Alkyl-oxygen scission

The highly strained 1,3-lactones give, in addition to the usual acid- and base-catalyzed hydrolysis typical of ordinary esters, an added reaction with water. This reaction proceeds through alkyl-oxygen scission and is accompanied by inversion as shown below. 

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).

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]. 

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. 

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-... 

The cationic polymerisation of lactones takes place in the presence of the usual cationic catalysts (Lewis acids and Bronstedt superacids) and the active propagating species are oxonium cations, formed by the attack of the exocyclic oxygen atoms of lactone and the ring opening of the lactone cycle takes place by alkyl - oxygen bond scission [31] ... 

The majority of polymers degrade photochemically or thermally by a free radical process. Thermal degradation of poly(ethylene terephthalate) and side group cleavage in PMA and PtBMA are examples where P-scission of the alkyl-oxygen bond occurs from a cyclic transition state without free-radical formation.[147]... 

One purpose of the present investigation was to obtain some additional information on structure-crystallinity relationships in this family of polymers by the preparation of stereoregular isotactic polyesters from a single asymmetric isomer of the chiral monomer that is, from an optically-active a,a-disubstituted-s-propiolactone. Because the polymerization reaction mechanism operates through scission of the alkyl-oxygen bond and does not involve bond reorganizations at the asymmetric center, it was fully expected that polymerization of the optically-active monomer... 

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. 

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]. 

The extent of decarboxylation primarily depends on temperature, pressure, and the stabihty of the incipient R- radical. The more stable the R- radical, the faster and more extensive the decarboxylation. With many diacyl peroxides, decarboxylation and oxygen—oxygen bond scission occur simultaneously in the transition state. Acyloxy radicals are known to form initially only from diacetyl peroxide and from dibenzoyl peroxides (because of the relative instabihties of the corresponding methyl and phenyl radicals formed upon decarboxylation). Diacyl peroxides derived from non-a-branched carboxyhc acids, eg, dilauroyl peroxide, may also initially form acyloxy radical pairs however, these acyloxy radicals decarboxylate very rapidly and the initiating radicals are expected to be alkyl radicals. Diacyl peroxides are also susceptible to induced decompositions ... 

Diperoxyketals. Some commercially available di(/ f2 -alkylperoxy)ketals and their corresponding 10-h half-life temperatures (deterrnined in dodecane) are hsted in Table 5 (39). Diperoxyketals thermally decompose by cleavage of only one oxygen—oxygen bond initially, usually foUowed by P-scission of the resulting alkoxy radicals (40). For acychc diperoxyketals, P-scission produces an alkyl radical and a peroxyester. 

This ladical-geneiating reaction has been used in synthetic apphcations, eg, aioyloxylation of olefins and aromatics, oxidation of alcohols to aldehydes, etc (52,187). Only alkyl radicals, R-, are produced from aliphatic diacyl peroxides, since decarboxylation occurs during or very shortiy after oxygen—oxygen bond scission in the transition state (187,188,199). For example, diacetyl peroxide is well known as a source of methyl radicals (206). 

The benzoin ethers (75, R-alkyl R H) and the ot-alkyl benzoin derivatives (75, R=H, alkyl R =alkyl) undergo a-scission with sufficient facility that it is not quenched by oxygen or conventional triplet quenchers.276 This means that the initiators might be used for UV-curing in air. Unfortunately, it does not mitigate the usual effects of air as an inhibitor (Section 5.3.2). The products of a-scission (Scheme 3.53) are a benzoyl radical (13) and an ( -substituted benzyl radical (76) both of which may, in principle, initiate polymerization, 76 2"... 

Present methods for solubilizing coal (including reductive alkylation in tetrahydrofuran (15) or liquid ammonia (8)) entail cleavage of oxygen ethers, scission of C-C bonds in certain polyaryl-substituted ethylenes and, in the case of reactions in tetrahydrofuran, extensive elimination of hetero-atoms (16). 

---