Bulk free radical polymerization of styrene with unsymmetrical bifunctional initiators

Abstract

An experimental study of the bulk free radical polymerization of styrene catalyzed by thermally unsymmetrical bifunctional initiator 4-((tert-butylperoxy)carbonyl)-3-hexyl-6-[7-((tert-butylperoxy)carbonyl)heptyl] cyclohexene is reported. The cyclic peroxyester group and the primary peroxyester group in the bifunctional initiator molecule have significantly different thermal stabilities. When this unsymmetrical bifunctional initiator is used for styrene polymerization a t high temperatures, it is possible to produce polymers of high molecular weight a t high reaction rate. It is observed that the bifunctional initiator has relatively lower initiating efficiency than conventional monofunctional initiators. The initiator efficiency also tends to decrease with an increase in its concentration. The experimental polymerization data are also compared with the kinetic model simulations, and very good agreement has been obtained.Bulk or low solvent free radical polymerization of styrene at high temperature is of significant industrial importance. With rapidly growing demands for the polymers of precisely controlled properties and the polymers of varying grades, the need for a more efficient polymerization process is becoming more acute than ever. The polymerization process innovations can be achieved by the use of more efficient initiators, novel reactor systems, and improved polymerization techniques. During the past decades, numerous papers have been published on the kinetics of styrene polymerization, reactor modeling, and optimal control of batch and continuous polymerization processes. Many of the works reported in the literature on styrene polymerization dealt with relatively low-temperature reactions and/or single monofunctional initiators such as benzoyl peroxide (BPO) and azobis(isobutyronitri1e) (AIBN). In practice, more complex initiator systems such as mixed initiators and bifunctional initiators or multifunctional initiators are widely used in the polymer industry in order to enhance the monomer conversion and to improve polymer properties or to reduce the batch time. This paper is concerned with the kinetics of styrene polymerization catalyzed by unsymmetrical bifunctional initiators.The multifunctional initiators are defined as free radical generating initiators containing more than one labile group (e.g., peroxy or azo) having distinctly different thermal decomposition characteristics. Unlike conventional monofunctional initiators, functional groups in the multifunctional initiators can be decomposed in a controlled manner by varying the polymerization conditions. Therefore, the multifunctional initiators offer an added degree of freedom to polymer reactor engineers in optimizing the polymerization processes more effectively by taking advantage of some unique characteristics of the initiators. For example, it has been illustrated by some workers that the multifunctional initiators can produce

Solution styrene polymerizations performed with multifunctional initiators

“…The characteristic linear increase of monomer conversions in thermal styrene polymerizations is related to the mild gel effect of styrene polymerizations . The increase of monomer conversion with temperature is related to the increase of rates of thermal initiation and propagation . The decrease of the average molar masses with the temperature increase is related to the faster increase of rates of chain termination, when compared with the rates of propagation.…”

Bulk polymerization of styrene catalyzed by bi‐ and trifunctional cyclic initiators

“…[17][18][19] However, this behavior was not observed for the bi-and trifunctional cyclic initiators used in this study, which indicated that there was no substantial difference in the reactivities of the two or three peroxide groups contained by initiator molecules under isothermal conditions. Another important feature was that there was no physical evidence of the gel effect in the temperature range of 90 -200°C and conversion range of 95-99%, in spite of it being demonstrated that termination reactions involving polymeric radicals became diffusion controlled and the termination rate constant decreased considerably with an increase in the monomer conversion.…”

Recent Advances in the Study of Multifunctional Initiators in Free Radical Polymerizations

“…Using polyethylene as an example, it has been found that size exclusion chromatography (SEC) can detect a minimum length for an alkane branch somewhere between six and twelve carbons 37,38. It is generally accepted that six carbons or longer can be detected by 13C NMR spectroscopy 39–41. In the case of rheological methods, a definite length has not been completely established.…”

“…Since that time, there have been numerous studies expounding the behaviour of difunctional initiators and their benefits towards polymer production, with most of the focus on styrene monomer 9–22. It has been shown that through repeated initiation, propagation and termination steps, multifunctional initiators are able to generate high rates of polymerization while maintaining high molecular weights (see Figure 1).…”