Polymerization of 1,2-epoxides

Mechanisms for polymerization of epoxides by Lewis acids are proposed in References 11—15. 

Davidson, S.R. and Wilkinson, S.A., electron-beam-induced polymerization of epoxides. J. Photochem. Photobiol. A Chem.,SS, 123-134 (1991). 

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

Yasuda, T., Aida, T., and Inoue, S., Reactivity of (por-phinato)aluminum phenoxide and alkoxide as active initiators for polymerization of epoxide and lactone. Bull. Chem. Soc. Jap.. 9, 3931-3934, 1986. 

Beyond these fundamental aspects, the great versatility of organozinc compounds can also be seen in the important roles they play in commercial applications. Organozinc complexes have been shown to be excellent catalysts and precatalysts for the polymerization of esters and the co-polymerization of epoxides and carbon dioxide they are also co-catalysts in polyolefin catalysis. 

Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

Organozinc diiminates of various ligand platforms (particularly /2-diketiminates (BDIs)) have been used extensively in catalysis, especially for the polymerizations of lactides and epoxides and the co-polymerization of epoxides with carbon dioxide. 

Schon, E. Zhang, X. Zhou, Z. Chisholm, M. H. Chen, P. Gas-Phase and Solution-Phase Polymerization of Epoxides by Cr(salen) Complexes Evidence for a Dinuclear Cationic Mechanism. Inorg. Chem. 2004, 43, 7278-7280. 

Curing of epoxy resins can also be achieved by ring-opening polymerization of epoxide groups using either Lewis acids (including those generated photochemically) or Lewis bases (Sec. 7-2). 

The anionic polymerization of epoxides such as ethylene and propylene oxides can be initiated by metal hydroxides, alkoxides, oxides, and amides as well as metal alkyls and aryls, including radical-anion species such as sodium naphthalene [Boileau, 1989 Dreyfuss and Drefyfuss, 1976 Inoue and Aida, 1984 Ishii and Sakai, 1969]. Thus the polymerization of ethylene oxide by M+A involves initiation... 

The anionic polymerization of lactams proceeds by a mechanism analogous to the activated monomer mechanism for anionic polymerization of acrylamide (Sec. 5-7b) and some cationic polymerizations of epoxides (Sec. 7-2b-3-b). The propagating center is the cyclic amide linkage of the IV-acyllactam. Monomer does not add to the propagating chain it is the monomer anion (lactam anion), often referred to as activated monomer, which adds to the propagating chain [Szwarc, 1965, 1966]. The propagation rate depends on the concentrations of lactam anion and W-acy I lactam, both of which are determined by the concentrations of lactam and base. 

Potassium carboxylate groups introduced onto the surface of carbon fibers initiated anionic polymerization of epoxides (e.g., styrene oxide, epichlorohydrin, and glycidyl phenyl ethers) and cyclic acid anhydrides (e.g., maleic anhydride, succinic anhydride, and phthalic anhydride) in the presence of 18-crown-6 [41]. 

Alternating Co-polymerization of Epoxides with Carbon Monoxide... 

PO proceeded in a living manner to yield highly regioregular polyethers with narrow MWDs. These authors also developed the immortal polymerization of epoxides where polymers with narrow MWDs were obtained with the number of polymer chains exceeding the number of initial aluminum-porphyrin complexes (Scheme I). The key in the immortal polymerization is a reversible chain transfer, which is much more rapid than the chain propagation. In the presence of an alcohol (R OH) as a chain-transfer reagent, an aluminum-porphyrin complex with a growing species reacts with R OH reversibly, so that the polymerization takes place from all the molecules of aluminum-porphyrin complex and R OH. 

Scheme 2 Polymerization of epoxide in a oationio mechanism and a possible side reaction.

Immortal polymerization of epoxides with la and an alcohol is also accelerated by co-use of bulky Lewis acid 2a. The polymerization of PO with la/2-propanol system ([PO]/[la]/[2-propanol] = 1000/1/49) in the presence of 2a ([PO]/[2a] = 1000/1) proceeds rapidly to achieve 86% conversion in 1.5 h, while the polymerization in the absence of 2a requires 380 h to reach 84% conversion (Table 1). The polyether produced in the presence of 2a has an of 900 gmoP and an MJM of 1.10, which indicates that almost all of la and 2-propanol participate in the initiation of the polymerization. Other protic chain-transfer reagents, such as methanol, benzyl alcohol, and 4-/ r/-butylphenol, are also applicable to the high-speed immortal polymerization to give similar results as 2-propanol. As a substrate, ECH is also employable. Polymerization of ECH ([EGH]/[la]/[2-propanol]/[2a] = 1000/1/49/1) gives a polymer with and/n of 1100gmol close to the value estimated from the conversion and [PO]/([la] + [2-propanol]) ratio, and a narrow M IM of 1.10, while the conversion is lower than the case of PO. 

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