Polymerisation of heterocyclic monomers

As far as the polymerisation of heterocyclic monomers is concerned, the situation is qualitatively similar, but quantitatively different. As a model for the active species in oxonium polymerisations, Jones and Plesch [10] took Et30+PF6 and found its K in methylene dichloride at 0 °C to be 8.3 x 10"6 M however, in the presence of an excess of diethyl ether it was approximately doubled, to about 1.7 x 10 5 M. This effect was shown to be due to solvation of the cation by the ether. Therefore, in a polymerising solution of a cyclic ether or formal in methylene dichloride or similar solvents, in which the oxonium ion is solvated by monomer, the ion-pair dissociation equilibrium takes the form... 

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. 

As mentioned above, such polymerisation involves trans ligand insertion which coincides with the polymerisation of heterocyclic monomers as regards the polymerisation mechanism. 

Characterise the stereoselective (enantiosymmetric) polymerisation and stereoselective (enantioasymmetric) polymerisation of heterocyclic monomers. Give examples. 

Reviews concerning ring-opening polymerisations of heterocyclic monomers were published by Billingham Penczek and Goethals and Schacht ... 

For cationic polymerisation of olefins in solvents of DC appreciably less than ca. 10 and for those of heterocyclic monomers in all solvents of DC up to perhaps 15-20, this is not so. For such systems the polymerisations are probably at least dieidic (free ions and ion-pairs) and a lowering of the temperature will increase the DC of the ion pairs. Thus in such systems the change of temperature affects not only k p and k"p, but also the relative abundance of the different types of chain-carriers therefore the proper interpretation of the apparent activation energies is difficult and by no means obvious. 

As far as the polymerisation of heterocyclic compounds with one hetero-atom is concerned (cyclic ethers and their analogues) there seems little doubt at present that the propagation involves a displacement at the positive propagating centre. The ring which is part of this -onium ion is opened between the charged atom and a carbon atom next to it, and this becomes attached to the hetero-atom of the monomer ... 

The differentiation between the coordination polymerisation of heterocyclic and heterounsaturated monomers and their nucleophilically initiated anionic polymerisation lies in the covalent nature of the metal-heteroatom bond (but being polarised as in the Mt 5+— Xs bond) in the coordination catalyst, which activates the monomer by its coordination, enhancing the nucleophilicity of the metal substituent simultaneously, and the ionic character of the metal-heteroatom bond in the nucleophilic initiators. 

Catalysts for the polymerisation of heterocyclic and heterounsaturated monomers contain a heteroatom, such as, for example, Cl (i.e. with an Mt-Cl active bond), or the group bound to the metal atom via the heteroatom (Mt X for example, X = 0, S, N) as the initiating substituent. However, there are also catalysts that possess an alkyl initiating substituent at the metal atom, especially when they are used for the polymerisation of heterounsaturated monomers. 

The coordination polymerisation of heterocyclic and heterounsaturated monomers consists in the nucleophilic attack of the metal initiating substituent (or the growing polymer chain) on the carbon atom of the coordinated monomer. Scheme 2.6 shows initiation and propagation steps in the coordination polymerisation of epoxides, as the most representative heterocyclic monomers with an endocyclic heteroatom, with catalysts containing an Mt-X active bond [68,114,115] ... 

The coordination polymerisation and copolymerisation of heterocyclic monomers have been restricted in industry to a much smaller volume than the polymerisation and copolymerisation of hydrocarbon monomers polyether elastomers from epichlorohydrin and ethylene oxide or propylene oxide, and allyl glycidyl ether as the vulcanisable monomeric unit, are produced on a larger scale [4-7],... 

As regards the coordination homopolymerisation of heterounsaturated monomers, it does not play such an important role as in the case of heterocyclic monomers (with the exception of carbon monoxide). This is because of the high polymerisability of heterounsaturated monomers in the presence of ionic initiators which is taken into account in some industrial processes (e.g. polyformaldehyde). 

