Oxacyclic monomers

Cationic copolymerization of cyclic ethers, formals, esters and anhydrides has been thoroughly studied in recent years and sufficient information about it is now available. The propagating species involved in the cationic copolymerization of these oxacyclic monomers are believed to be the oxonium ions in most cases, but their detailed nature is dependent on monomer structure. From their copolymerization behavior, these monomers can be arranged in the following order of increasing car-bocationic character of the propagating species ... 

In the copolymerization of five- and six-membered oxacyclic monomers, the effective monomer concentration in the propagation reaction decreases because only the monomer in excess of equilibrium is available for copolymerization. However, it is not easy to determine the equilibrium monomer concentration in a copolymerization system. The following equilibrium is expected to exist in the copolymerization of THF. 

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. 

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]. 

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. 

Although thiacyclic monomers do not play such an important role in coordination polymerisation as oxacyclic monomers, some polymerisation of tiiranes with coordination catalysts deserves attention, especially considering the polymerisation stereochemistry ... 

Polymerization of Substituted Oxiranes, Epoxy Aldehydes, and Derived Oxacyclic Monomers... 

This paper is a review of studies on the ring-opening polymerization of cyclic ethers, e.g., styrene oxide, phenyl glycldyl ethers, epoxy aldehydes and derived oxacyclic monomers. Model reactions involving ring-opening processes occurring in those compounds have alsobeen discussed. 

Many papers have been published concerning the structure of the active centers in anionic and cationic ring-opening polymerization reactions of oxacyclic monomers. Recently, attention has been paid in our laboratory to the influence of the structure of complex carbonium salt initiators, especially of the dioxolanyllum salts used for initiating the cationic polymerization reactions of trioxane, tetrahydrofuran and dioxolane, on the course of the polymerization ( ). 

In the present report some examples of the influence of monomer structure, especially of steric hindrance and electronic effects, on the mechanism of polymerization and on the nature of the active centers formed in the anionic polymerization of certain oxacyclic monomers are discussed. 

In particular the polymerization of 1.2.- and 1.3-epoxides (l)-(5) (18) (19) tetrahydrofurane (1)-(4) (6) (2o) Hioxolane (2177(22) and trioxane (11) (23)-X26) was thoroughly TnvestTgated. For reviews see (7T (8) (27) (28) (3o). It should be emphasized, that different oxacyclic monomers can also be copolymerized by cationic catalysts. Of great practical importance is e.g. the copolymerization of trioxane with ethylene oxide or dioxolane (31). Macromolecules with a statistic distribution of oxymethylene- and oxy-ethylene-units are formed in this way. On the other hand, however, the homopolymerization of dioxolane yields a polymer consisting of strictly alternating oxymethylene- and oxyethylene units (21) (32) therefore it can formally be considered as an alternating copolymer (eq.i). 

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... 

In regard to the copolymerization process, we have to describe the rates of incorporation of comonomers and distribution of different units in the resulting copolymer (microstructure). In this section we briefly discuss characteristic features of copolymerization of TXN with cyclic formals and then with other oxacyclic and some vinyl monomers. 

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