Hexaarylbiimidazoles as visible light thiol-ene photoinitiators

Abstract

Objectives: The aim of this study is to determine if hexaarylbiimidazoles (HABIs) are efficient, visible light-active photoinitiators for thiol-ene systems. We hypothesize that, owing to the reactivity of lophyl radicals with thiols and the necessarily high concentration of thiol in thiol-ene formulations, HABIs will effectively initiate thiol-ene polymerization upon visible light irradiation.

Methods: UV-vis absorption spectra of photoinitiator solutions were obtained using UV-vis spectroscopy, while EPR spectroscopy was used to confirm radical species generation upon HABI photolysis. Functional group conversions during photopolymerization were monitored using FTIR spectroscopy, and thermomechanical properties were determined using dynamic mechanical analysis.

Results: The HABI derivatives investigated exhibit less absorptivity than camphorquinone at 469nm; however, they afford increased sensitivity at this wavelength when compared with bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide. Photolysis of the investigated HABIs affords lophyl radicals. Affixing hydroxyhexyl functional groups to the HABI core significantly improved solubility. Thiol-ene resins formulated with HABI photoinitiators polymerized rapidly upon irradiation with 469nm. The glass transition temperatures of the thiol-ene resin formulated with a bis(hydroxyhexyl)-functionalized HABI and photopolymerized at room and body temperature were 49.5±0.5°C and 52.2±0.1°C, respectively.

Significance: Although thiol-enes show promise as continuous phases for composite dental restorative materials, they show poor reactivity with the conventional camphorquinone/tertiary amine photoinitiation system. Conversely, despite their relatively low visible light absorptivity, HABI photoinitiators afford rapid thiol-ene photopolymerization rates. Moreover, minor structural modifications suggest pathways for improved HABI solubility and visible light absorption.

Keywords: Camphorquinone; HABI; Photoinitiator; Thiol–ene.

Fig. 1

(a) The radical-mediated thiol–ene polymerization mechanism proceeds via alternating propagation and chain transfer events, where a thiyl radical initially propagates to a vinyl group, yielding a thioether and carbon-centered radical reaction product. This radical subsequently abstracts a hydrogen from a thiol, regenerating a thiyl radical. (b) Upon irradiation, the inter-imidazole HABI bond undergoes homolytic cleavage, generdsating two relatively stable, long-lived lophyl radicals which then abstract a hydrogen from thiol to produce thiyl radicals.

Fig. 2

Materials used: (a) bisGMA (512.6 g·mol⁻¹), (b) TEGDMA (286.3 g·mol⁻¹), (c) PETMP (488.6 g·mol⁻¹), (d) TATATO (249.3 g·mol⁻¹), (e) CQ (166.2 g·mol⁻¹), (f) EDAB (193.2 g·mol⁻¹), (g) Irgacure 819 (418.5 g·mol⁻¹), (h) o-Cl-HABI (609.6 g·mol⁻¹), and (i) p-HOH-HABI (891.9 g·mol⁻¹).

Fig. 3

UV-vis absorbance spectra and EPR spectra (inset) for (a) o-Cl-HABI and (b) p-HOH-HABI prior to (solid line) and during (dashed line) irradiation with 405 nm at 10 mW·cm⁻².

Fig. 4

Absorbance at λ_max versus time for o-Cl-HABI (solid line) and p-HOH-HABI (dashed line), irradiated with 405 nm at 1 mW·cm⁻² from 0.5 – 10 minutes.

Fig. 5

Conversion versus time for the photopolymerization of bisGMA/TEGDMA irradiated with 469 nm light at intensities of 1 (black, squares), 3 (red, circles), 10 (blue, triangles), and 20 (green, diamonds) mW·cm⁻² and formulated with 1 wt% (a) CQ/EDAB and (b) Irgacure 819. No photopolymerization was observed for bisGMA/TEGDMA formulations containing 1 wt% o-Cl-HABI or p-HOH-HABI under these irradiation conditions.

Fig. 6

Conversion versus time for the photopolymerization of PETMP/TATATO irradiated with 469 nm light at intensities of 1 (black, squares), 3 (red, circles), 10 (blue, triangles), and 20 (green, diamonds) mW·cm⁻² and formulated with 1 wt% (a) CQ/EDAB, (b) Irgacure 819, (c) o-Cl-HABI, and (d) p-HOH-HABI. Vinyl conversions are indicated by solid lines, whereas thiol conversions are indicated by dashed lines.

Fig. 7

Conversion versus time for the photopolymerization of (a) bisGMA/TEGDMA, and (b) PETMP/TATATO, formulated with 1 wt% CQ/EDAB (black, squares), Irgacure 819 (red, circles), o-Cl-HABI (blue, triangles), and p-HOH-HABI (green, diamonds) and irradiated with 469 nm light at 10 mW·cm⁻². Vinyl conversions are indicated by solid lines, whereas thiol conversions are indicated by dashed lines.

Fig. 8

Conversion versus time for the photopolymerization of bisGMA/TEGDMA irradiated at 1 mW·cm⁻² with 469 (black, squares), 405 (red, circles), and 365 (blue, triangles) nm light and formulated with 0.1 wt% (a) CQ/EDAB and (b) Irgacure 819. No photopolymerization was observed for bisGMA/TEGDMA formulations containing 0.1 wt% o-Cl-HABI or p-HOH-HABI under these irradiation conditions.

Fig. 9

Conversion versus time for the photopolymerization of PETMP/TATATO irradiated at 1 mW·cm⁻² with 469 (black, squares), 405 (red, circles), and 365 (blue, triangles) nm light and formulated with 0.1 wt% (a) CQ/EDAB, (b) Irgacure 819, (c) o-Cl-HABI, and (d) p-HOH-HABI. Vinyl conversions are indicated by solid lines, whereas thiol conversions are indicated by dashed lines.

Fig. 9

Conversion versus time for the photopolymerization of PETMP/TATATO irradiated at 1 mW·cm⁻² with 469 (black, squares), 405 (red, circles), and 365 (blue, triangles) nm light and formulated with 0.1 wt% (a) CQ/EDAB, (b) Irgacure 819, (c) o-Cl-HABI, and (d) p-HOH-HABI. Vinyl conversions are indicated by solid lines, whereas thiol conversions are indicated by dashed lines.

Fig. 10

Storage modulus (E′, solid lines) and tan δ (dashed lines) versus temperature for photopolymerized films, irradiated for 20 minutes at 10 mW·cm⁻² with 469 nm and at 23°C (black, squares) or 37°C (red, circles), of (a) bisGMA/TEGDMA formulated with 1 wt% CQ/0.5 wt% EDAB ((b) second temperature ramp), and (c) PETMP/TATATO formulated with 1 wt% p-HOH-HABI ((d) second temperature ramp).

Fig. 11

Conversion versus time, scaled assuming that (a) R_p ~ R_i^0.50, and (b) R_p ~ R_i^0.84, for the photopolymerization of PETMP/TATATO formulated with 1 wt% p-HOH-HABI and irradiated with 469 nm light at intensities of 1 (black, squares), 3 (red, circles), 10 (blue, triangles), and 20 (green, diamonds) mW·cm⁻². Vinyl conversions are indicated by solid lines, whereas thiol conversions are indicated by dashed lines.