Highly reactive, liquid diacrylamides via synergistic combination of spatially arranged curing moieties

Highly reactive, liquid diacrylamides via synergistic combination of spatially arranged curing moieties Maximilian Maier1, Magnus S. Schmidt2, Markus Ringwald2 and Christoph P. Fik*1 Full Research Paper Address: 1Dentsply Sirona Restorative, De-Trey-Str. 1, 78467 Konstanz, Germany and 2MCAT GmbH, Raiffeisenstr. 35, 78166 Donaueschingen, Germany Email: Christoph P. Fik* - * Corresponding author Keywords: acrylamide; allyl; cyclopolymerization; photopolymerization; spatial effect Open Access Beilstein J. Org. Chem. 2017, 13, 372–383. doi:10.3762/bjoc.13.40 Received: 19 November 2016 Accepted: 02 February 2017 Published: 27 February 2017 This article is part of the Thematic Series "Spatial effects in polymer chemistry". Guest Editor: H. Ritter © 2017 Maier et al.; licensee Beilstein-Institut. License and terms: see end of document. Abstract Six polymerizable N,N’-diacylamides containing spatially arranged N-acryl, N-allyl and/or N-alkyl groups were prepared via twostep syntheses and characterized by 1H/13C NMR-spectra, refractive index (RI) and viscosity measurements. Photo DSC measurements on activated samples provided reactivity parameters ∆Hp, Rp,max and tmax, while FTIR spectra before and after curing elucidated the underlying polymerization mechanism. Mechanical testing of the obtained polymers exhibited gradual differences in network densities, depending on the intramolecular arrangement and number of functional groups. Overall, a general building principle for highly reactive, liquid diacrylamides via synergistic combination of optimally arranged functional groups could be identified. The highest possible level of intramolecular synergism was found for low viscous N,N'-diacryloyl-N,N'-diallyl-1,4-but-2enediamine. Introduction The selection of suitable monomers is a critical step for any free-radical polymerization approach. Particularly for (in situ) photo-induced polymerizations, monomers should comprise sufficient solubility in a given matrix, moderate viscosity, matching refractive indices as well as an optimized reactivity – the proper design of these features ensures continuous light transmittance, adequate propagation rates and, ultimately, thorough polymerization [1,2]. The number of applications for UV–vis curable monomer systems has greatly increased over the last decades [3]. At the same time, the selection of new monomers and crosslinkers remained limited [4]. Mono-, di-, tri- and multifunctional (meth)acrylates are among the first choices for photopolymerized mixtures as they exhibit a favorable balance between reactivity and thermal stability upon storage [5-7]. Moreover, they comprise compatibility with different matrices/solvents together with an adequate reactivity in a broad temperature range [8-10]. In general, acrylate monomers exhibit a higher reactivity than the respective methacrylates [11-13], but tend to be more sensitive to oxygen inhibition [14]. A major drawback of many (meth)acrylate-based compositions, however, is their susceptibility to premature hydrolysis when used in aqueous solutions, especially at pH values <2.5 [15,16]. 372 Beilstein J. Org. Chem. 2017, 13, 372–383. One strategy to improve the hydrolytic stability is the oxygento-nitrogen substitution. The obtained class of (meth)acrylamides is of interest in the field of biomedical applications, e.g., for dental materials, artificial cornea, or drug-delivery systems, for which contact with body fluids is inevitable [17,18]. Whilst some of the resulting secondary di(meth)acrylamides end up being solids, tertiary di(meth)acrylamides can be obtained as relatively low viscous, highly soluble/compatible liquids [19]. Furthermore, acrylamides are generally more reactive than the respective methacrylamides. Regarding the substitution pattern, N-monosubstituted acrylamides tend to homopolymerize more readily than their N,N-disubstituted analogues [20]. Yet, acrylamides are particularly affected by the solvent regarding propagation reaction in free radical polymerization, even more so, if water is present [21]. Factors such as hydrogen bonding, hydrogen abstraction and the overall electronic characteristics are crucial in the design of improved monomer structures [22]. In this sense, Bowman et al. demonstrated increased photo-polymerization rates for monoacrylates equipped with secondary functionalities, yet limiting discussion to oxygen-based (meth)acrylate derivatives [23]. In this study, we present the synthesis and characterization of tailor-made, liquid N,N’-diacyl diacrylamides with enhanced reactivity through synergistic combination of spatially arranged curing moieties. The obtained structures were investigated in terms of underlying building principle, chemical and physical properties as well as polymerization behavior upon photoinitiation. Results and Discussion As stated earlier [24] we strive to investigate the unique physical properties and reactivity of tertiary N,N’-diallyl-diacrylamides. Closely related to this class of crosslinkers are bifunctional N-alkyl-N-allylacrylamides, which are known to undergo radical cyclopolymerization due to their adjacent double-bond functionalities [25-27]. The propagation reaction of these structures proceeds intramolecularly between acryl and allyl groups and intermolecularly (mostly) between polymerradical and acrylamide groups. Cyclo- is preferred over linear polymerization due to the preformed five or six-membered lactams and gets even more predominant with increasing chain length of N-alkyl groups [28]. Expanding this concept in view of an optimized spatial layout, we synthesized molecules with additional “internal” (at the molecules’ center), symmetrical allyl functions, connecting two N-allylacrylamide groups, thus adding a two-way, intramolecular reaction site. In order to individually assess the effect of “internal” and “external” (at the molecules’ periphery) N-allylic functions on the physical/polymerization properties, a systematic variation of the molecular structure has been realized. When allyl- and acrylamide functionalities were spatially adjacent, a “synergistic potential” beneficial in radical polymerization was expected (Scheme 1). Scheme 1: Top: Overview of the synthesized crosslinkers 1–6 and their correlation to each other via formal reactions. Bottom: Schematic of 1–6 in terms of their structural synergistic potential due to adjacent acrylamide and allyl functions. 373 Beilstein J. Org. Chem. 2017, 13, 372–383. Synthesis Six derivatives of highly functionalized crosslinkers 1–6 were synthesized as outlined in Scheme 2. We started from dibro- mide 7 to gain access to the corresponding compounds 1, 2 and 5. In case of the alpha-methyl compound 4, we started from trans-1,3-pentadiene (14) and synthesized the dibromide 15 ac- Scheme 2: Synthetic pathways to structurally related compounds 1–6. 374 Beilstein J. Org. Chem. 2017, 13, 372–383. cording to the work of Heasley et al. [29]. Intermediates (...truncated)