anhydrides. A mechanism is suggested to explain the formation of cyclic products in terms of the "back-biting" reactions (eqns. 10–12).