Elsevier

European Polymer Journal

Volume 40, Issue 7, July 2004, Pages 1451-1460
European Polymer Journal

Kinetic aspects of the Diels–Alder reaction between poly(styrene-co-furfuryl methacrylate) and bismaleimide

https://doi.org/10.1016/j.eurpolymj.2004.01.036Get rights and content

Abstract

The crosslinking Diels–Alder reaction between styrene–furfuryl methacrylate copolymer samples (poly(ST-co-FM)) and bismaleimide (BM) at 25 °C in chloroform was studied by following the decay in UV absorbance of the maleimide (MI) group at 320 nm. Reaction conditions were changed by using copolymers with different mole fraction of FM, FFM, and by employing different initial molar ratios of reactants (furan group within FM and MI group within BM). Second order kinetics were obeyed. 13C NMR spectra showed that, even when all reactants had been converted to an insoluble crosslinked network, unreacted MI groups remained, presumably in the form of singly reacted pendant BM molecules. The fractions of MI groups remaining unreacted were found to be 0.49, 0.34 and 0.22 for FM:MI mole ratios in the initial mixture of 2, 1 and 0.5 respectively, when using a copolymer of FFM=0.1354. An attempt was also made to follow the kinetics of network formation by 13C NMR spectroscopy, using the peak areas for reacted and unreacted MI and FM groups, but many of the findings were subject to some uncertainty for reasons, which are discussed. However, because the peak areas were considered reliable for unreacted MI groups, the rate constant, k, was evaluated, thereby. Overall using UV and NMR the values of k lay within the interval (0.8–3.6) × 10−5 dm3 mol−1 s−1.

Introduction

The Diels–Alder (D–A) reaction is a very useful reaction in organic synthesis [1], [2] in which a conjugated diene adds to a dienophile forming a cyclic product. This reaction is reversible on heating. The application of D–A process in polymer chemistry has also received some attention. Polymers have been synthesised via consecutive D–A reaction between (i) AB monomers [3], [4], [5], (ii) AA and BB monomers [6], [7], [8] or (iii) AA monomers [9], [10] where A and B respectively denote reactive diene and dienophile groups in the monomers. In the last case the same group A behaves as diene and dienophile, e.g. a cyclopentadiene group.

Another interesting application of this reaction in polymers is the formation of thermally reversible networks. Due to the reversible nature of the reaction, it might be possible to control the molecular weight of polymers and therefore some mechanical properties. Polymers bearing pendant diene or dienophile groups have been crosslinked by reaction with a bisdienophile or bisdiene, respectively [11], [12], [13]. In a recent paper, Gheneim et al. [13] synthesised copolymers with pendant maleimide groups and then reacted them with a bisdiene containing furan groups. Other workers [14], [15], [16], however, have used two functionalised polymers, e.g. Imai et al. [16] mixed maleimide-modified and furan-modified poly(2-methyl-2-oxazoline) to yield networks. The use of only one modified polymer has also been possible when cyclopentadiene group was the pendant group in a polymer as described by Miura et al. [17]. None of the above papers [11], [12], [13], [14], [15], [16], [17] dealt with the kinetics of the crosslinking reaction.

In a previous publication [18], we described the copolymerization of styrene (ST) with furfuryl methacrylate (FM). Some preliminary work on crosslinking this copolymer via the D–A reaction was included. Subsequently the thermal breakdown of the crosslinked poly(ST-co-FM) network via the retro D–A reaction was shown to obey first order kinetics [19]. The crosslinked copolymers were found to swell to organogels of interesting behaviour [20]. An appreciation of the necessary time scale required to synthesise the unswollen crosslinked materials is of practical relevance.

It is the purpose of the present paper to investigate the kinetics of the reaction between poly(ST-co-FM) and 1,1-(methylenedi-4,1-phenylene) bismaleimide (BM). The technique of UV spectroscopy has been employed mainly, although use of 13C NMR was also attempted. The findings from the 13C NMR proved subject to uncertainty, but it seems of interest to summarize them for the benefit of others in the field and to include briefly possible causes of this uncertainty.

Section snippets

Materials

ST (Aldrich) was distilled under vacuum and 2,2-azobis-(isobutyronitrile) (AIBN) (Aldrich) was purified by recrystallization from ethanol. Rectified toluene (BDH laboratories) was used without further purification. FM (Aldrich) was purified by high vacuum distillation at 64–66 °C and 3 mmHg, cuprous chloride being added to suppress polymerization. BM (95%) and deuterated chloroform (Aldrich) were used as received.

Apparatus

Gel permeation chromatography with universal calibration (courtesy of Dow) was

Gel point

The technique described in Section 2.5 was employed only for a MI:FM mole ratio of 1. The gel point was observed to occur after 100, 45 and 21 h of reaction for the copolymers with a FFM=0.0126, 0.0856 and 0.1354, respectively. Hence the onset of gelation occurs more quickly the higher the concentration of reacting groups.

Conversion ratio

The variation of C with reaction time is illustrated in Fig. 2 for copolymers of different composition using a MI:FM mole ratio of 1 and in Fig. 3 for the cases in which MI:FM

Conclusions

  • 1.

    The D–A process between poly(ST-co-FM) and BM yielded a network. The reaction obeyed second order kinetics. The overall range of k derived from using copolymers of different composition and using different concentrations of BM was (0.8–3.6) × 10−5 dm3 mol−1 s−1.

  • 2.

    At the time all the initial linear copolymer was recovered as insoluble crosslinked gel, there was some BM that had not been consumed.

  • 3.

    The crosslinked networks possessed pendant MI groups. 13C NMR was used to evaluate the fraction of pendant

Acknowledgements

Financial support from The Dow Chemical Co. is gratefully acknowledged.

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