General Mechanisms in Ring-Opening Polymerization

Endo T (2009) General mechanisms in ring-opening polymerization. In Dubois P, Coulanbier O, Raquez JM (eds) Handbook of Ring-Opening Polymerization. Wiley, Weinheim. p 53... 

Mechanisms of ring-opening polymerization vary according to monomer type and initiator, but in most instances they fit one of the two general forms ... 

Chain Initiation and Propagation In general, two mechanisms have been suggested for initiation and chain propagation in ring-opening polymerization by the cationic process. One mechanism involves the formation of an onium ion by interaction of the catalyst system with the monomer, as shown here for the cationic polymerization of tetrahydrofuran ... 

The results show that the presence of bulky substituent on a polymer chain may effectively inhibit the termination proceeding by this mechanism. The results presented at this point may be summarized as follows chain transfer to polymer is a general feature of cationic ring-opening polymerization although for different systems the contribution of this reaction may vary only in some systems this process results in termination (These systems involve, e.g., cyclic amines (3- and 4-membered) and cyclic sulfides (3- and 4-membered) and the contribution of the reaction is reduced for substituted chains. 

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]. 

Mechanism of Equilibration. The generally accepted mechanism for the base-catalyzed ring-opening polymerization of cyclosiloxanes involves attack of the basic catalyst at the silicon atom (15). It has been proposed, and generally accepted, that the active species is a partially dissociated siloxanolate anion (13). In the results presented in this chapter, significant differences in reaction rates were observed as the corresponding cation of the siloxanolate species was varied. The more rapid disappearance of D4 and aminopropyldisiloxane in the presence of these catalysts increased in the following order ... 

Two general mechanisms for the chain propagation in cationic ring-opening polymerizations have been suggested. 

Cyclic olefins undergo a very unusual type of ring-opening polymerization in the presence of certain transition metal catalysts [253-257]. This is illustrated in Eq. (90) for the ring-opening metathesis polymerization (ROMP) [255] of cyclooctene to form polyoctenamer. Quite surprisingly, the double bond is maintained in the polymer, i.e., it is not a normal addition polymer. The generally accepted mechanism for these... 

The generally accepted propagation mechanism for cationic ring-opening polymerization for ethylene oxide is shown in Figure 5. According to this mechanism, the proton of the superacid (produced from the initiation step as explained in... 

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