Epoxide copolymerization alternating

An alternative method for generating enriched 1,2-diols from meso-epoxides consists of asymmetric copolymerization with carbon dioxide. Nozaki demonstrated that a zinc complex formed in situ from diethylzinc and diphenylprolinol catalyzed the copolymerization with cyclohexene oxide in high yield. Alkaline hydrolysis of the isotactic polymer then liberated the trans diol in 94% yield and 70% ee (Scheme 7.20) [40]. Coates later found that other zinc complexes such as 12 are also effective in forming isotactic polymers [41-42]. 

The synthesis of polycarbonates from the alternating copolymerization of epoxides with C02 was first reported in 1969 using a ZnEt2/H20 mixture.954 Subsequent studies have focused upon a... 

Abstract Development in the field of transition metal-catalyzed carbonylation of epoxides is reviewed. The reaction is an efficient method to synthesize a wide range of / -hydroxy carbonyl compounds such as small synthetic synthons and polymeric materials. The reaction modes featured in this chapter are ring-expansion carbonylation, alternating copolymerization, formylation, alkoxycarbonylation, and aminocarbonylation. 

It is also possible to desymmetiize a meso epoxide in the alternating copolymerization. Thus, asymmetric alternating copolymerization of cyclohexene oxide with CO2 catalyzed by a dimeric zinc complex provides a polycarbonate in which the diol unit is optically active with 80% ee. (See Scheme 4.24.)... 

Lu X-B, Wang Y (2004) Highly active, binary catalyst systems for the alternating copolymerization of CO2 and epoxides under mild conditions. Angew Chem Int Ed 43 3574—3577... 

Shi L, Lu X-B, Zhang R, Peng X-J, Zhang C-Q, Li J-E, Peng X-M (2006) Asymmetric alternating copolymerization and terpolymerization of epoxides with carbon dioxide at mild conditions. Macromolecules 39 5679-5685... 

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... 

Some copolymerizations have been studied where one of the reactants is a compound not usually considered as a monomer. These include copolymerizations of epoxides and higher cyclic ethers with carbon dioxide, episulhdes with carbon dioxide and carbon disulhde, and epoxides with sulfur dioxide [Aida et al., 1986 Baran et al., 1984 Chisholm et al., 2002 Inoue and Aida, 1989 Soga et al., 1977]. The copolymers are reported to be either 1 1 alternating copolymers or contain 1 1 alternating sequences together with blocks of the cyclic monomer. 

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. 

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. 

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... 

Figure 3.10. Highly active Zn(II)-based catalysts for the alternating copolymerization of C02 and epoxides. R = CH(CH3)2.

Carbon dioxide is an attractive renewable resource being of low toxicity, abundant and the waste product of many processes. There is precedent, dating back to 1969, for the copolymerization of C02 and strained heterocycles, most notably epoxides [18]. The focus on this review will be on understanding the polymerization catalysts for the alternating copolymerization. 

TPPAIOR (1b) was effective for the copolymerization of epoxide with C02 (8-9) or with cyclic acid anhydride emd produced copolysiers with ester and ether linkages. These copolymers have a narrow moleculeir weight distribution, but do not yield alternating structures. 

In order to enhance the reactivity of aluminum porphyrins (J ) especially towards C02 in the copolymerization with epoxide (Equation 4), the effect of addition of an amine or phosphine as a possible sixth ligand to the aluminum porphyrin was examined. The enhancement in reactivity by the addition of a tertiary amine such as N-methyl-imldazole was actually observed for the epoxlde-C02 reaction. The product, however, was a cyclic carbonate (JO), not a linear copolymer. On the other hand, J he addition of trlphenylphosphlne was very effective in the formation of an alternating copolymer from epoxide and... 

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