Kinetics and Mechanisms of Zwitterionic Polymerizations of Alkyl Cyanoacrylates

Abstract

ABSTRACT:The paper reviews work in the author's laboratory on the polymerization of alkyl cyanoacrylates, chiefly the butyl ester, in THF, by organic bases, phosphines and amines. It presents evidence that the bases initiate by addition, thus forming zwitterionic propagating species. Polymerizations initiated by Ph3P are confirmed as "living polymerizations," but with relatively slow initiation (e.g., k;/k", x 10-5 at -80°C). Acid-inhibition studies of amine-initiated reactions show that rates of initiation are complex and have negative temperature dependence. This makes unnecessary an earlier postulate that these polymerizations involved a combinationtermination process. A "slow-initiated, non-terminated'' kinetic scheme quantitatively describes polymerizations by pyridine, 4-vinylpyridine, 2,6-lutidine, and benzyldimethylamine, yielding kP values ((2-8) x 10 5 I mol-1 s -I at 20°C) consistent among themselves and with that from the Ph 3 Pinitiated kinetics.KEY WORDSCyanoacrylates I Zwitterions I Phosphines I Tertiary Amines I Slow-Initiated-Non-Terminated Kinetics I When these polymerizations were first reviewed in 1977, 1 the major unsettled questions were seen to be: 1) What is the electrochemical nature of the growing species when the polymerizations are initiated by covalent bases, amines or phosphines? Are they simple anions (with ammonium or phosphonium counter cations), as would follow if initiation somehow (not obviously) involved proton abstraction? Or are they, as expected, genuine zwitterions, formed by addition of the bases to the monomer?2) What explains the striking difference between the overall kinetics of polymerizations initiated by phosphines (which appeared to be near-ideal "living polymerizations," i.e., terminationless) and by amines, which showed a negative temperature coefficient of overall rate, at that time interpreted by the postulate of a highly temperature dependent (combination) termination process.As will be described below, evidence for zwitterion formation has now been provided by spectroscopic demonstration of the presence of phosphonium, pyridinium and carbanion groups in the polymer 2 and by the demonstration that graft copolymers are formed, when polymerization is by poly(vinylpyridine? or poly(styryldiphenylphosphine).4The different overall kinetics of phosphine-and amine-initiated polymerizations can now be reconciled by the conclusion that in neither case is there any intrinsic terminatipn process, and that the differences arise from different complexities in the processes which generate the active species. This conclusion follows from the discovery that, under appropriate conditions in the presence of added strong acids, amine-initiated polymerizations showed clear-cut inhibition periods, from which the rates of chain-initiation could be deduced. These are found to have a negative temperature coefficient capable of explaining that of the overall rates, i.e., permit evaluation of the concentration of active species, and hence of propagation rate constants, virtually iden...

Fast underwater bonding to polycarbonate using photoinitiated cyanoacrylate

“…Owing to the poor reproducibility of the data, each curve in Figure 3 actually represents the average of ten separately measured exotherms. The exotherms in Figure 3 show that the cure advances faster at lower reaction temperatures in agreement with previous studies dealing with ferrocene derivatives [21,22], or aliphatic amines and pyridine derivatives [19] as initiators. These results indicate that this adhesive system will cure faster in cold water conditions.…”

“…Cyanoacrylates are among the most reactive monomers to be examined kinetically. Pepper and co-workers [18,19] studied their cure kinetics and the mechanism of polymerisation. Despite the fact that they employed carefully controlled experimental conditions, problems were experienced with respect to reproducibility.…”

Polycyanoacrylates

“…In an adhesive context, polymerization or curing of the adhesive is brought about by the presence of nucleophilic species present on the substrate surface. The nucleophile can be an ion or a neutral molecule (such as an amine, alcohol, or phosphine) which adds to the cyanoacrylate to form a macrozwitterionic species and a zwitterionic polymerization ensues as documented by Pepper (17).…”

“…Because the rate of propagation associated with cyanoacrylate polymerization kinetics is typically much higher than the rate of initiation (17), this results in polymerization being complete before all potential initiating species present on the substrate surface have been consumed. The presence of such residual basic species can induce thermal degradation/depolymerization at the polymer-substrate interface, resulting in loss of mechanical strength and adhesion.…”

Degradation and stabilization of polycyanoacrylates

“…With amines, however, the rate of Michael addition of the initiator to the monomer is slow compared to the rate of propagation. Pepper 9 called this situation a "slow-initiation, no termination mechanism." Most of the amine remains unreacted after polymerization is complete (this result has implications on the degradation of the polymer and is discussed later).…”