Lactones, polymerisation

The mode of lactone ring opening depends on the kind of catalyst. It is characteristic that -lactone polymerisation with a catalyst containing a metal alkoxide active bond (Mt-X X = OR) involves C(0)-0 bond scission in the coordinating monomer (via the metal orthocarbonate species) with regeneration of the metal alkoxide active bond [scheme (7)] [87]. On the other hand, the application of a catalyst with a metal carboxylate active bond [Mt-X X = 0C(0)R] for -lactone polymerisation results in Cp — O bond scission in the coordinating monomer with regeneration of the metal carboxylate active bond [scheme (8)] [88-90],... 

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). 

In the case of lactone polymerisation with the (tpp)A10R catalyst, ring opening via C(0)-0 bond cleavage involves the participation of two molecules of the catalyst [36,136] which can be presented schematically for the polymerisation of /1-lactone as follows ... 

Some of the more important monomers whose ring opening polymerisations have been induced by stable cation salts include, 1,4-epoxides, notably tetra-hydrofuran (20,112,113), 1,2-epoxides (114), 1,3-episulphides (thietans) (33,53), 1,2-episulphides (thiiranes) (53), azetidines (115,116), aziridines (117), the cyclic formals, 1,3-dioxolan (23,54, 118-120), and 1,3-dioxepan (118,119), trioxane (121,122) and more recently lactones (123). Aldehydes (124) may also be included since these molecules can be regarded as the smallest possible oxygen hetero-... 

Mention may be made, finally, of an unsuccessful attempt, recently described by Berkowitz and Bylander,141 to prepare the still unknown substance glycollic lactone by ruthenium tetroxide oxidation of ethylene oxide in carbon tetrachloride at 0°. Tarry products were obtained, which could have been Canned by ruthenium dioxide-oatalyred polymerisation f thiB highly strained a-lactone. 

L. Chen and D. E. Kiely, D-glucaric acid esters/lactones used in condensation polymerisation to produce hydroxylated nylons, a qualitative equilibrium study in acidic and basic conditions, J. Carbohydr. Chem., 13 (1994) 585-601. 

Tin organohalogenides can be used as catalysts to produce ethers of phosphoric acid and to polymerise lactons forming colourless polyesters. Various halogen derivatives of dibutyltin have been put forward as catalysts for the solidification of silicone elastomers, as agents preventing the cracking of polystyrene, as inhibitors of metal corrosion in silicone polymers. 

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. 

Heterocyclic monomers containing both endocyclic and exocyclic heteroatoms such as cyclic esters (lactones, lactide, carbonates) and cyclic anhydrides undergo coordination polymerisation or copolymerisation involving complex formation between the metal atom and the exocyclic heteroatom [100,124]. Polymerisation of /1-lactones is representative of such coordination polymerisations with catalysts containing an Mt-X active bond the initiation and propagation steps are as follows ... 

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. 

Oxacyclic monomers constitute the most widely investigated class of heterocyclic monomers regarding both academic and industrial interest. In particular, the coordination polymerisation of cyclic ethers such as epoxides (oxiranes) and of cyclic esters such as lactones, lactides and cyclic carbonates has been considered. 

The coordination polymerisation of cyclic esters concerns mostly lactones, especially those containing a four-membered ring in the molecule. There is, however, an interest in the coordination polymerisation of such oxacyclic ester monomers as lactide and alkylene carbonate and, to a lesser extent, in the... 

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). 

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] ... 

When catalysed by bimetallic /(-oxoalkoxides, the polymerisation of / -, 3-and e-lactones involves the alkoxide group as the attacking nucleophile. Thus, the mean degree of association of the catalyst exerts a further decisive effect on the kinetics of the polymerisation [122,125], Bimetallic / -oxoalkoxides and aluminium alkoxides [106] have the great advantage of polymerising /f-propio-lactone and e-caprolactone in a living manner. 

Of interest is the fact that a Lewis acid with bulky substituents, such as methylaluminium di(2,6-di-t-butyl-4-methylphenoxide), also has an accelerating effect on the polymerisation of /1-lactone, the extent of acceleration being dependent on the mode of lactone ring cleavage. The polymerisation of /i-butyrolactone in the presence of (tpp)AlOMe [scheme (9)] was slower than that in the presence of (tpp)AlCl [scheme (10)], but the accelerating effect of the bulky Lewis acid was more significant for the (tpp)A10Me catalyst [125]. Thus, the acceleration effect is considered [125] to be due to the coordination of... 

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. 

Morpholine-2,5-dione (l,4-oxazacyclohexane-2,5-dione) is characterised by the appearance in its molecule of both the <5-lactam and -lactone functions. The coordination polymerisation of this monomer [scheme (22)] has been reported for systems with diethylzinc [178] and tin bis(ethylhexanoate) [150] as catalysts ... 

Since oxiranes are representative heterocyclic monomers containing an endo-cyclic heteroatom, and the most commonly polymerised of such monomers, they have been subjected to copolymerisations with heterocyclic monomers containing both an endocyclic and an exocyclic heteroatom. Coordination copolymerisations of heterocyclic monomers with different functions are focused on oxirane copolymerisation with cyclic dicarboxylic acid anhydride and cyclic carbonate. However, the statistical copolymerisation of heterocyclic monomers with an endocyclic heteroatom and monomers with both endocyclic and exocyclic heteroatoms have only a limited importance. Also, the block copolymerisation of oxirane with lactone or cyclic dicarboxylic acid anhydride is of interest both from the synthetic and from the mechanistic point of view. Block copolymerisation deserves special interest in terms of the exceptionally wide potential utility of block copolymers obtained from comonomers with various functions. It should be noted, however, that the variety of comonomers that might be subjected to a random, alternating and block polymerisation involving a nucleophilic attack on the coordinating monomer is rather small. 

Block copolymers characterised by different backbone structures of well-defined block lengths have been obtained from oxiranes and other heterocyclic monomers in the presence of catalysts that are effective at bringing about living polymerisations. Aida et al. [127,188,189,195,196] applied aluminium porphyrins and Teyssie et al. [125,197,198] applied bimetallic /i-oxoalkoxidcs for block copolymerisations in systems involving oxirane lactone, oxirane oxirane/cyclic acid anhydride, and oxirane/cyclic acid anhydride lactone as block forming units and obtained respective polyether polyester and polyester polyester block copolymers. Such copolymers seem to be of exceptionally wide potential utility [53]. 

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

The polyester polyols are obtained by the polycondensation reactions between dicarboxylic acids (or derivatives such as esters or anhydrides) and diols (or polyols), or by the ring opening polymerisation of cyclic esters (lactones, cyclic carbonates). 

The lactones, the cyclic esters of hydroxy acids have been polymerised with various catalysts anionic catalysts, cationic catalysts and organometallic catalysts [28, 29, 39, 45, 54]. Five member lactone rings are unpolymerisable. Four, seven or eight member lactones are polymerisable [28, 29, 39, 45, 54]. 

By the attack of an anion to the lactone cycle (anionic polymerisation), carbonyl - oxygen scission and carbon - oxygen scission are possible ... 

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