Cationic ring-opening polymerization studies

The cationic ring-opening polymerization of cyclic ethers has been the subject of many recent investigations (1.. Nuclear magnetic resonance (NMR) methods, particularly carbon-13 techniques, have been found most useful in studying the mechanism of these polymerizations ( ). In the present review we would like to report some of our recent work in this field. 

The system in cationic ring-opening polymerization, which most closely approaches this ideal situation, is the polymerization of cyclic siloxanes [86-88]. The chain, composed of —O—Si— units is highly flexible and provides a reaction sites for cyclization. It is also well suited for experimental studies, because concentrations of cyclic oligomers in equilibrium, due to their volatility, can be determined with accuracy by gas-liquid chromatography. The slope of the experimentally derived plot is indeed very close to the theoretically predicted value of -2.5. 

This characteristic feature of cationic polymerization of THF allows the important synthetic application of this process for preparation of oli-godiols used in polyurethane technology and in manufacturing of block copolymers with polyesters and polyamides (cf., Section IV.A). On the other hand, the cationic polymerization of THF not affected by contribution of chain transfer to polymer is a suitable model system for studying the mechanism and kinetics of cationic ring-opening polymerization. 

It is generally agreed that both processes, namely addition polymerization (the nature of active species still of much debate) and acidolysis/ condensation reaction, occur simultaneously in the cationic ring-opening polymerization [247,248], although the contribution of both mechanisms is still a matter of discussion. Kinetics of the acid-catalyzed condensation of silanol groups was studied in detail [249,250]. 

The synthesis of polystyrene-g-polytetrahydrofurane [188] was achieved by ATR copolymerization of methacrylic PTHF macromonomer, MA-PTHF, with styrene (Scheme 105). The PTHF macromonomer was synthesized by cationic ring opening polymerization of THF with acrylate ions, formed by the reaction of methacryloyl chloride and AgC104. The polydispersity indices of the graft copolymers determined by SEC ranged between 1.3-1.4. Kinetic studies revealed that the relative reactivity ratio of the macromonomer to St was independent of the molecular weight of PTHF. 

Photoinitiated cationic polymerization has been the subject of numerous reviews. Cationic polymerization initiated by photolysis of diaryliodonium and triarylsulfonium salts was reviewed by Crivello [25] in 1984. The same author also reviewed cationic photopolymerization, including mechanisms, in 1984 [115]. Lohse et al. [116], reviewed the use of aryldiazonium, diphenyliodonium, and triarylsufonium salts as well as iron arene complexes as photoinitiators for cationic ring opening polymerization of epoxides. Yagci and Schnabel [117] reviewed mechanistic studies of the photoinitiation of cationic polymerization by diaryliodonium and triarylsulfonium salts in 1988. Use of diaryliodonium and sulfonium salts as the photoinitiators of cationic polymerization and depolymerization was again reviewed by Crivello [118] in 1989 and by Timpe [10b] in 1990. 

Initiation. The most recent classification of initiators for cationic ring-opening polymerization was presented and discussed by Penczek et al. (IJ. Only a few classes of initiators that are very useful both for mechanistic studies as well as for synthesis of well-defined polymers will be presented here. 

Various authors, who studied the mechanism of cationic ring-opening polymerization, observed the formation of some specific cyclic oligomers there were, however, no attempts to rationalize observed distribution on theoretical grounds, i.e. the preferential formation of certain oligomers or the apparent absence of cyclic oligomers in some systems. Several reviews have been published on macrocyclization but usually they concentrate either on the thermodynamic 5) or synthetic aspects 4). Indeed, both these approaches, thermodynamic and synthetic , were developed simultaneously but have not correlated with each other. In the most papers concerned with synthesis the Jacobson-Stockmayer theory was not even quoted. 

Chain transfer to polymer and macrocyclization, frequently observed in cationic ring-opening polymerization, has also been studied for lactones 4). In the polymerization of PPL cyclic oligomers were not observed, whereas in the polymerization of e-CL initially high-molecular-weight polymer is formed which then is slowly degraded to cyclic oligomers, mostly from dimer to pentamer. 

Cyclic ethers are the class of heterocyclic monomers that provide suitable models for mechanistic studies. Polymerization of several monomers of this class leads to polymeric materials that are produced on an industrial scale. The most prominent examples are polymers of ethylene oxide (EO), propylene oxide (PO), epichlorohydrin (ECH), or tetra-hydrofiiran (THE). Cationic ring-opening polymerization (CROP) of cyclic ethers is thus interesting from both an academic and industrial point of view. 

Homogeneous poiymerization of cydic acetals had been studied mostly before the first edition of Comprehensive Polymer Science was published and since then there has been limited activity in this area. Therefore, the first part of this chapter is based (sometimes in extenso] on the chapter Cationic Ring-Opening Polymerization Acetals from the first edition. There are several reviews and book chapters in which cationic polymerization of cyclic acetals (including bicyciic acetals) has been discussed. " ... 

In the domain of the cationic ring-opening polymerization in dispersion, until now only one system has been investigated. In 1968, Penczek et al published results of the studies of the cationic copolymerization of 1,3-dioxolane and 1,3,5-trioxane initiated with BF3 and carried out in cyclohexane in the presence or the absence of poly(ethylene oxide). Hie initial concentration of 1,3-dioxolane in these studies was 20 times lower than the initial concentration of 1,3,5-trioxane. The former monomer was used with the purpose of protecting poly (1,3,5-trioxane) from depolymerization. It was found that depolymerization stops when 1,3-dioxolane monomeric unit is the terminal one. 

Onium salt cationic photoinitiators present many unique and interesting opportunities for basic studies of cationic ring-opening polymerizations. Since they are latent photochemical sources of strong Bronsted adds, they can be dissolved in the subject monomers and then precisely tri ered on demand by the application of light. Mixing problems and the use of complex stopped-flow devices and other apparatuses required to overcome them are thus avoided. Only the rate of initiation is different in a photoinitiated cationic polymerization as compared to a conventional thermally initiated polymerization. The rate of initiation for an onium salt-photoinitiated cationic potymerization (eqn [68]) is... 

S. Neffgen, H. Keul, H. Hocker, Cationic ring-opening polymerization of trimethylene urethane a mechanistic study. Macromolecules 30 (1997) 1289-1297. 

Cationic ring-opening polymerizations are also nucleophilic substitution reactions e.g. equation (7). Two experiments are frequently employed to follow the cationic polymerization of heterocyclic monomers. The first is based on the polymerization of a low molar ratio of monomer and initiator directly in an NMR sample tube. This experiment requires a low rate of polymerization and allows direct identification of all growing species and determination of the rate constants of all individual reaction steps by HNMR spectroscopy. Cyclic imino ethers were the first to be studied by this method. Subsequently, cationic polymerization of other heterocyclic monomers has been investigated." ... 

---