Acyl-oxygen scission

NMR studies on the alkoxide initiators confirm that all the lactones polymerize via an acyl— oxygen scission, including /3-PL (which, by contrast, opens at the alkyl—oxygen bond with (251)). Monomer coordination and subsequent ring opening may be observed by 111 NMR spectroscopy. Coordination is also observed with 7-BL and 7-VL, although these adducts are stable to insertion and polymerization does not proceed. 

Carothers and colleagues were the first to explore the ROP of lactones. Many research laboratories have now been involved in this research area. The ROP of lactones is the method of choice for the production of biocompatible and biodegradable polyesters. Lactones are ambidentate and the polymerization may proceed by either alkyl-oxygen or acyl-oxygen scission. Evidence in favor of both types of scission is reported in the literature. 

The anionic polymerization of 49 initiated with lithium benzophenone ketyl gave the polymer 50 having a cisjtrans ratio of 42/58. Probably it proceeded through the acyl-oxygen scission of the ester linkage of the monomer. The formation... 

Figure 21.1 Possible mechanisms for anionic ring-opening polymerisations of non-substituted lactones showing both acyl-oxygen scission (a) as well as alkyl-oxygen scission (b).

The metal-insertion ROP owes its name to the propagation mechanism [Fig. 21.2]. After coordination of the metal-alkoxide with the carbonyl group of the lactone, the addition of the nucleophilic alkoxides takes place onto the electrophilic ester bond. Subsequently, an elimination reaction occurs via acyl-oxygen scission. The novel alkoxide will act as the newly generated propagating species. 

In (blactones, scission of either the acyl-oxygen bond or the alkyl-oxygen bond may take place leading to the formation of alkoxide- or carboxylate-grow-ing chains [ 115]. Methylene chloride end groups were observed in the ZnCl2-in-... 

Recently, tin(II) butoxide was used in the polymerization of l-LA [88]. The initiation is fast and quantitative and no transesterification or back-biting reactions are observed. The reaction proceeds with acyl-oxygen bond scission with retention of the configuration, and can be used both in bulk and solution (THF, 20-80 C) polymerization. It is possible to control the molecular weight in the range of 103 to 106 with a MWD of 1.15-1.85. The polymerization is very fast, kp=5 x 10 1 mol-1 L s-1, with only the rare earth alkoxides being faster. 

In the first scheme (Fig. 21.1, reaction a), the alkoxide attacks the ester group after which an acyl-oxygen bond is broken. In the other case [Fig. 21.1, reaction b), the alkoxide attacks the carbon atom adjacent to the alcohol residue of the ester, after which an alkyl-oxygen scission takes place. This last mechanism is of particular importance for the polymerisation of four-membered lactones (3-lactones] in which the first mechanism is disfavoured by stereo-electronic effects [32]. 

Initiation of [3-lactone polymerization with ionic alkoxides proceeds in such a way that it initially results in acyl-oxygen and alkyl-oxygen bond scission in c. 1 1 proportion (Scheme 2). However, independent of the chemistry at earlier stages, the eventual active species are carboxylate... 

The alkyl-oxygen bond scission is known only for P lactones, whereas for higher laaones and LAs exclusively the acyl-oxygen bond scission proceeds and the active species are alkoxide anions. 

On the other hand, in the purely anionic polymerization of five- or higher-membered cyclic esters, the carbonyl carbon of the monomer is attacked with subsequent acyl-oxygen bond scission and reformation of the alkoxide anion. In the coordination polymerization, this is also the carbonyl carbon that is now first coordinated with alkoxide species and then the acyl-oxygen bond is broken with reforming of the covalent alkoxide chain end. In the already formed macromo-lecular chains, the same ester bonds are present as those being the site of the nucleophilic attack in the monomer molecules. These processes are illustrated in Scheme 12, where the active centers are shown as. ..-OMt, for both anionic and covalent centers. 

The nature of the active centers formed at various stages of the polymerization has also been elucidated in several model experiments. The authors concluded that the propagation would proceed on alkoxide and carboxylate active centers, respectively formed by the acyl-oxygen and alkyl-oxygen bond scissions of the monomer. The number of carboxylate active sites was also found to increase with the DP, such that they became prevalent on completion of the reaction (Scheme 9.14) [48]. 

A possible mechanism for the formation of the furanones 6 and 7 is illustrated in Scheme 2. The initial alkoxy radical generated from the alcohol 5 and lead tetraacetate (LTA) undergoes /3-scission to produce the acyl radical intermediate 9. Subsequent cyclization to 10 proceeds through attack of the radical at the carbonyl oxygen. The resulting Pb(IV) intermediate 11 finally collapses via the reductive... 

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