Zinc dimethoxide

Oxetane, a four-membered cyclic ether, is highly susceptible to cationic polymerisation [83]. However, this monomer also undergoes coordination polymerisation in the presence of catalysts such as zinc dimethoxide [84], triethylaluminium water acetylacetone [85-87], aluminium isopropoxide zinc chloride and di-ethylzinc water [87,88], as well as tetraphenylporphinatoaluminium chloride methylaluminium di(2,6-di-/-butyl-4-methylphcnoxidc) [89]. Studies of the microstructure of the polymer derived from the polymerisation of 2-methylox-etane with the triethylaluminium-water-acetylacetone (2 1 2) catalyst showed that the polyether obtained consisted of regioregular monomer unit sequences, fairly rich in isotactic triads [87] ... 

The active species of the diethylzinc-methanol system was previously proven to be zinc dimethoxide (1 ) Contrary to the zinc-water system, no trace of cationic nature was observed in the zinc-methanol system at any ratio of the two components. 

When racemic methyloxirane is polymerized with zinc dimethoxide, D-and L-monomers are separately incorporated into growing chains to form an isotactic polymer consisting of poly(D-methyl-oxirane) and poly(L-methyloxirane). This stereoselective polymerization can be satisfactorily explained in terms of the enantio-morphic catalyst sites model (1 ). The d -sites accept D-methyl-oxirane in preference to the L-monomer, resulting in the formation of -DDDD- isotactic sequences. The same situation is valid for the l -catalyst sites. 

Similar results with respect to the initiation step were obtained with dibutyl magnesium [13], aluminum trialcoholates [14], dibutyltin dimethoxide [14], diethyl zinc [14], tin octoate and rare-earth metal compounds as initiators [15-22]. With these initiators, a coordination-insertion mechanism is active which, for some of the initiators, has been studied in detail [30, 31]. A special initiator with an aluminum alcoholate group is that of tetraphenylporphyrin aluminum this has an alkoxy group as the third ligand (RO—Al—TPP) at the aluminum, and is known as the Inoue catalyst [32]. This initiator is active for various monomers, and is therefore used for the preparation of block copolymers with one or two polycarbonate blocks. Initiation occurs by the nucleophiUc addition of RO—Al—TPP to the carbonyl group of the carbonate. 

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