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Chapter 6

Downloaded by COLUMBIA UNIV on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch006

Propagation and Termination in Free Radical Polymerization of Styrene to High Conversion Investigated by Electron Spin Resonance Spectroscopy Per B. Zetterlund*, Hirotomo Yamazoe, Satoru Yamauchi, and Bunichiro Yamada* Department of Applied and Bioapplied Chemistry, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan

Abstract: The propagation and termination processes in the bulkfreeradical polymerization of styrene to high conversion have been investigated by electron spin resonance (ESR) spectroscopy. The focus has been the very highest conversion range where the propagation step is diffusion-controlled; at 70 °C, both the values of the propagation rate coefficient (kp) and the termination rate coefficient ( < k t > ) increased with increasing initiator concentration. At 120 °C, the same trend with initiator concentration was observed for kp, although remained unchanged. Both kp and f fall dramatically at approximately 80% conversion at 120 °C even though the temperature is above the glass transition temperature. The addition of a small amount of divinylbenzene has no major effect on kp, which still remains under chemical control up to approximately 80% conversion at 70 °C. The effect on is more dramatic, especially at intermediate conversion.

© 2003 American Chemical Society

In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction Major efforts have over the years been directed towards elucidating and quantifying the details of free radical bulk polymerization. Although significant progress has been made in recent years with improvements in the electron spin resonance (ESR) method (I) and the development of pulsed laser-based techniques (PLP) (2,3), several aspects of the process are not fully understood, especially at high conversion where P L P methods are not suitable. The E S R technique is extremely valuable because it is in principle possible to estimate both the propagation rate coefficient (kp) and the chain-length averaged termination rate coefficient ( < * t > ) even as the conversion approaches its limiting value by directly measuring the free radical concentrations as functions of time. PLP methods allow the determination of kp under conditions of low conversion levels of a few percent. The value of can be estimated as a function of conversion up to high conversion levels, although current P L P methodology does not allow access to in the region where kp falls due to diffusion control of the propagation step setting in. The current ESR study deals with a very wide conversion range, with specific attention being paid to the very highest conversion regime, which is currently the least understood. Determination of kp by ESR is in principle straightforward, although caution is warranted as there exist several pitfalls (4). Determination of is more complicated, and involves either steady-state or non-steady state measurements. In the current study we have taken the former approach, which allows direct access to the ratio lf, where/denotes the initiator efficiency. It follows that/ needs to be estimated independently to enable estimation of . We have investigated the propagation and termination kinetics in the bulk free radical polymerization of styrene at 70 and 120 °C, the significance of these two temperatures being that at the latter temperature, vitrification will not occur even at the very highest conversion level. Furthermore, the effects of adding a small amount of the crosslinker divinylbenzene ( D V B ) have been investigated, considering the introduction of crosslinks on monomer and macroradical mobility.

Experimental Materials Commercially available St and D V B were purified by distillation under reduced pressure before use. D V B consisted of the p- and w-isomers of D V B (50.8 %) and ethylstyrene (49.2 %) according to *H N M R analysis. Dimethyl


74 2,2'-azobisisobutyrate (MAIB) was recrystallized from hexane, and f-butyl peroxide (TBP) was used as received (Wako Pure Chemicals). Polymerizations Bulk polymerizations o f St were initiated by 0.05, 0.10 and 0.20 M M A I B at 70 °C, and by 0.05, 0.10 and 0.15 M T B P at 120 °C. Bulk polymerizations of St/DVB (0.05,0.10 and 0.20 M D V B ) at 70 C were initiated by 0.10 M M A I B . FT-NIR measurements were carried out in a 5 mm o.d. Pyrex tube in an aluminum furnace at 70 and 120 °C. The consumption of St was monitored by the absorbance at 6150 cm' (assigned to the overtone absorption of v _ ), employing a Jasco INT-400 Spectrometer with a M C T detector. Polymerizations for molecular weight measurement were conducted in glass ampules sealed under vacuum. The polymer was precipitated in methanol, and the conversion was determined by gravimetry. Molecular weights were measured with a Tosoh-800 series H P L C with G P C columns calibrated with polystyrene standards. The conversion range within which gelation occurred for St/DVB was determined by observing formation of insoluble polymer. e

1


c==c

H

ESR ESR spectra were recorded on a Broker ESP300 during polymerization in the cavity in a 5 mm o.d. quartz tube sealed under vacuum. The line width and apparent splitting pattern change with conversion as a result of increasing viscosity (Figure 1). The values o f the hyperfine coupling constants for the poly(St) radical, recorded under conditions of high resolution, have been reported previously (1). 2,2,6,6-Tetramethylpiperidinyl-l-oxyl (TEMPO) in benzene was used for calibration of the relationship between ESR spectral signal intensity and radical concentration. The sensitivity of the ESR instrument is a function of conversion in the case of polar monomers such as methyl methacrylate ( M M A ) in bulk (5). The sensitivity remains constant over the entire conversion range for the non-polar St (1,5).

Methods Propagation Rate Coefficient The value of kp as a function of conversion was determined from the FT-NIR and ESR data by use of eq 1 :


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c

Figure 1. ESR spectra at différent conversions during bulk free radical polymerization of St initiated with 0.20 M MAI Β at 70

Living Radical Polymerization - American

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