Cyclic Ester Related Monomers

In addition to (substituted) lactones various cyclic esters related monomers were polymerized via enzyme-catalyzed ROP. 

For instance, in the last decade synthesis of poly(ester-alt-ether) was intensively studied. A common enzyme used in these syntheses is CALB. Polymerization of l,5-dioxepan-2-one (DXO) was performed by enzyme-catalyzed ROP in order to avoid contamination of product polymers by toxic organometallic catalysts [92], High molecular weight of poly(DXO) was obtained (Mn = 56000 Mw = 112000, 97% yield) at 60 °C for 4h. The polymerization had the characteristics of a living polymerization, as indicated by the linearity of plots between M and monomer conversion, meaning that the product molecular weight could be controlled by the stoichiometry of the reactants. Similarly, Nishida et al. [91] carried out enzymatic ROP of l,4-dioxan-2-one at 60 °C catalyzed by Novozym 435 that resulted in a polymer with Mw = 41000 in 77% yield. 

The lipase-catalyzed polymerization of the disubstituted TMC analog 5-methyl-5-benzyloxycarbonyl-l,3-dioxan-2-one (MBC) was also studied [89]. The bulk polymerization, catalyzed by Pseudomonas fiuorescens lipase for 72 h at 80 °C, gave 97% monomer conversion and product in 97% yield with M = 6100. The benzyl 

Enzymatic ROP of 2-oxo-12-crown-4-ether (OC) was studied by Meijer and coworkers [106], OC is different from other ether containing lactone monomers previously studied as it combines high hydrophilicity with a large ring size. Using Novozym 435 as catalyst, at 60°C for 90min in a mixture of toluene and tri-t-butylbenzene, homopolymerization of OC was successfully accomplished giving poly(OC) in yields 95% with M and Mw/M values of 3400 and 2.1, respectively. 

Substituted oxo-crown-ethers were studied as the starting monomers for cross-linked polymers gels. For instance, the CALB-catalyzed ROP of 2-methylene-4- 

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A compilation of the lipase-catalyzed lactone, cyclic ester related monomers and copolymers, the enzymes used, and the corresponding citation(s) are given in Table 4.4. 

Coordination polymerisation of heterocyclic monomers comprises polymerisation and copolymerisation processes of such monomers as oxacyclic monomers, especially epoxides [2,61-71], thiacyclic monomers like episulphides [72-76], azacyclic monomers [77,78] and phosphacyclic monomers [79]. Monomers with an exocyclic oxygen atom, such as cyclic esters like lactones [80-90] and lactide [90-92], cyclic acid anhydrides [93-98], cyclic carbonates [99,100] and related monomers, belong to oxacyclic monomers undergoing coordination polymerisation or copolymerisation. 

In parallel, Sanchez-Barba and coworkers also reported a related family of hybrid scorpionate/cyclopentadienyl Mg species, bpzcpjMgR (R = Me, 47 Et, 48 Bu, 49 tBu, 50 CH2SiMe3 51, Bn, 52) (Fig. 14) [69]. These complexes were moderately competent for the ROP of cyclic esters, and their behavior was reminiscent of that of the non-bulky heteroscorpionate-supported initiators the authors had previously described [67]. The polymerization of 500 equiv of CL proceeded smoothly (0-20 °C, 1-10 min) with 51, but was much more sluggish with the other complexes (2-48 h, 20-70 °C) the polydispersity was usually in the range 1.19-1.49. Complex 51, the most efficient initiator in this series, also enabled the ROP of l-LA (200 equiv) in a controlled fashion at 90 °C in toluene (2.5 h) or at 65 °C in THE (5 h) the polydispersity of the resulting PLLAs was low (M /M 1.02-1.15) and the molecular weight of the material was fully predictable on the basis of monomer-to-metal ratio (Mn,obs 34 caic 5,800-27,600 g mol-i). The ROP of d,l-LA was even slower (conversion <59 % after 8 h at 90 °C) but only returned atactic PLA. 

In this chapter we have shown that studies of the thermodynamics and kinetics of ROP play an indispensable role in our understanding of polymerization mechanisms. The results of these investigations have helped to establish controlled polymerization conditions, allowing the preparation of polymers with required molar masses and microstructures. The presence of various heteroatoms within the macromolecular main chain introduces an almost infinite number of possible homopolymeric and copolymeric properties. A recent development has been the controlled synthesis of aliphatic polyesters, mostly via ROP, based on their potential applications as biodegradable thermoplastics or as biomedical polymers. Moreover, as some cyclic ester monomers are prepared from renewable resources, some of the examples provided here have related to the ROP of aliphatic cyclic esters. 

In addition to traditional anionic polymerization techniques for the ROP of cyclic esters, the metal-free anionic ROP of lactide has also been reported (60). Bredereck s reagent [tert-butoxybis (dimethylamino) methane] and related compounds were shown to be latent anionic initiators capable of dissociation to a formatni-dinium cation and an alkoxide anion (Scheme 14.16). These species are then able to propagate PLA formation, resulting in a well-controlled polymerization at 70 °C that produces polymers with predictable molecular weights by monomer conversion and monomer initiator ratio, with narrow polydispersities (<1.18). Both, NMR and mass spectroscopic investigations have demonstrated incorporation of the alcohol initiator at the end of the polymer chain. The authors proposed that, as a consequence of the electrophilic nature of the formamidinium counterion, a reversible capture of the propagating anion (Scheme 14.16) was possible, and may be a hkely source of the excellent control observed. 

Benzoin derivatives are used as initiators for the photochemical curing of printing inks, lacquers and other surface coatings, since the intermediate radicals in a reaction such as (4.7) can be diverted to initiate the polymerization of vinyl monomers. The use of an unsvm-metrical ketone (4.8) also shows that discrete radicals are produced in the cleavage reaction, since the ratio of hydrocarbon products is close to that expected tor a random combination of separated radicals. Esters that give rise to similar stabilized radicals undergo loss of carbon dioxide (decarboxylation) by a closely related mechanism, and this has proved useful in making quite strained cyclic systems bv irradiation of readily prepared cyclic diesters (4.9). 

In addition to the polymerisation of pure lactones, these cyclic [di] esters have already been included as co-monomer to improve the properties of closely related polymers like polyamide obtained starting from lactams [86]. 

The H-spectra of isotactic and syndiotactic polymers derived from cyclic ethers, thioethers and esters should consist only of the resonances expected for the spin systems present in the individual monomers. AB patterns are typically observed for the backbone protons in poly(a-deuterio-propylene oxide) [21], poly(a-deuterio-propylene sulfide) [22], polylactide [23], and related materials. 

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