However, the most important goal that might be reached by the application of coordination catalysts for the polymerisation of heterounsaturated monomers is the possibility of the enchainment of heterounsaturated monomers, not susceptible to homopropagation, via their copolymerisation with heterocyclic monomers. This concerns primarily the coordination copolymerisation of carbon dioxide and oxacyclic monomers such as epoxides, leading to aliphatic polycarbonates [8 12]. Representative examples of the copolymerisations of heterocyclic monomers and hardly homopolymerisable heterocumulenes, in the presence of coordination catalysts, are listed in Table 9.4 [1]. 

It is worth mentioning in this connection that the anionic polymerisation of heterocyclic and heterounsaturated monomers requires the application of nucleophilic initiators, involving mostly alkali metal compounds, which are characterised by a high nucleophilicity of the monomer attacking agent and by low Lewis acidity of the positive counterion in the initiator. Thus, nucleophilic initiation of anionic polymerisation does not require any monomer coordination on to the metal, although interaction of the monomer with an electrophilic counterion is considered commonly to occur to some extent. 

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. 

Table 9.5 Representative examples of coordination catalysts for polymerisations of heterocyclic and heterounsaturated monomers 2...

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. 

Characterise the main features of catalysts for the coordination polymerisation of heterocyclic and heterounsaturated monomers. 

Neither for olefins nor for heterocyclic monomers do we yet have a sufficiently extensive body of activation energies of the kp-s to make a detailed discussion profitable. It is worth noting, however, that for the cationic (as opposed to the pseudo-cationic) polymerisation of olefins in solvents of DC greater than about 10, it is likely that a reduction of the temperature does not affect the rate except through its effect on k p, since these reactions are mainly carried by free ions only. 

Polymerisations and copolymerisations of heterounsaturated and heterocyclic monomers in the presence of coordination catalysts constitute a distinct group of coordination polymerisation processes. Considering the nature of the coordination bond of the a type between the monomer heteroatom (beyond carbon monoxide [60]) and the metal atom, the complexes formed differ essentially from the re complexes of unsaturated hydrocarbon monomers with transition metals. 

Coordination catalysts, which are usually applied for the polymerisation and copolymerisation of heterocyclic and heterounsaturated monomers, involve a wide range of metal derivatives characterised by a moderate nucleophilicity and relatively high Lewis acidity. Compounds of group 2 and 3 metals, such as zinc, cadmium and aluminium, as well as transition metals, such as iron, are representative coordination catalysts. The appropriate Lewis acidity of the metal and the appropriate nucleophilicity of the metal substituent in the catalyst make the monomer coordination favourable prior to the nucleophilic attack of the metal substituent on the monomer not yet coordinated. 

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 ... 

The progress in the polymerisation of epoxides, which are considered to be very important heterocyclic monomers, concerns both mechanistic studies and technological studies aimed at commercialising certain epoxide polymerisation processes. Although wide studies have dealt with the catalyst structure, only a limited number of catalysts have been structurally determined in the solid state as well as in solution. The epoxide polymerisation mechanism, including stereochemical aspects, has been satisfactorily explained on the molecular... 

Polymerisation of Non-hydrocarbon (Heterocyclic and Heterounsatu rated) Monomers... 

Although any coordination polymerisation involves a monomer coordination in each step, literature data that might concern complex formation between the monomer heteroatom and the metal atom at the active site are rather scant. This is due to the lability of such complexes of monomers with active sites if they were stable enough, they would not undergo any further rearrangement, which might lead to polymer chain growth. Thus, models have been studied that have consisted of a metal complex, of no effectiveness as a catalyst, and a monomer, or of a catalyst and a non-polymerisable heterocycle. 

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]. 

Heterogeneous and homogeneous catalysis in polymer chemistry - polymerisation of hydrocarbon, heterocyclic and heterounsaturated monomers... 

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