Alternating epoxide copolymerization mechanism

The copolymerization mechanisms show that the propagation reactions are bimolecular in two alternating steps and that the rate-determining step is the slower propagation reaction. In our case, the slower reaction is the addition of the epoxide (Eqs. (38), (43), (47), (52), (59), (67), and (71) in the respective schemes) 99) as has also been found for the initiation by ammonium salts56). Since the tertiary amine does not directly take part in growth reactions (cf. 3.3.3), a more suitable expression for the copolymerization rate is Eq. (84) where the tertiary amine should be replaced by an active centre. 

Fig. 16 Proposed mechanism for the epoxide/CO alternating copolymerization, including pyridine mediation...

Epoxides readily undergo anionic copolymerization with lactones and cyclic anhydrides because the propagating centers are similar—alkoxide and carboxylate [Aida et al., 1985 Cherdron and Ohse, 1966 Inoue and Aida, 1989 Luston and Vass, 1984]. Most of the polymerizations show alternating behavior, with the formation of polyester, but the mechanism for alternation is unclear. There are few reports of cationic copolymerizations of lactones and cyclic ethers other than the copolymerizations of [5-propiolactone with tetrahydrofuran and... 

All authors accept the alternating incorporation of epoxide and anhydride into the macromolecular chain 36 39.40.45 52.73-74). However, the mechanisms of termination and chain transfer have not yet been elucidated. Although the lability of the nitrogen atom is obvious 39 40 44> and its salts or associates are readily thermally decomposed 89), Fischer 39 detected its presence in precipitated polyesters by elemental analysis. A simple calculation confirms the presence of the nitrogen atom in almost every tenth macromolecule. In this case, the isolated polyester might be a living polymer and, on the addition of monomers, it might initiate another copolymerization. Similar experiments have not been reported so far. 

Coates and coworkers have carried out kinetic studies of the alternating copolymerization of CHO and C02 catalyzed by several of the P-diiminate zinc derivatives [29]. These authors have proposed a bimetallic mechanism to be operative, which is consistent with their experimental observations, including the large differences in activity noted for a series of structurally closely related catalysts. It was proposed that one zinc center would coordinate and activate the epoxide substrate, while the second zinc center would provide the propagating carbonate species to ring-open the epoxide. This proposal is represented by the transition state depicted in Figure 8.3a. 

Metal-catalyzed reactions of C02 and epoxides that give polycarbonates and/or carbonates have been extensively investigated as a potentially effective C02 fixation (Beckman, 1999 Inoue, 1987). The possible reaction mechanism is illustrated in Figure 3.8 (Darensbourg et al., 1999). The repetition of the reaction sequence in which C02 inserts into a metal-alkoxide bond, followed by ring-opening of the epoxide with the metal carbonate forms the alternating copolymer. In 1969, this copolymerization was first reported by Inoue and Tsuruta who used a Zn catalyst derived from... 

Under the same conditions, the reactivity of three-membered cyclic ethers in anionic copolymerization with cychc anhydrides is higher than that of four-membered ethers Higher membered cyclic ethers can polymerize or copolymerize with anhydrides only by a cationic mechanism whereby not only alternating copolymer but also a great number of polyether sequences are formed. This difference in reactivity is evidently associated with the basicity of cychc ethers, three-membered ethers having the lowest basicity The lower basicity causes a lower reactivity of the epoxide (cychc ether) in competitive reactions or in copolymerization with other cychc monomers compared with the expected reactivity which follows from the strain in the ring. The strain energy, taken as the difference between the experimental and calculated heats of formation was found to be 54.4kJ/mol for ethylene oxide... 

Moore, D. R. Cheng, M. Lohkovsky, E. B. Coates, G. W. Mechanism of the alternating copolymerization of epoxides and CO2 using beta-diiminate zinc catalysts Evidence for a bimetallic epoxide enchainment. J. Am. Chem. Soc. 2003,125, 11911-11924. 

The reaction mechanism for the copolymerization is initiated as shown in Scheme 6.24, left and once the first coupling epoxide-C02 is performed, the growth of the chain should proceed with a perfect alternate insertion of epoxide and CO2. 

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