No 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...

Problems of interpretation of the mechanisms in the polymerisation of styrene by HCIOJ in CH2CI2, especially of chain propagation are discussed and experimental results from recent studiesofthe bimodal molecular weight distributions found in polymers prepared at 0 C and below. and of the kinetics of the rapid. non-stationary, precursor polymerisation at -80 C and other low temperatures are prescnted. The former indicate simultaneous. but independent, propagation at 0 C by pseudo-ionic and ionic species. The latter reveal the transient existence at low tcmperatures of an ionic species (absorbing at 340 nm) identified as the (po1y)styryl carbenium ion. The measurements permit estimates ofthe free-ion propagation rate constant at -80 C to -45 C, and suggcst the unexpected conclusion that propagation by ion pairs is several fold faster than by free ions. Both species however appear to be not simple, but strongly complexed by monomer. It is speculated also that monomer complexation of the perchloric acid initiator plays an important r61e in determining whether pseudo-or true ionic mechanisms predominate. ZUSAMM ENFASSUNG :Es werden Probleme bei der Deutung des Mcchanismus insbesondere des Kettenwachstums der Polymerisation von Styrol mit HCIOJ in CH2CI2 diskutiert und experimentelle Ergebnisse aus neueren Untersuchungen sowohl der bimodalen Molekulargewichtsverteilungen von Polymeren, die bei 0'C und unterhalb hergestellt wurden, als auch der Kinetik der schnellen, nicht stationaren Vorpolymerisation bei -80' C und anderen tiefen Temperaturen dargestellt. Erstere deuten auf gleichzeitiges, aber unabhiingiges Wachstum der pseudoionischen und ionischen Spezics bei 0 C. Letztere zeigen das vorubergehende Vorhandensein einer ionischcn Spezies bei tiefen Temperaturen (die bei 340 nm absorbiert) und die als (Po1y)styryIcarbeniumionen identifiziert wurde. Die Messungen gestatten eine ungefihre Berechnung der Wachstumsgeschwindigkeitskonstante des freien Ions bei -80'C bis -45'C und erlauben den unerwarteten SchluR. daR das Wachstum iiber Ionenpaare mehrfach schneller verliiuft als iiber freie Ionen. Beide 1077 D. C. Pepper scheinen jedoch keine einfachen, sondern stark durch das Monomere komplexierte Spezies zu sein. Es wird vermutet, dal3 auch die Komplexierung des Perchlorsiiure-Initiators durch das Monomere eine wichtige Rolle bei der Entscheidung spielt, ob ein pseudooder ein rein ionischer Mechanismus uberwiegt. * Communication no. 23, 1st Intern. Symp. Cationic Polymerisation, Rouen, Sept. 1973.

A theory of the overall rates and molecular weights in slowly-initiated, non-terminated polymerizations is developed for cases of complex initiation of alkyl cyanoacrylates by tertiary amines. Results with butyl cyanoacrylate (BCA) in the solvent THF, and five different amines (chiefly pyridine and derivatives) show widely differing overall rates. The theory, with the aid of previously determined initiation rate constants, permits the same value for the propagation rate constant (k,) to be derived in all cases, corroborated by values derived from the simpler overall kinetics of polymerisation initiated by Ph,P. The rate constant kp shows very slight temperature dependence, less than one order of magnitude between 20 and -80 "C.

This paper describes a re-examination of the kinetics and molecular weights formed in the zwitterionic polymerization of butyl cyanoacrylate by triphenylphosphine ( P h P) and pyridine (Py) in THF, and an extension to include the solvents, diethyl ether (EGO), dimethoxyethane (DME) and hexane. In THF, the previously observed kinetics were confirmed, yielding evidence for absence of termination, and values for rate constants for initiation (4 ) and propagation (la) with substantial agreement between the values for /$, obtained from both systems. At 20"C, the mean value found for kp in THF was 3 . lo5 1 . mol-' . s -' ; in EGO, some 10 times lower, and in hexane, 10 times lower again. The temperature dependence of /$ in THF was confirmed to be extremely low and anomalous; in EbO rather greater and near-normal, corresponding to an energy of activation of ca. 5 kcal. mol-' . A correction procedure enabled the large experimental scatter in the kp values to be reduced sufficiently to reveal trends downwards at higher concentrations of propagating species, i. e. evidence for their dissociation on dilution.In Py-initiated polymerizations, excess of pyridinium salts did not depress kp ; evidence that the propagating species are preponderately paired ions, not free. It is concluded that at 20°C these species are mainly 'tight' ion-pairs in all solvents, and remain so in EGO at all temperatures down to -80 "C. In THF, over this temperature range, there is increasing solvation producing an increasing proportion of more reactiveLsolvent-separated' ion-pairs. The molecular weights produced under all conditions were high ( M , usually 1 to 3 . lo6). Those produced by high concentration of Ph,P did not conform to theory; others followed the 'no-termination' theory as modified to allow for a small effect of transfer. I COOBu which propagate anionically without termination, provided acidic contaminants or additives are excluded;2) that in THF, triethyl and triphenyl phosphines initiate sufficiently fast as to be effectively completely converted to propagating species, so that the overall kinetics 9 In what follows, th charge signs w i l l be omitted, and the concentration of zwitterions of all chain lengths be represented simply by [Z].