Advances in the major areas of the cationic ring-opening polymerizations are discussed. In the first part synthesis of new polymers is reviewed; this includes mainly novel block and graft copolymers based on the di- and multicationically ended living polyacetals and polyethers (polyTHF).



Recent data on thermodynamics of nonideal systems, giving dependence of [M]e on [M]o, are summarized with special emphasize of the differences between monomers due to variation of their nucleophilicities.



Discussion of kinetics and mechanisms of polymerization of cyclic acetals and ethers is focused on the macroring formation and reactivities of active centers.



The reactivity of covalent and various ionic growing species are compared in the polymerization of cyclic ethers (THF and oxepane) and oxazolines. Reasons for much lower reactivity of covalent than ionic species and reasons for similar reactivities of macroion-pairs and macroions are presented.



It is shown, that the kinetically controlled proportions of macrorings in the polymerization of these and other heterocyclic monomers depend on the structure of the end-groups, formed upon initiation. Initiation with acids, giving H-X∼ (where X=heteroatom) end-groups, leads to the kinetically enhanced proportion of macrorings. This is particularly important for the low degrees of polymerization.

Cationic ring-opening polymerization

Polymerization of THF in CCl4 solvent was initiated with 1,3-dioxolan-2-ylium eations with AsF6−, PF6−, and SbF6− anions as well as with esters of fluorosulfonic and trifluoromethanesulfonic acids. With these esters polymerization proceeds with a marked acceleration period, due to slow initiation. The corresponding rate constants of initiation and their dependence on the polarity of the THF/CCl4 mixture were determined. The rate constant of propagation on the macroion-pairs (kp±) of the polytetrahydrofurylium cation with AsF6−, PF6−, and SbF6− and CF3SO3−, anions was found to be independent in CCl4 solvent on the anion structure and given by the expression: kp± = 2.93 × 10−2 exp {−4.7 × 103/T} at [THF]0 = 8.0M. This constant depends on the polarity of the polymerization mixture, and at 25°C for the THF-CCl4 system, kp± = 1.78 × 10−2 exp {−4.9/D}; thus, in CCl4 at [THF]0 = 8.0M, and at 25° kp± = 4.0 × 10−21/mole-sec. In the polymerization with derivatives of CF3SO3H (able to form the corresponding macroester) the overall polymerization rate is much lower than that with complex anions because of the reversible conversion of the macroion-pairs into the macroester (internal return). The macroester is much less reactive than the macroionpair (102–103 times) in the monomer addition reaction. At [THF]0 = 8.0M and at 25°C, 96.5% of the growing species exists in the macroester form.

Polymerization of THF initiated with derivatives of CF3SO3H is a subject of a strong special salt-effect. At a sufficiently high ratio of [AgSbF6] to [I]0, where the initiator I is C2H5OSO2CF3, the overall polymerization rate is equal to that observed for the polymerization of THF on the macroion-pairs, since the internal return within the triflate ion-pair (the macroester formation) is eliminated and polymerization proceeds on the macroion-pairs with SbF6- anions exclusively.

Kinetics and mechanism of the cationic polymerization of tetrahydrofuran in solution. I. THF–CCI4 system

Cationic polymerization of tetrahydrofuran (THF) in CH2Cl2 solvent and in mixed CH2Cl2/CH3NO2 solvent was initiated with 1,3-dioxolan-2-ylium cations with AsF and SbF anions. Dissociation constants of the polytetrahydrofuranium ion pairs into ions were measured (e.g., KD = 1.5 × 10−5M at 25°C and [THF]0 = 7.0M; CH2Cl2 solvent) and were found to be more than 100 times lower than in CH3NO2 solvent at the same [THF]0 and temperature. The rate constants k and k, measured for degrees of dissociation ranging from 0.03 to 0.35 in CH2Cl2, were the same within an experimental error of measurements (±15% of the value of kp). Dependence of k( = k = k) on the dielectric constant was a monotonous function in three different solvents, namely, CCl4, CH2Cl2, and CH3NO2, which covered a large range of dielectric constants of the medium (from D = 5 to D = 22) and degrees of dissociation of the macroion pairs, α (from 0.03 to more than 0.70). Thus a decrease in the dielectric constant increases the rate constant k in the whole range of studied polarities of the medium. This result confirms an earlier conclusion that the rate constant of propagation does not depend on the state of aggregation of ions and k = k.

Kinetics and mechanism of the cationic polymerization of tetrahydrofuran in solution. THF-CH2Cl2 and THF-CH2Cl2/CH3NO2 systems

There are several comprehensive reviews, covering the cationic polymerization of cyclic ethers, which have been published recently.1–4 In the present survey we summarize the major findings of the field, giving priority to the new advances (e.g. polymerization involving the activated monomer mechanism) as well as the problems overlooked by previous reviews (e.g. cationic cyclopolymerization of diepoxides or application of cationic initiators in curing of epoxy resins).

Cationic Ring-opening Polymerization: Ethers

A convenient real-time monitoring of monomer concentration during living cationic ring-opening polymerizations of tetrahydrofuran (THF) initiated with methyl triflate (MeOTf) has been developed for kinetic investigation and determination of equilibrium monomer concentration ([M]e) via in situ FTIR spectroscopy in combination with a diamond tipped attenuated total reflectance (ATR) immersion probe. The polymerization rate was first order with respect to monomer concentration and initiator concentration from the linear slope of ln([M]0-[M]e)/([M]-[M]e) versus polymerization time at different temperatures in various solvents. [M]e decreased linearly with initial monomer concentration while increased exponentially with increasing polymerization temperature. The equilibrium also strongly depends on solvent polarity and its interaction with monomer. The equilibrium polymerization time (te) decreased with increasing solvent polarity and decreased linearly with increasing [M]0 in three solvents with different slopes to the same point of bulk polymerization in the absence of solvent. Equation of Mn,e = 72.1/(0.14-0.04[M]e) has been established to provide a simple and effective approach for the prediction for the number-average molecular weight of polyTHFs at equilibrium state (Mn,e) in the equilibrium living cationic ring-opening polymerization of THF at 0 °C.

Real-time monitoring of living cationic ring-opening polymerization of THF and direct prediction of equilibrium molecular weight of polyTHF

The equilibrium cationic polymerization of tetrahydrofuran in benzene is investigated in the 15 to 50°C range Polymerizations are initiated with triethyloxonium hexafluorophos-phate (Et3O+PF5 −) and performed using high vacuum techniques It is shown that, at a given temperature, the equilibrium monomer concentration varies linearly with polymer concentration Values of ΔGlc, the free-energy change upon the polymerization of 1 mole of liquid monomer to 1 base-mole of liquid amorphous polymer of infinite chain length, is deduced from these results Values of ΔGlc, obtained are in good agreement with those found in the literature for the equilibrium polymerization of tetrahydrofuran in bulk, However, there is a large discrepancy between these values of

Equilibrium Cationic Polymerization of Tetrahydrofuran in Benzene

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Equilibrium Cationic Polymerization of Tetrahydrofuran in Benzene