Polymerization, anionic lactones

In contrast to the fact that cyclic acetals can be polymerized only by cationic initiators, lactones undergo polymerization both cationically and anionically, and therefore a wide variety of initiators including coordinated catalysts can be used. In this section, the polymerization of bicyclic lactones is described, although only a limited number of papers on this subject have been published. 

A very broad range of initiators and catalysts are reported in the scientific literature to polymerize lactones. The polymerization mechanisms can be roughly divided into five categories, i.e., anionic polymerization, coordination polymerization, cationic polymerization, organocatalytic polymerization, and enzymatic polymerization. 

It is worth noting that another transfer reaction to the monomer is reported in the case of the anionic polymerization of (3-lactones, as shown in Fig. 9. This transfer reaction takes place during the polymerization of (3-propionolactone initiated by carboxylates even though molar masses up to 10,000 can be reached, even at room temperature [8]. The situation is even more critical in the case of the polymerization of substituted (3-butyrolactones, as witnessed by a ratio equal to 10, which is the highest number average degree of polymerization that can be reached under these conditions [8]. 

The range of monomers that can be incorporated into block copolymers by the living anionic route includes not only the carbon-carbon double-bond monomers susceptible to anionic polymerization but also certain cyclic monomers, such as ethylene oxide, propylene sulfide, lactams, lactones, and cyclic siloxanes (Chap. 7). Thus one can synthesize block copolymers involving each of the two types of monomers. Some of these combinations require an appropriate adjustment of the propagating center prior to the addition of the cyclic monomer. For example, carbanions from monomers such as styrene or methyl methacrylate are not sufficiently nucleophilic to polymerize lactones. The block copolymer with a lactone can be synthesized if one adds a small amount of ethylene oxide to the living polystyryl system to convert propagating centers to alkoxide ions prior to adding the lactone monomer. 

Anionic polymerization proceeds by acyl-oxygen cleavage for almost all lactones, consistent with the mechanism for alkaline saponification of esters [Duda and Penczek, 2001 Hofman et al., 1985 Kricheldorf et al., 1988], For example, initiation by methoxide ion proceeds as... 

Anionic polymerization of -caprolactone has been studied in several laboratories and it was found that considerable amounts of oligomers were found as by-products.— — We have studied the formation of oligomers in the anionic polymerization of 6-capro-lactone by gpc technique.A In view of the very facile intra- and intermolecular transesterification reactions in this system- -, the product distribution seems very interesting to check the validity of the thermodynamic equilibrium. 

Successive addition of monomers to the end of macromolecular initiator is the usual technique for the synthesis of tailored blockcopolymers. Anionic polymerization of pivalolactone, a-pyrrolidone— and the NCA of T-methyl-D-glutamate -2 was started from the end group of a prepolymer consisting carboxylate group or acyl lactam group or amino group. Living polymer of C-capro-lactone was expected to be formed by the initiated polymerization from polymer carbanion under kinetic controlled condition. 

Thus, in the present paper we review the available data, pertinent to the kinetics and mechanism of anionic polymerization of lactones and discuss the recent data of our own, giving eventually an access to the rate constants of propagation on macroions and macroion--pairs. 

Alkoxide-Type Initiators. Using the guide that an appropriate initiator should have approximately the same structure and reactivity as the propagating anionic species (see Table 1), alkoxide, thioalkoxide, carboxylate, and silanolate salts would be expected to be useful initiators for the anionic polymerization of epoxides, thiiranes, lactones, and siloxanes, respectively (106—108). Thus low molecular weight poly(ethylene oxide) can be prepared... 

Kricheldorf et al. reported an anionic polymerization of y-D,L-butyro-lactone or D,L-lactide with cyclic dibutyltin initiators, such as 2,2-dibutyl-2-stanna-l,3-dioxepane, to give cyclic polymers [ 138-140]. Figure 41 shows the ring expansion polymerization of lactone for synthesizing a cyclic polymer as an example. They also synthesized the cyclic polymer with a living mechanism in the polymerization of e-caprolactone [141]. 

The depolymerization reaction is suppressed in the presence of a more associated alkoxide such as lithium alkoxide. y-Lactones are difficult to polymerize by ROP. However, anionic polymerization of bicyclic bis(y-lactones) [76] can be carried out according to Scheme 8. 

PL, unlike other lactones, undergoes polymerization with weakly nucleophilic initiators such as metal carboxylates, tertiary amines, phosphines, and a variety of other initiators [81-83]. This is primarily due to the high ring-strain in the four-membered ring. Pyridine and other tertiary amines initiate the anionic polymerization via a betaine that rapidly transforms into a pyridinium salt of acrylic acid. In order to minimize the chain transfer reactions, the polymerization is performed at a temperature between 0 and 10 °C (Scheme 9). 

Some heterocycles have both nucleophilic and electrophilic atoms in their molecule. Thus they can be opened and polymerized by the anionic, cationic or coordination mechanisms. Examples are lactams, lactones, and cyclic siloxanes. Investigations of the mechanism of lactam propagation are complicated by the occurence of side reactions. In principle, the mechanism described in Chap. 3 by the schemes (55)—(57) and (71) is accepted. Anionic polymerization of cyclic esters consists, in most cases (see Chap. 4, Sect. 2.2) of repeated reversible attacks on the carbonyl carbon by the anion 0]-. From e-caprolactone, polyester chains grow according to [315]... 

The kinetic depression is well known in the anionic polymerization of e-capro-lactone, where the formation of the cyclic dimer, trimer and tetramer starts when monomer is almost completely exhausted ... 

The anionic polymerization of A(-benzyloxycarbonyl-L-serine 3-lactone leads to poly(A -acyl-L-serine ester) My, = ca. 40 000), from which poly(L-serine ester hydrochloride) can be obtained by hydrogenation. ... 

The anionic polymerization reactions of a.Oi-disubstituted- -propio-lactones, and the properties of the resulting polyesters, with the structures shown on the next page, have been under investigation in this laboratory for over ten years (1,2). 

These and related methods allowed us recently to reevaluate the structure of active centers in anionic polymerization of simple, unsubstituted lactones, 5-propiolac-tone. The rationale was put forward in terms of stereo -electronic factors to explain why g-propiolactone propagates on carboxylate and e-caprolactone on alcoholate anions. This is shown in scheme below ... 

As a result. In the anionic polymerization of lactones, low pol3rmer yield, uncontrollable molecular weight, broad distribution, coupling linkage, and cyclic ester oligomers contamination have been frequently encountered O In this paper, a new Improved process was revealed to eliminate such trensesterlflcatlons has been developed. Block copolymers with styrene and butadiene have been prepared and characterized by different techniques. 

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