Degradation and Stabilization of Polycyanoacrylates
Total Page：16
File Type：pdf, Size：1020Kb
Degradation and Stabilization of Polycyanoacrylates DOUGLAS R. ROBELLO, TERESA D. ELDRIDGE, MICHAEL T. SWANSON Eastman Kodak Company, Research Laboratories, Rochester, New York 14650-2116 Received 3 May 1999; accepted 11 August 1999 ABSTRACT: Polycyanoacrylates were found to be inherently unstable. Even in the absence of a deliberately added strong base, their molecular weights decreased drasti- cally on standing in solution in accord with observations by Ryan and McCann (Mak- romol Chem Rapid Commun 1996, 17, 217). The initial high molecular weight polymer disappeared over the course of a few hours in solution and was replaced by a much lower molecular weight material. For polymers made by anionic polymerization, the entire sample degraded, but for polymers made by free-radical polymerization, only a portion of the sample was affected. This behavior was consistent with the mechanism proposed by Ryan and McCann, in which the polymer chains are in dynamic equilib- rium with their monomers and the polymer degrades from its chain terminus. Surpris- ingly, the degradation in molecular weight even occurred slowly in the solid state. The degradation was inhibited by acids and could be prevented by free-radical copolymer- ization with small amounts of more stable monomers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4570–4581, 1999 Keywords: polycyanoacrylates; copolymers; copolymerization; degradation; stabili- zation; equilibration; depolymerization–repolymerization; methacrylic acid; methyl methacrylate; molecular weight; base catalysis; size exclusion chromatography; tactic- ity; mechanism; microstructure; stereochemistry; NMR INTRODUCTION changes and obtained evidence in support of the mechanism proposed by Ryan and McCann.1 In a recent communication, Ryan and McCann1 Moreover, our experiments indicated that this described rapid changes in the molecular weight equilibration behavior is even more general than (Mr) of poly(butyl cyanoacrylate) (PBCA) when it previously indicated and affects similar polymers was treated in solution with a strong base, tet- made with nucleophilic initiators and (to a lesser rabutylammonium hydroxide. These authors sug- extent) those made with free-radical initiators. gested that the added base deprotonated the We discovered that added base is not necessary; chain end, leading to the rapid depolymerization adventitious base is sufficient to promote molec- of the polymer backbone, followed by the simul- ular weight degradation. Unexpectedly, the depo- taneous repolymerization of the liberated mono- lymerization–repolymerization reaction was ob- mer to produce a “daughter” polymer of much served even in the solid polymer. We also describe lower molecular weight. herein a simple method for producing polycyano- Their publication prompted us to describe our acrylates (PCAs) that are immune to this degra- own parallel studies of cyanoacrylate polymers in dation reaction. which we reproduced these molecular weight Background Correspondence to: D. R. Robello (E-mail: drobello@ Cyanoacrylates (1) are remarkable compounds. kodak.com) Best known as “instant adhesives” or “super Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37, 4570–4581 (1999) © 1999 John Wiley & Sons, Inc. CCC 0887-624X/99/244570-12 glues,” they were discovered serendipitously by 4570 DEGRADATION AND STABILIZATION OF POLYCYANOACRYLATES 4571 zwitterionic, although the only formal difference between this mechanism and simple anionic po- lymerization is that the counterion is bound co- valently to the origin of the polymer chain. We refer to this class of polymerization as anionic because the propagation step takes the form of the addition of an anionic chain end to a Michael Figure 1. Anionic polymerization of cyanoacrylates. acceptor monomer, regardless of counter ion. Cyanoacrylates also can be polymerized by us- ing conventional free-radical initiators.14 In this Coover2 in 1953 at what was then the Tennessee case, it is common to include some acid in the Eastman Chemicals Division of the Eastman reaction mixture to suppress accidental anionic Kodak Company. The mechanism of adhesive ac- polymerization. For free-radical polymerization, tion is anionic polymerization (Fig. 1). The mono- the propagation step is probably much slower mers are polymerized rapidly on contact with nu- than for anionic polymerization, and the corre- cleophiles, including even water at a sufficiently sponding molecular weights are generally lower. high concentration. As many who have worked Free-radical polymerization is a viable method for with these compounds can attest, the nucleo- synthesizing PCAs; however, the predominate philes present in skin proteins are especially good worldwide