Polymerisation of Lactones

The coordination polymerisation of lactones with a six- and seven-membered ring (5- and e-lactones respectively) occurs via ring opening at the C(0)-0 linkage to generate metal alkoxide chain terminals, following a reaction analogous to that presented by scheme (9). 

Aluminium alkoxides (especially aluminium isopropoxide), dialkylalumi-nium alkoxides, yttrium alkoxides, zinc alkoxides, aluminoxanes, zincoxanes, bimetallic -oxoalkoxides, aluminium porphyrins and aluminium Schiff s base complexes are the most representative coordination catalysts, containing multi-nuclear or mononuclear species, for lactone polymerisations (Table 9.5). 

Of the catalysts evaluated, those effective for the polymerisation of l -lactones with ring opening by C(0)-0 bond cleavage [scheme (9)], as well as for the polymerisation of 8- and e-lactones (also with C(0)-0 bond cleavage), have 

The polymerisation of lactones via C(O) O bond cleavage has been explained in terms of the nucleophilic attack on the carbon atom of the carbonyl group in the coordinated monomer molecule. This attack can be carried out by the adjacent metal atom of the catalyst containing the multinuclear species [82]  

More recently, a very efficient yttrium-based catalyst, yttrium 2-methoxyeth-oxide, has been applied successfully for /f-butyrolactone polymerisation which proceeded easily at room temperature [99], It is worth mentioning that rare-earth metal alkoxides (derived from yttrium and lanthanum) exhibit outstanding efficiency as catalysts for the polymerisation of cyclic esters such as e-caprolactone [132] and lactide [133]. 

---

Analogously to the mechanism of epoxide polymerisation, the mechanism of the polymerisation of -lactone and other heterocycles with both endocyclic and exocyclic heteroatoms involves multicentred transition states with the participation of at least two metal atoms. 

Apart from the coordination polymerisation of lactones, the polymerisation of lactide (3,6-dimethyl-l,4-dioxacyclohexane-2,5-dione) with various coordination catalysts is of growing interest. This is connected with some advantageous properties of polyesters obtained from rac-D,L-lactide and mexo-D,L-lactide as potentially useful materials for biomedical and pharmaceutical applications in terms of their low toxicity and favourable biodegradability. A number of catalysts containing multinuclear species, such as aluminium isopropoxide [108,138,139], triethylaluminium-neopentyl (1 1) [139], triethylaluminium-(+)-menthol (1 1) [139], methylaluminoxane [139] and bimetallic /i-oxoalkoxide [140], as well as catalysts containing mononuclear species, such as (tpp) AlOMe [141] and (sal)AlOMe [130], have been used for the polymerisation of lactide. 

Polymerisation of Lactones Yu. N. Sazanov, Russ. Chem. Rev. Engl. Transl), 1968, 37, 463-469. 

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] ... 

During the anionic or anionic coordinative polymerisation of lactones, there is a permanent equilibrium (8.31) between the alcoholate groups and hydroxyl groups from the reaction system ... 

The polymerisation of lactones initiated by hydroxyl groups is possible in the absence of any catalyst, but it needs higher reaction temperatures (160-180 °C). 

The last type of ring-opening polymerisation of lactones discussed in the present work Involves the use of organic compounds as catalysts. Although alcohols and amines are generally not nucleophilic enough to initiate polymerisation, in some cases, tertiary amines are able to initiate the polymerisation of highly reactive -lactones [30]. 

The polymer synthesis of long chain polyhydroxyacids can be performed either in bulk via condensation reactions or by the ring-opening polymerisation of lactones/ macrolactones obtained by co-hydroxyfattyacid cyclisation [30]. 

Cyclic dicarbonates, cyclo /s(hexamethylene carbonate) and cyclo /s(diethylene glycol carbonate) were polymerised by lipase from C. antarctica and P. fluorescens [62]. CALB was used for the ROP of cyclo /s(decamethylene carbonate) (DMC2) giving a polymer with a MW of 5.4 x 10 and 99% yield, and an ultralow enzyme/substrate weight ratio of 1/200. Compared with six-membered trimethylene carbonate, a much lower reaction activity of large-sized DMC2 vvas observed, the opposite of the enzymatic polymerisation of lactones with different ring sizes [63]. 

Uyama and co-workers [126,127] utilised PFL in the single-step ROP of DDL and acylation of hydroxy termini with different vinyl esters to produce polymers containing polymerisable groups only at one terminus of the polymer chain. Kobayashi and coworkers [128] have reported that the lipase-catalysed (CAL and PCL) polymerisation of lactones (12-, 13- and 16-membered), divinyl esters of adipic and sebacic acid and a,(o-glycols using one-pot synthesis produced the corresponding ester copolymers in which two different types of polymerisation, ROP and polycondensation as well as transesterification, simultaneously occurred via the same enzyme intermediate to provide random copolymers. Polymerisation of macrolides (DDL and PDL) in the presence of preformed polyester (polycaprolactone) produced the corresponding copolyesters [129]. 

Table 12.6 Lipase catalysed ring opening co-polymerisation of lactones ...

RING-OPENING POLYMERISATION OF LACTONES, LACTIDES CABOXYANHYDRIDES AND SIMILAR MONOMERS... 

Ring-opening polymerisation is receiving much interest presently probably due to the range of monomers that can be polymerised with this procedure. A number of recent reviews have appeared on aspects of ROP including polymerisation of lactones, lactides, and cyclic esters [138], cyclic carbonates and block copolymers with ureas and PTHE [139], and the use of a wide range of macroinitiators in ROP [140]. 

The second process, ring-opening polymerisation of lactones, lactides and glycolide (Fig. 4.3), is free from these limitations. High molecular weight polyesters can be easily prepared under mild conditions from lactones of different ring size, substituted or not by functional groups (Lou et al, 2003). 

Ring-opening polymerisation of lactones, lactides and glycolide. 

Supercritical carbon dioxide as a medium for the ring-opening polymerisation of lactones and lactides and a processing aid for aliphatic polyesters... 

---