A review of semi-rigid, stilbene-containing alternating copolymers

Appl Petrochem Res (2015) 5:27–33 DOI 10.1007/s13203-014-0055-0 KACST FORUM A review of semi-rigid, stilbene-containing alternating copolymers Alice M. Savage • Xu Zhou • Jing Huang • S. R. Turner Received: 3 March 2014 / Accepted: 17 March 2014 / Published online: 16 April 2014 Ó The Author(s) 2014. This article is published with open access at Springerlink.com Abstract The synthesis and properties of sterically congested, stilbene-containing alternating copolymers are reviewed. Persistence lengths (2–6 nm) determined by size exclusion chromatography and small angle X-ray scattering techniques show that these are semi-rigid copolymers. Fully characterized polyanions and polyampholytes, prepared from organic-soluble precursors were studied, along with their respective salt and pH response behavior resulting from the semi-rigid polymer backbone. The solidstate characterization from these studies reveals a high degree of hindered rotation along the polymer backbone. The contorted structure and the hindered rotation of the polymer backbone generate inefficient chain packing, which leads to an increase of nanoporosity and higher surface areas. The semi-rigid stilbene-containing copolymers are a new class of copolymers where the increased polymer backbone rigidity leads to a wide range of polymer properties not attained with flexible polymers. Keywords Stilbene  Alternating copolymers  Copolymerization  Maleic anhydride  Maleimides Introduction Substituted stilbenes (1,2-diphenylethylenes) are widely studied and used organic compounds for the optical properties arising from their conjugated ethylene structure, e.g. optical brighteners, dyes, etc. [1, 40]. Many applications of A. M. Savage  X. Zhou  J. Huang  S. R. Turner (&) Department of Chemistry MC0212 and Macromolecules and Interfaces Institute MC0344, Virginia Tech, Blacksburg, VA 24061, USA e-mail: stilbene derivatives have resulted in a large library of substituted stilbene small molecules and facile synthetic techniques to enable synthesis of a variety of derivatives many of which can serve as copolymerizable monomers. However, the addition polymerization and copolymerization of this family of 1,2-disubstituted olefins, like all 1,2substituted ethylene structures, is difficult due to the steric constraints of the 1,2-structure. Homopolymerization of stilbene and substituted stilbenes to the corresponding poly (benzyl) backbone has not been achieved; however, alternating copolymerization of stilbene both anionically [48] and radically [44] is well documented. In alternating copolymerizations, the cross-propagation step is greatly favored since the steric interactions in the cross propagation step are depressed compared to those required for a successful self-propagation of the stilbene monomers. In addition the electronic effects resulting from electron rich and electron poor comonomers cross-propagating with an electron rich or electron poor terminal radical favor fast cross-propagation. Stilbene copolymers have been claimed in the patent literature to be useful in lubricants [37], lubricating oil additives [2, 31], thermal stability additives for photoresists [42], and improved stain resistant polyamide textiles [11], although successful commercialization of these potential applications is not known. The radical alternating copolymerization of unsubstituted stilbene is the most studied stilbene polymerization [6, 16, 35]. It was found in 1930 that stilbene copolymerized with maleic anhydride [44]. Since neither monomer can undergo radical homopolymerization, these are strictly alternating copolymers with no stilbene–stilbene or maleic anhydride-maleic anhydride dyads present [21]. This study attracted many groups to study the comonomer reactivity ratios [21], monomer sequences [16], thermal stability [32], and copolymerization mechanism [9, 17, 34]. However, 123 28 Appl Petrochem Res (2015) 5:27–33 surprisingly limited solubilities and uninteresting brittle materials have slowed detailed investigations of the copolymerizations and the properties of the resulting copolymers [41]. N-substituted maleimides readily radically polymerize with stilbene into predominately alternating copolymer structures—maleimides can radically homopolymerize, so some maleimide–maleimide dyads can be present in these copolymers [19, 35, 46]. Many of these copolymers have limited solubilities in common organic solvents so solution studies are limited. Our recent work involves the design and synthesis of new stilbene comonomers with specifically chosen functional groups for effecting the copolymerization with maleic anhydride or N-substituted maleimides. Radical polymerization processes are compatible with many functional groups [5] and the alternating nature of the copolymerization enables precise placement of functionality into the copolymer backbones for study of the structure/property relationships of these novel sterically congested backbone polymers in both solution and the solid state. Monomer substituent effect on copolymerization The type and position of substituents on the aromatic groups of stilbene were found to have a significant effect on both the overall copolymerization rates as well as the solubility of the copolymers [24]. Several methyl-substituted stilbene monomers were prepared by using the Wittig-Horner reaction and the resulting monomers were copolymerized with maleic anhydride. Feed ratio/composition studies and 13C NMR confirmed the strictly (b) (a) (c) Fig. 1 Structures of organic-soluble, main-chain stilbene-containing copolymer precursors. a poly((E)-2-methylstilbene-alt-maleic anhydride) (2MeSti-alt-MAn) [24], b poly(N,N,N0 ,N0 -tetraalkyl-4,40 -diaminostilbene-alt-maleic anhydride) (TADASti-alt-MAn) [29], 123 alternating sequences in the backbone [24]. The methylsubstitution increased the solubility of the copolymers in organic solvents by disrupting copolymer aggregation. This allowed for thorough characterization of these copolymers via SEC, NMR and DLS (dynamic light scattering). We note that with unsubstituted stilbene and maleic anhydride copolymers, strong aggregation in DLS was observed which likely accounts for the ‘‘insolubility’’ of these copolymers as reported in the literature. The location of the methyl groups on the phenyl rings was found to have a strong influence on the overall rate of copolymerization. Methyl groups in the para position led to increase copolymerization rates due to the inductively electron donating characteristics which lead to a more reactive stilbene radical to cross propagate with the electron deficient double bond of maleic anhydride. The ortho-methyl stilbene monomers were found to copolymerize at significantly slower rates due to the loss of conjugation of the aromatic groups with the radical due to steric interactions of the methyl groups forcing the phenyl groups out-of-plane (Fig. 1). DLS measurements showed that the para-substituted stilben (...truncated)