use of these compounds is as adhesives initiators. Because of their rapid bonding of tis- where anionic polymerization is the operative sue, cyanoacrylates also have been investigated mechanism. as surgical adhesives.3 The details of the mechanism of polymerization of cyanoacrylates by nucleophiles have been stud- MOLECULAR WEIGHT MEASUREMENTS ied extensively by Pepper and coworkers.4–13 These researchers published evidence that, for Before studying the degradation behavior of uncharged nucleophiles such as tertiary amines PCAs, we needed a reliable means of assaying and phosphines, the polymerization proceeds via their molecular weights, especially because rela- a macrozwitterion and has living character (Fig. tively rapid degradation can occur in solution un- 2). With amines, however, the rate of Michael der certain circumstances (discussed later). Pep- addition of the initiator to the monomer is slow per11 wrote the following: compared to the rate of propagation. Pepper9 called this situation a “slow-initiation, no termi- “A problem of a different kind is the difficulty of nation mechanism.” Most of the amine remains finding suitable solvents, especially for molecular unreacted after polymerization is complete (this weight determination of the polymers. The strongly result has implications on the degradation of the polar solvents, nitromethane, acetonitrile and dimeth- polymer and is discussed later). Because of the ylformamide, dissolve the polymers from methyl, ethyl and n-butyl esters (PMCA, PECA, PBCA) but in vari- unusual kinetics with amine initiators, there is ous cases are either known or suspected to cause deg- not the expected correspondence between the radation. Solutions of high molecular weight PECA in amount of initiator and the polymer molecular THF show remarkable viscosity and GPC anomalies. weight as is observed for true living polymeriza- (p 67)” tions. Nonetheless, tertiary phosphines are much more rapid initiators of cyanoacrylates, and these Related problems were encountered by Guthie systems exhibit the hallmarks of ideal living po- et al.,15 who tested the molecular weight of an lymerization. Note that, in all cases, the propaga- ethyl cyanoacrylate (ECA) adhesive after periods tion step is extremely rapid and has an unusually low activation energy. For example, Johnston and Pepper13 found that cooling the reaction to 295 °C only reduced the rate by half. Very high mo- lecular weights, up to 106, typically are obtained by anionic polymerization. When uncharged nucleophilic initiators such as tertiary amines or phosphines are employed, Figure 2. Mechanism of the anionic polymerization the polymerization perhaps is described better as of cyanoacrylates by nucleophiles. 4572 ROBELLO, ELDRIDGE, AND SWANSON of heating. These researchers found a gradual isolated and dried) dissolved in acetonitrile, held decrease in tensile shear strength of the PCA at room temperature, and sampled periodically. bond between steel plates over 2 days at 90 °C. Even more remarkable was the nature of the deg- Because they attempted to perform size exclusion radation. Rather than gradual shifting to a lower chromatography (SEC) of the PCA in tetrahydro- molecular weight, the original high molecular furan (THF), their measurements were con- weight material slowly disappeared and was re- founded by artifacts, and they were unable to placed by a much lower molecular weight fraction resolve a substantial low molecular weight frac- (typically 10,000–40,000 g/mol). This daughter tion. They concluded, possibly erroneously, that fraction gradually increased slightly in molecular the observed decrease in bond strength was not weight as the equilibration proceeded; however, caused by molecular weight degradation. no material of intermediate size could be de- We found that the degradation of PCAs in so- tected. Once the high molecular weight material lution could be inhibited by the addition of acid had disappeared, there were no further changes (discussed later). Fortunately, an acidic eluent for in the low molecular weight fraction, even after SEC was available from previous work by Mourey prolonged incubation. Similar results were ob- and coworkers,16–17 namely 20/80 dichloroacetic tained by Ryan and McCann;1 in our experi- acid/dichloromethane containing 0.01 M of ammo- ments, however, no additional base was added to nium acetate. Not only were all of the PCAs that the samples. we studied soluble in this aggressive medium, but We deconvoluted the SEC curves for the sam- no change in the molecular weight of the PCAs ples incubated for 10 and 55 min, using Gaussian occurred in solution, and the SEC was free from functions for peak fitting (Figure 4). This analysis artifacts caused by specific interactions between indicated that the samples were 42 and 47% de-