title: Extended mechanistic aspects on photoinitiated polymerization of 1,6-hexanediol diacrylate by hexaarylbisimidazoles and heterocyclic mercapto compounds.
abstract: Chlorine substituted hexaarylbisimidazole (o-Cl-HABI) efficiently initiates radical polymerization of multifunctional acrylic esters in the presence of a heterocyclic mercapto compound if the latter can form its tautomeric thione. Exposure of o-Cl-HABI results in lophyl radicals, which efficiently add to the thione in the first step while the second step releases a highly reactive thiyl radical from this intermediate. LC-MS and CID-MS measurements support this reaction scheme. Furthermore, photo-DSC experiments applying UV light between 320 and 380 nm showed that mercaptotriazole and phenylmercaptotriazole exhibited the best reactivity in the monomer 1,6-hexanediol diacrylate (HDDA) while alkyl substituted mercaptotriazoles showed less reactivity. Change of the triazole heterocycle by mercaptoimidazole resulted in a significant decrease of photoinitiation efficiency. This heterocycle does not form the corresponding thione in HDDA as shown by NMR measurements. Replacement of mercaptotriazole by an alkylthiol leads to a system showing the lowest photoinitiation efficiency in this series. Formation of thione structure in the case of heterocyclic mercapto compounds may cause higher reactivity of the heterocyclic mercapto compounds with the lophyl radical in the monomer chosen.
authors.affiliation: Department of Chemistry and Institute for Coatings and Surface Chemistry, Niederrhein University of Applied Sciences, Adlerstr. 32, D-47798 Krefeld, Germany. bernd.strehmel@hsnr.de veronika.strehmel@hsnr.de.
authors.forename: Stefan
authors.initials: S
authors.lastname: Berdzinski
authors.forename: Nadine
authors.initials: N
authors.lastname: Strehmel
authors.forename: Heike
authors.initials: H
authors.lastname: Lindauer
authors.forename: Veronika
authors.initials: V
authors.lastname: Strehmel
authors.forename: Bernd
authors.initials: B
authors.lastname: Strehmel
mesh_terms: D000179:Acrylates / Q000138:chemical synthesis* / Q000737:chemistry; D055664:Electrochemical Techniques; D006571:Heterocyclic Compounds / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D015394:Molecular Structure; D055668:Photochemical Processes; D010782:Photolysis; D058105:Polymerization; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D014466:Ultraviolet Rays
chemical_list: D000179:Acrylates; D006571:Heterocyclic Compounds; D007093:Imidazoles; D013438:Sulfhydryl Compounds; C020355:1,6-hexanediol diacrylate