Polymerisation of Heterounsaturated Monomers

Heterounsaturated monomers that undergo coordination polymerisation or copolymerisation with other monomers can be divided into two classes monomers with a carbene-like structure such as isocyanides and carbon monoxide which are coordinated by n complex formation with the transition metal atom at the catalyst active site, and monomers such as isocyanates, aldehydes, ketones and ketenes which are coordinated via 5-bond formation with the metal atom at the catalyst active site. 

---

The coordination polymerisation of heterounsaturated monomers, such as aldehydes [101-103] and ketones [104], isocyanates [105] and ketenes [106,107], in homopolymerisation systems has not been widely described in the literature. However, the coordination copolymerisation of heterounsaturated monomers not susceptible to homopropagation, such as carbon dioxide [71,108-113], with heterounsaturated monomers such as cyclic ethers has been successfully carried out and is of increasing interest. 

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. 

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]. 

Heterounsaturated monomers such as aldehydes or carbon dioxide polymerise and/or copolymerise with the participation of at least two metal atoms in multicentred transition states. Scheme (9) shows the initiation and propagation steps in the coordination polymerisation of carbonyl monomers with catalysts containing an Mt-X active bond [125] ... 

Monomer coordination at the active site of the catalyst may occur in varied ways, essentially reducing to a twofold mechanism which is dependent on both the kind of monomer and the catalyst. This appears to encompass cases where unsaturated hydrocarbon monomers coordinate at the metal of transition metal-based catalysts, involving % complex formation, as well as cases where heterocyclic and heterounsaturated monomers are subjected to polymerisation with various coordination catalysts in which monomer complexation proceeds via a bond formation between the heteroatom and the metal atom. 

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. 

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. 

Heterounsaturated monomers such as aldehydes, ketones, ketones, isocyanates and isocyanides, which have been reported to undergo a polymerisation in the presence of coordination catalysts, are listed in Table 9.3 [1,3]. 

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. 

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

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

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

Kuran, Witold. Principles of Coordination Polymerisation Heterogeneous and Homogeneous Catalysis in Polymer Chemistry—Polymerisation of Hydrocarbon, Heterocyclic, and Heterounsaturated Monomers. New York Wiley, 2001. 

---