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.

Hexaarylbiimidazoles as visible light thiol–ene photoinitiators

“…1b) that are unreactive with oxygen and show slow recombination rates [38], attributable to steric hindrance and electron delocalization [39]; indeed, HABI photoinitiators show no initiation activity in (meth)acrylate formulations without the presence of a hydrogen-donating coinitiator [40]. Thiols are commonly used as coinitiators in conjunction with HABIs [40], where hydrogen abstraction by the HABI-derived lophyl radicals yields initiating thiyl radicals (Fig. 1b), suggesting a particular suitability for HABI photoinitiators in thiol–ene formulations.…”

Application of ionic liquids in synthesis of polymeric binders for coatings

“…Photo-DSC investigations were carried out using a differential scanning calorimeter DSC TA Q300 (TA Instruments) equipped with a UV-vis light source and a broadbandfilter of 320-380 nm (Omnicure) [27,29]. Samples were prepared from 5 and 7 by dissolution of o-chlorohexaarylbisimidazole (1.77 × 10 −3 mol% with respect to the monomer) and 3-mercapto-1,2,4H-triazole (6.69 × 10 −3 mol% with respect to the monomer) in the ionic liquid monomer during stirring at 50 • C for 2-5 h. An amount of 5 mg sample was investigated in an aluminum pan without a lid.…”

“…This time includes 5 min data recording during irradiation using a wavelength region between 320 and 380 nm followed by 15 min dark reaction. The maximum polymerization rates (R max p ) were determined as described elsewhere [27,29]. .…”

“…Furthermore, rate of polymerization of ionic liquid monomers differs from traditional monomers as well. Photo-DSC investigation results in the maximum of the polymerization rate (R max p ), which gives information regarding the reactivity of the photopolymer composition [27,29]. The R max p values depicted in Fig.…”

“…1). The reason ) during photoinitiated polymerization of ionic liquid monomers 5 and 7 compared with hexamethylene dimethacrylate (HDDMA) at room temperature using a photoinitiator system composed of o-chlorohexaarylbisimidazole and 3-mercapto-1,2,4-4H-triazole [29,27]. may be the same substituent bound at the cation of the monomers 5 and 7 that is a butyl group in case of both monomers.…”