Mechanistic Kinetic Modeling of Thiol–Michael Addition Photopolymerizations via Photocaged “Superbase” Generators: An Analytical Approach

Abstract

A kinetic mechanism and the accompanying mathematical framework are presented for base-mediated thiol–Michael photopolymerization kinetics involving a photobase generator. Here, model kinetic predictions demonstrate excellent agreement with a representative experimental system composed of 2-(2-nitrophenyl)propyloxycarbonyl-1,1,3,3-tetramethylguanidine (NPPOC-TMG) as a photobase generator that is used to initiate thiol–vinyl sulfone Michael addition reactions and polymerizations. Modeling equations derived from a basic mechanistic scheme indicate overall polymerization rates that follow a pseudo-first-order kinetic process in the base and coreactant concentrations, controlled by the ratio of the propagation to chain-transfer kinetic parameters (kp/kCT) which is dictated by the rate-limiting step and controls the time necessary to reach gelation. Gelation occurs earlier as the kp/kCT ratio reaches a critical value, wherefrom gel times become nearly independent of kp/kCT. The theoretical approach allowed determining the effect of induction time on the reaction kinetics due to initial acid–base neutralization for the photogenerated base caused by the presence of protic contaminants. Such inhibition kinetics may be challenging for reaction systems that require high curing rates but are relevant for chemical systems that need to remain kinetically dormant until activated although at the ultimate cost of lower polymerization rates. The pure step-growth character of this living polymerization and the exhibited kinetics provide unique potential for extended dark-cure reactions and uniform material properties. The general kinetic model is applicable to photobase initiators where photolysis follows a unimolecular cleavage process releasing a strong base catalyst without cogeneration of intermediate radical species.

Modeling the Strategies for Improved Efficacy and Crosslink Depth of Photo-Initiated Polymerization and Phototherapy

“…As previously reviewed by Lin [26,27] for CXL, the photochemical kinetics has three pathways which are revised for a more general polymer system and briefly summarized as follows. We will limit the kinetic to the simple case of radical-mediated mechanism, although more complex, two-step thiol-Michael mechanism, involving anionic centers reactive intermediates may also occur [19,20].…”

“…They also ignored the contribution from the type-I term, g or k8[A], since type-II is dominant in their anti-cancer process. Most of the previous models [18][19][20][21] have also ignored the dynamic absorption factor, A (z, t) shown by Eq. (1.d), due to the strong depletion of PI concentration [25].…”

“…The kinetics of photoinitiated polymerization systems (PPS) have been studied by many researchers for uniform photoinitiator distribution or for the over simplified cases that the photolysis product becomes completely transparent after polymerization or constant light intensity [14][15][16][17]. Previous models of PPS [18][19][20][21] assumed either a constant light intensity [20] (for thin polymers), or a conventional Beer-Lambert law [18,19,21] for the light intensity. For more realistic systems, the distribution of the photoinitiator is non-uniform and the UV light may still be absorbed by the photolysis product besides the absorption of the monomer.…”

Enhancing UV Photopolymerization by a Red‐light Preirradiation: Kinetics and Modeling Strategies for Reduced Oxygen Inhibition

“…Physical methods include working in an inert or closed environment, increasing the PI concentration, increasing the light dose, or light intensity (for reduced induction time), use of multiple PIs with different rates 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 . To overcome oxygen inhibition, phthalocyanines were explored as an alternative to previously dyes (such as methylene blue, riboflavin, Eosin Y, and rose Bengal) because of their photostability compared to other dyes, with a relatively long triplet‐state lifetime (5–350 μs) and a high quantum yield (0.58–0.65) …”

Modeling the Dynamic Concentration Profiles and Efficacy of Type-I and Type-II Photopolymerization

“…The kinetics of photoinitiated polymerization systems (PPS) have been studied by many researchers for uniform photoinitiator distribution or for the over simplified cases that the photolysis product becomes completely transparent after polymerization or constant light intensity [18][19][20][21][22][23].…”