title: Reduced shrinkage stress via photo-initiated copper(I)-catalyzed cycloaddition polymerizations of azide-alkyne resins.
abstract: 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.
  
  
  
  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.
  
  
  
  A glassy CuAAC polymer (Tg=62°C) exhibited 15-25% lower flexural modulus of 2.5±0.2GPa and flexural strength of 117±8MPa compared to BisGMA-based polymer (Tg=160°C) but showed considerably higher energy absorption around 7.1MJ×m-3without fracture when strained to 11% via three-point bend compared to the flexural toughness of 2.7MJ×m-3obtained 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.
  
  
  
  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.
authors.affiliation: Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder, CO, United States.
authors.forename: Han Byul
authors.initials: HB
authors.lastname: Song
authors.affiliation: Materials Science and Engineering Program, University of Colorado Boulder, 596 UCB, Boulder, CO, United States.
authors.forename: Nancy
authors.initials: N
authors.lastname: Sowan
authors.affiliation: Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder, CO, United States.
authors.forename: Parag K
authors.initials: PK
authors.lastname: Shah
authors.affiliation: Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder, CO, United States.
authors.forename: Austin
authors.initials: A
authors.lastname: Baranek
authors.affiliation: Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder, CO, United States.
authors.forename: Alexander
authors.initials: A
authors.lastname: Flores
authors.affiliation: Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder, CO, United States; Department of Craniofacial Biology, School of Dental Medicine, Anschutz Medical Campus, Aurora, CO, United States.
authors.forename: Jeffrey W
authors.initials: JW
authors.lastname: Stansbury
authors.affiliation: Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder, CO, United States; Materials Science and Engineering Program, University of Colorado Boulder, 596 UCB, Boulder, CO, United States. Electronic address: Christopher.bowman@colorado.edu.
authors.forename: Christopher N
authors.initials: CN
authors.lastname: Bowman
mesh_terms: D000480:Alkynes; D001386:Azides; D003188:Composite Resins; D003300:Copper; D061565:Cycloaddition Reaction; D055119:Elastic Modulus; D008422:Materials Testing; D008689:Methacrylates; D018583:Pliability; D058105:Polymerization; D013314:Stress, Mechanical
chemical_list: D000480:Alkynes; D001386:Azides; D003188:Composite Resins; D008689:Methacrylates; D003300:Copper
keywords: Click chemistry; CuAAC; Dental resins; Flexural strength; Modulus; Photopolymerization; Shrinkage; Step-growth polymerization; Stress