Tetrahydrofuran polymerization kinetics

The polymerization kinetics of alkali salts of living vinyl polymers In ethereal solvents, such as tetrahydrofuran CD, tetrahydropyran (2), dlmethoxyethane Q), oxepane (4) and dloxane... 

In the polymerization of 1,3-dioxolane and tetrahydrofuran it has been shown additionally that concentration of active centers is constant throughout the polymerization (both by direct determination and from analysis of polymerization kinetics). In some other polymerizations, believed to proceed as living processes, only the moderate molecular weights regions (M < 105) were studied thus, for example, no very high molecular weight polymers were obtained in the polymerization of oxazolines. 

This is not the first time that the kinetics of bulk polymerizations has been analysed critically. Szwarc (1978) has made the same objection to the identification of the rate constant for the chemically initiated bulk polymerization of tetrahydrofuran as a second-order rate constant, k, and he related the correct, unimolecular, rate constant to the reported by an equation identical to (3.2). Strangely, this fundamental revaluation of kinetic data was dismissed in three lines in a major review (Penczek et al. 1980). Evidently, it is likely to be relevant to all rate constants for cationic bulk polymerizations, e.g., those of trioxan, lactams, epoxides, etc. Because of its general importance I will refer to this insight as Szwarc s correction and to (3.2) as Szwarc s equation . 

When a cychc monomer such a tetrahydrofuran or caprolactam is used as the monomer, the polymerization can be made to occur primarily by monomer reacting with the polymer rather than aU polymers reacting with each other. These kinetics are more tike addition polymerization, where only the monomer can react with the polymer. However, we stiU call this condensation polymerization because it produces this type of polymer. 

A review is given on the kinetics of the anionic polymerization of methyl methacrylate and tert.-butyl methacrylate in tetrahydrofuran and 1,2-dimethoxy-ethane, including major results of the author s laboratory. The Arrhenius plots for the propagation reaction+are linear and independent of the counterion (i.e. Na, Cs). The results are discussed assuming the active centre to be a contact ion pair with an enolate-like anion the counterion thus exhibiting little influence on the reactivity of the carbanion. 

Remarkably few systematic studies have been made of the kinetics of anionic polymerization in non-polar solvents containing small amounts of ethers in contrast, studies of bulk ether systems abound. Several studies have appeared 156 158) in which the propagation reactions involving styryllithium were measured in mixtures of benzene or toluene with ethers. The kinetic orders, in some cases, of the reactions were identical to those observed in the absence of the ether. Thus, in part, the conclusion was reached 157,1581 that the ethers did not disrupt the dimeric degree of aggregation of poly(styryl)lithium. The ethers used were tetrahydrofuran 156), anisole 157), diphenyl ether 158), and the ortho and para isomers of ethylanisole157). 

The use of a precision digital density meter as supplied by Mettler Instruments (Anton Paar, Ag.) appeared attractive. Few references on using density measurements to follow polymerization or other reactions appear in the literature. Poehlein and Dougherty (2) mentioned, without elaboration, the occasional use of y-ray density meters to measure conversion for control purposes in continuous emulsion polymerization. Braun and Disselhoff (3) utilized an instrument by Anton Paar, Ag. but only in a very limited fashion. More recently Rentsch and Schultz(4) also utilized an instrument by Anton Paar, Ag. for the continuous density measurement of the cationic polymerization of 1,3,6,9-tetraoxacycloundecane. Ray(5) has used a newer model Paar digital density meter to monitor emulsion polymerization in a continuous stirred tank reactor train. Trathnigg(6, 7) quite recently considered the solution polymerization of styrene in tetrahydrofuran and discusses the effect of mixing on the reliability of the conversion data calculated. Two other references by Russian authors(8,9) are known citing kinetic measurements by the density method but their procedures do not fulfill the above stated requirements. 

Wicke and Elgert studied the mode of a-methylstyrene placement in chains polymerized with butyllithium in tetrahydrofuran. They called attention to the possibility of various addition and depropagation rates for isotactic and syndiotactic monomer placement. Isotactic attachment is harder to form, and easier to decompose. For reversible processes this circumstance must be manifested at higher temperatures by thermodynamic, and at lower temperatures by kinetic, control of product stereoregularity [363],... 

Cationic polymerization of 2-methylpropene at temperatures about 170 K may be almost flash-like the transformation of tetrahydrofuran to an equilibrium polymer-monomer mixture may last tens to hundreds of hours at 260 K. Evidently the overall polymerization rate is a function of many factors which may be interconnected or may act separately. The aim of kinetic measurements is to describe the polymerization, and to find conditions under which it would proceed in the desired manner. This is usually only possible after the various factors and their consequences have been isolated and investigated. The rate of monomer consumption during polymerization mostly depends on the generation rate of active centres, and on their concentration and reactivity. 

