Makromol. Chem, Rapid Commun. 14,213 -215 (1993) 21 3

Pulsed laser study of the propagation kinetics of tert-butyl methacrylate

Philippe Pascal"), Mitchell A. Winnik

Department of Chemistry and Erindale College, Universtiy of Toronto, Toronto, Ontario, M5S 1Al Canada

Donald H. Napper, Robert G. Gilbert*

School of Chemistry, University of Sydney, Sydney, N. S . W., 2006 Australia

(Received: September 11, 1992; revised manuscript of November 9, 1992)

Introduction

This article presents the temperature dependence of the propagation rate coefficient, k,, for the free-radical polymerization of bulk tert-butyl methacrlyte (t-BMA) at low conversion. The motivation for this work was to provide reliable data which could be used for kinetic modelling of more complex polymerization systems such as emulsion polymerization, as well as to provide data for future testing of models for the transition states for the propagation process. The pulsed-laser polymerization (PLP) technique (whose first appearance in the literature seems to be a suggestion in a theoretical paper in 1977 but which was developed independently and as a method for the determina- tion of k, by Olaj et al.2*3)) adopted for this study has been shown to yield reliable values of k, for various monomer systems and is more direct than conventional means such as SIP (spatially intermittent polymerization) method3-*). In summary, the action of pulsing a laser beam through a monomer solution containing a photoinitiator produces a population of polymer chains which tend to have a degree of polymerization v,, given by:

vp = k , [MI tf (1)

where [MI is the concentration of monomer and t, is the dark time between pulses (some radicals produced by this method may survive more than one period, leading to values of 2 v,, 3 v,, etc.). Hence, given that one is able to determine the value of vp by gel-permeation chromatography (GPC), k, can be obtained from Eq. (1). In practice (because of effects such as stochastic broadening of chain length during propagation, the occurrence of transfer, and instrumental broadening in the GPC analysis), v, must be inferred from the GPC trace using the point of inflection, an assumption justified elsewhere's *).

a) Current address: RhBne-Poulenc Recherches, 52, rue de la Haie-Coq, 9338 Aubervilliers, France.

0 1993, Hiithig & Wepf Verlag, Base1 CCC 0173-2803/93/$02.00

214 P. Pascal, M. A. Winnik, D. H. Napper, R. G. Gilbert

Experimental part

Details of the pulsed-laser polymerization (PLP) and gel-permeation chromatography (GPC) procedure and equipment used in these laboratories have already been outlined '). The terf-butyl methacrylate (t-BMA) (99V0, Aldrich) was further purified by distillation under reduced pressure (40 "C; 18 mmHg). The value of k, was determined at five temperatures, ranging from 9 to 66 "C. Photopolymerizations in bulk I-BMA were initiated by mol . dm-3 benzoin ethyl ethera) (99%. Aldrich) and quenched by turning the laser off and inhibiting the reaction mixture with 2,6-di-fert-butyl-4- methylphenol (99'70, Aldrich). The optical density across 1 cm of the reaction solution was 0,12. The laser was pulsed at 15 Hz, producing about 45 mJ per pulse, for 15 -20 min. Polymerizations were carried out tq no greater than 0,3 wt.-Yo polymer. For GPC, a p- Styragel column (pore size: lo4 A; Millipore) and tetrahydrofuran (99,5+ To, spectrophotometric grade; Aldrich) eluent were used. Molecular weight calibration was performed using uniform poly(t-BMA) standards (Polymer Standards) with molecular weights: 9,1.103, 25.103, 34~103and71,7~103g~mol-'.

Results and discussion

A typical GPC trace is shown in Fig. 1, which also shows the differential of this trace clearly indicating the point of inflection as the minimum in the differential. The k , values resulting from such data are plotted in Arrhenius form in Fig. 2. The Arrhenius parameters are: frequency factor A = 107,4'0,4 dm3 . mol-' * s and activation energy Ep = 27,7 t- 2,5 kJ mol-I. Our value of k, at 25 "C (380 dm3 * mol-I - s - ' ) is in accord with that reported over 30 years ago by Grant and Grassie') (350 dm3 - mol-' - s - l ) , who used the SIP method. Their E, (18,4 kJ mol-I), however, is

Fig. 1 . Fig. 2.

Fig. 1. Line: gel-permeation chromatography (GPC) trace from pulsed-laser polymerization experiment. Solvent was tetrahydrofuran at a flow rate of 1,0 cm3 . min -' on a lo4 A Millipore p-Styragel column, with refractive index detection. Laser repetition was 15 Hz, dark time between pulses tf = 0,0667 s, temperature = 23°C. Points: differential of trace, showing point of inflection of GPC traceas minimum in differential plot at 9,14 min, which corresponds to a degree of polymerization v, = 113 for the experimental conditions

Fig. 2. Arrhenius plot of propagation rate coefficient k, for polymerization of tert-butyl methacrylate in bulk

a) Systematic IUPAC name: a-ethoxybenzyl phenyl ketone.

Pulsed laser study of the propagation kinetics of tert-butyl methacrylate 215

significantly smaller than the value found here. Unfortunately, error limits were not quoted by Grant and Grassie, and therefore it is difficult to make any conclusions about this discrepancy.

Our Arrhenius parameters appear reasonable compared to other monomer systems: for example, for styrene4' 1 1 )

k,/(dm3. mol-' . s- ' ) = lo7,' exp[-29,5 k J . mol-'/(RT)].

The value for methyl methacrylate is lo)

dm3 .mol - ' - s - ' exp[-18,2 kJ.mol-'/(RT)].

The reason that the Arrhenius parameters (and hence transition state properties) for 1-BMA more resemble those for styrene than for methyl methacrylate, whereas one would expect greater similarity to the latter, is not immediately apparent; nevertheless, the differences appear much greater than the experimental uncertainties. Acquisition of data such as those presented here for a wider range of monomers is clearly necessary before reliable a priori qualitative and quantitative models for the energies and enthalpies of activation for propagation of macroradicals can be developed.

We thank NSERC Canada and ARCS Australia for their financial support. An Australian Postgraduate Research Award for P. P. is gratefully acknowledged, as is helpful interaction with Mark Piton.

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