Reduced shrinkage stress via photo-initiated copper(I)-catalyzed cycloaddition polymerizations of azide-alkyne resins

Abstract

Objectives Polymerization shrinkage stress and factors involved in the stress development such as volumetric shrinkage and modulus were investigated in photo-CuAAC (photo-initiated copper(I)-catalyzed azide-alkyne cycloaddition) polymerization and compared with conventional BisGMA-based methacrylate polymerization for their use as alternative dental resins. Methods Tri-functional alkyne and di-functional azide monomers were synthesized for photo-CuAAC polymerization. Conversion kinetics, stress development and polymerization shrinkage were determined with FTIR spectroscopy, tensometery, and with a linometer, respectively, for CuAAC and BisGMA-based monomer mixtures using a camphorquinone/amine visible light photoinitiator system. Thermo-mechanical properties for the cured polymer matrices were characterized by dynamic mechanical analysis and in three-point bending on a universal testing machine. Polymerization kinetics, polymerization shrinkage stress, dynamic volumetric shrinkage, glass transition temperature (Tg), flexural modulus, flexural strength, and flexural toughness were compared between the two different resin systems. Results A glassy CuAAC polymer (Tg = 62 °C) exhibited 15-25% lower flexural modulus of 2.5±0.2 GPa and flexural strength of 117±8 MPa compared to BisGMA-based polymer (Tg = 160 °C) but showed considerably higher energy absorption around 7.1 MJ×m−3 without fracture when strained to 11% via three-point bend compared to the flexural toughness of 2.7 MJ×m−3 obtained from BisGMA-based polymer. In contrast to BisGMA-based polymers at 75% functional group conversion, the CuAAC polymerization developed approximately three times lower shrinkage stress with the potential to achieve quantitative conversion under ambient temperature photocuring conditions. Moreover, relatively equivalent dynamic volumetric shrinkage of around 6-7 % was observed via both CuAAC and dimethacrylate polymerization, suggesting that the low shrinkage stress of CuAAC polymerization was due to delayed gelation along with slower rate of polymerization and the formation of a more compliant network structure. Significance CuAAC crosslinked networks possessed high toughness and low polymerization shrinkage stress with quantitative conversion, which eliminated obstacles associated with BisGMA-based dental resins including limited conversion, unreacted extractable moieties, brittle failure, and high shrinkage stress.

Kinetics and mechanics of photo-polymerized triazole-containing thermosetting composites via the copper(I)-catalyzed azide-alkyne cycloaddition

“…This behavior arises at least in part because the CuAAC systems had much narrower glass transition widths than their methacrylate counterparts (Fig. S5) [29,39]. …”

“…Fusion silane coupling agent was used as received from George Taub Products & Fusion Co., Inc. Schott glass (mean particle size of 0.4 μm) with both untreated surface and surface treated with γ–methacryloxypropyltrimethoxysilane were used as received from ESSTECH. Bis(6-azidohexyl) (1,3-phenylenebis(propane-2,2-diyl))dicarbamate (BZ-AZ), 1-(prop-2-yn-1-yloxy)-2,2-bis((prop-2-yn-1-yloxy)methyl)butane (AK), and prop-2-yn-1-yl (3-(triethoxysilyl)propyl)carbamate (Si-AK) were synthesized according to previously reported procedures [29,39,43]. All synthetic procedures and NMR predictions are provided in the Supporting Information.…”

“…Over the past decade, alternative chemistries/mechanisms to minimize or eliminate the detrimental characteristics associated with traditional methacrylate-based dental resins have been explored. Ring-opening polymerizations utilizing epoxy, silorane, or cyclopropyl functional groups in resins as well as on nanofillers generated low volumetric shrinkage [23–26], and step-growth polymerization of thiol-ene or CuAAC (copper(I)-catalyzed azide-alkyne cycloaddition) resins achieved reduced polymerization shrinkage stress via delayed gelation [27–29]. In addition, hybrid polymerizations of ternary systems including chain-growth polymerization of methacrylates with step-growth polymerization of thiol-ene/thiol-yne, produced interpenetrating polymer networks (IPNs) with a dramatic reduction in shrinkage stress [30–32].…”

“…Previously, we introduced the photo-initiated copper(I)-catalyzed azide-alkyne cycloaddition (photo-CuAAC) polymerization [37,38] (Scheme 1) as a means to prepare a glassy thermoset with significantly reduced polymerization shrinkage stress at complete conversion while simultaneously maintaining improved mechanical performance [29]. The preeminent benefits of the photo-CuAAC polymers over conventional BisGMA-based dimethacrylate polymers lie not only in the nature of the step-growth polymerization and the associated delayed gel point conversion, which contributes to reduced shrinkage stress [29], but also on the formation of rigid triazole linkages as a product that significantly enhance the overall mechanical behavior [38–40].…”

“…The preeminent benefits of the photo-CuAAC polymers over conventional BisGMA-based dimethacrylate polymers lie not only in the nature of the step-growth polymerization and the associated delayed gel point conversion, which contributes to reduced shrinkage stress [29], but also on the formation of rigid triazole linkages as a product that significantly enhance the overall mechanical behavior [38–40]. Furthermore, owing to the “click” chemistry of a highly orthogonal CuAAC reaction without forming a byproduct [41,42], the kinetics and mechanics of the CuAAC resin-based crosslinked photopolymers have been explored with varying monomer structures, photoinitiators, copper salts, and/or light exposure conditions [38,39,43–45].…”