FIGURE 14.9 Kinetic traces of the TREPR signal from main-chain polymeric radical la in various solvents. (A) Methylene chloride, (B) chloroform, (C) acetonitrile, (D) tetrahydrofuran, and (E) dioxane. The dashed line represents the baseline for each kinetic trace. 

Cationic polymerization of tetrahydrofuran is one of the few systems in cationic ring polymerization in which chain transfer to polymer may be practically avoided. The reasons for that are of purely kinetic nature. 

Kinetics and Mechanism of Methyl Methacrylate Polymerization Photoinitiated by Benzophenones in Tetrahydrofuran... 

Although some of the kinetic data mentioned above indicate the selective reaction of the optical antipode of NCA in the polymerization, the direct evidence for this was obtained for the first time by Matsuura, Inoue and Tsuruta (45-47). In the polymerization of alanine NCA (8, R = CH3) with the content of S antipode of 77.3%, initiated by methanol in tetrahydrofuran, the absolute optical rotation value of the polymer in trifiuoracetic acid (TFA) increased with conversion through a maximum, then decreased (Fig. 14). This fact clearly indicates that the S antipode of the NCA polymerizes preferentially in the early stage, diile in the later stage the residual R ant de polymerizes whidi becomes relatively rich in amount. 

Only one kinetic study exists on initiation of methacrylate polymerization by a sodium compound. The initiator was the disodium oligomer ( tetramer ) of a-methylstyrene and polymerization was investigated at 25°C in toluene in presence of 0.05—0.2 mole fraction of tetrahydrofuran [181]. An internal first order disappearance of monomer was observed, the first order coefficient being directly proportional to active chain and tetrahydrofuran concentrations. The rate coefficients evaluated, e.g. fep = 3.1—13 X 10 1 mole sec at various tetrahydrofuran concentrations, are much lower than those for lithium initiators. They were, however, evaluated using a methyl iodide titration technique to estimate the active chain concentration. In view of the reactivity of tritiated acetic acid with many short chains which are clearly not active in chain propagation, there must be suspicion of similar behaviour with methyl iodide. If this happens, the active chain concentration would be over-estimated and the derived fep value would be too low. Unfortunately no molecular weights of the precipitable polymer were determined, so that it is impossible to check on active chain concentration using this alternative method. 

In many instances the polymerization of tetrahydrofuran seems to proceed without termination, and living polymers, in which transfer reactions are also absent, have been prepared [40, 41]. A simple kinetic scheme suffices therefore to describe the situation, i.e. 

Alkali metal naphthalene complexes have also been used to initiate epoxide polymerizations. Solov yanov and Kazanski [25] studied the polymerization of EO in tetrahydrofuran using sodium, potassium or cesium naphthalene as initiator. A living polymer was produced there is no chain rupture or transfer. The rate of polymerization depends on the concentration of active centres in a complex manner. The kinetic order varies from 0.23 for Na" (or 0.33 for K and Cs" ) up to full first order as initiator concentration decreases. The polymerization is first order in monomer, but deviations are observed at high concentrations. 

In a more recent study Sigwalt et al. [41] investigated the use of carbazyl sodium to initiate PS polymerization in tetrahydrofuran (THF). This initiator produced only one living end per chain. Excellent agreement with the results obtained on amphianionic polymer was obtained. At levels of living ends below 10 molel the rate coefficient found for ion pairs was 3 x 10" 1 mole sec and for free ions 4 1 mole sec . The dissociation constant (determined kinetically) was 6.4 x 10 mole 1 . ... 

The polymerization of tetrahydrofuran has been reviewed in detail [5, 58]. There a thorough discussion of many aspects of the mechanism of polymerization and the very many initiators used was presented. Interested readers are referred to the earlier reviews for these aspects of the polymerization. Here attention is ain focused primarily on the kinetics of polymerization. 

Problem 10.4 (a) The kinetics of polymerization of tetrahydrofuran was studied [6] with the use of triethyloxonium tetrafluoroborate, (C2Hs)30+BF4, as im tiator and dichloromethane as solvent. Conversion versus time was measured at 0°C with initial catalyst concentration [I]o = 0.61 x 10 mol/L and monomer concentration [M] varying from 3 to 9 mol/L. The initial rates, Rp, determined from these data are given in Table A. 

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