Studies on polyimides: 2. Formation of high molecular weight poly(N-(hydroxyphenyl) maleimides)
Introduction
The free radical chain polymerization of 1H-pyrrole-2,5-diones, more commonly referred to as maleimides, and the N-substituted derivatives has been extensively studied [1], [2], [3], [4], [5]. Despite the 1,2-disubstituted ethylene structure of the maleimide ring, both homo- and copolymerizations have been reported to occur with a variety of N-substituents and comonomers [6], [7], [8]. The resulting polymers generally exhibit both good thermal and chemical stability [9], [10].
Poly(N-(hydroxyphenyl) maleimides) (1) have been extensively used either individually or in composite formulations with other phenolic resins in applications where good thermal properties are desirable [11], [12], [13]. However, the free radical polymerization of N-(hydroxyphenyl) maleimide (2) monomers gives polymers in relatively poor yields and low molecular weights. This has previously been attributed to the presence of the free phenolic substituent during the free radical polymerization [14]. However, there are reports in the literature concerning the free radical polymerization of monomers containing free phenolic groups which range from evidence of no effect [15], [16] to observations of considerable problems associated with the phenolic group [15], [17], [18], [19].
The choice of a solvent for the polymerizations of the maleimides is limited by the poor solubility of both monomeric and polymeric materials. Consequently, polar solvents are often used for the polymerizations, many of which are undesirable as solvents for free radical polymerization. It is therefore of interest to produce maleimide-based polymers of improved solubility in common solvents and of high molecular weight, both of which are highly desirable engineering properties for their use in industrial applications.
In this article, we describe the synthesis and controlled free radical chain polymerization of a series of novel N-(substituted phenyl) maleimides to form high molecular weight polymeric materials in inert solvents. The process is applicable generally to other phenolic monomers.
Section snippets
Materials
Maleic anhydride (AR), sodium acetate (99%), nickel(II) acetate tetrahydrate (98%), 3,4-dihydro-2H-pyran (97%) and acetic anhydride (98%) were obtained from Aldrich and used as received. Pyridinium p-toluenesulfonate (PPTS) was prepared as previously described [20]. The aminophenols (Aldrich) were recrystallized from ethyl alcohol. 2,2′-Azobis(isobutyronitrile) (AIBN) was recrystallized twice from diethyl ether. N,N-Dimethylformamide (DMF) was dried over CaH2. Solvents were purified by
Results and discussion
The free radical polymerization of the N-(hydroxyphenyl) maleimides was initially investigated to determine substituent effects and the effect of the free phenolic group.
Conclusions
New methodology has been developed for the synthesis in high yields of N-(hydroxyphenyl) maleimides of differing substitution patterns. Polymerization in DMF typically gave low DPs, which was attributed to the presence of the free phenolic group and chain transfer to the solvent. Protection of the phenolic group with an acetoxy group marginally improved the observed DPs, but the effects of the solvent were still controlling the polymerizations. Protection of the phenolic group with a
Acknowledgements
The authors wish to express their appreciation to Stuart Bateman, Jamie Munton and Mark Looney for their productive discussions. We also acknowledge Comalco Aluminium Limited and the Australian Research Council for financial assistance.
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