Thiol–Ene Photopolymerization: Scaling Law and Analytical Formulas for Conversion Based on Kinetic Rate and Thiol–Ene Molar Ratio

Abstract

Kinetics and analytical formulas for radical-mediated thiol–ene photopolymerization were developed in this paper. The conversion efficacy of thiol–ene systems was studied for various propagation to chain transfer kinetic rate-ratio (RK), and thiol–ene concentration molar-ratio (RC). Numerical data were analyzed using analytical formulas and compared with the experimental data. We demonstrated that our model for a thiol–acrylate system with homopolymerization effects, and for a thiol–norbornene system with viscosity effects, fit much better with the measured data than a previous model excluding these effects. The general features for the roles of RK and RC on the conversion efficacy of thiol (CT) and ene (CV) are: (i) for RK = 1, CV and CT have the same temporal profiles, but have a reversed dependence on RC; (ii) for RK >> 1, CT are almost independent of RC; (iii) for RK << 1, CV and CT have the same profiles and both are decreasing functions of the homopolymerization effects defined by kCV; (iv) viscosity does not affect the efficacy in the case of RK >> 1, but reduces the efficacy of CV for other values of RK. For a fixed light dose, higher light intensity has a higher transient efficacy but a lower steady-state conversion, resulting from a bimolecular termination. In contrast, in type II unimolecular termination, the conversion is mainly governed by the light dose rather than its intensity. For optically thick polymers, the light intensity increases with time due to photoinitiator depletion, and thus the assumption of constant photoinitiator concentration (as in most previous models) suffers an error of 5% to 20% (underestimated) of the crosslink depth and the efficacy. Scaling law for the overall reaction order, defined by [A]m[B]n and governed by the types of ene and the rate ratio is discussed herein. The dual ratio (RK and RC) for various binary functional groups (thiol–vinyl, thiol–acrylate, and thiol–norbornene) may be tailored to minimize side effects for maximal monomer conversion or tunable degree of crosslinking.

Modeling the Kinetics, Curing Depth, and Efficacy of Radical-Mediated Photopolymerization: The Role of Oxygen Inhibition, Viscosity, and Dynamic Light Intensity

“…There are many conventional strategies to reduce oxygen inhibition in photoinduced polymerization: working in an inert or closed environment, increasing the photoinitiator concentration, increasing the light dose or light intensity (for reduced induction time), use of multiple photoinitiators with different rate of initiation, or addition of oxygen scavengers. Chemical mechanisms incorporate additives or suitably functionalized monomers which are insensitive to oxygen, such as the thiol-ene and thiol-acrylate-Michael additive systems (Claudino et al, 2016; Chen et al, 2019a,b). Additive enhancer monomer was proposed to improve the curing (cross-link) efficacy by either reducing the oxygen inhibition effect by stable monomer or increase the lifetime of radicals in clinical applications (Chen et al, 2019b; Wertheimer et al, 2019).…”

Recent advances on carbazole-based photoinitiators of polymerization

“…Classically, thiol-ene photopolymerizations is carried out with UV light [105][106][107][108]. In the present case, the polymerization profiles ob tained for the copolymerization of trithiol/DVE-3 upon irradiation with a laser diode at 457 nm (100 mW/cm 2 ) or a LED bulb at 462 nm (10 mW/cm 2 ) were similar, even though the light intensity of the LED was 10 times lower than that of the laser diode (See Fig.…”

Dual-Wavelength (UV and Blue) Controlled Photopolymerization Confinement for 3D-Printing: Modeling and Analysis of Measurements

“…Although the mechanism for radical-mediated polymerization initiation and inhibition in dualwavelength system and the simple formulas for the associated printing speed and inhibition volume thickness [11] were reported, there is no detailed kinetics or the conversion efficacy have been theoretically reported in a dual-wavelength, thick polymer system. We have previously reported the kinetics and modeling of a single-wavelength radical-mediated photopolymerization in singleinitiator [12][13][14], two-initiator [15] and two-component system [16,17]. This study will extend our previous modeling to a 2-wavelength, 3-initiator system.…”

“…(13), and using the light intensities given by Eq. (17) are shown as follows. We will first show the conversion for the case of blue-light only, i.e., when B2=0 (no UV light) for various concentration of the initiator, C10= (0, 0.5, 1.0, 3.0) %, and coupling parameter b1 which is given by the absorption coefficient and blue-light intensity; and also the role of the crosslink rate constant (k') which gives the conversion in Eq.…”