Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerization

TLDR: Type II triplet quenching of aryl thiols was found to be dependent on the sensitizer, viscosity, and hydrogen-donating capability of the monomer and on the presence of oxygen.

Abstract: hydrogen from thiol, and as a result reinitiation occurs by a thiyl radical. While the thiol−ene reaction in Scheme 14 implies the formation of linear polymer from difunctional enes and difunctional thiols, the situation is in reality much more complicated with reactive vinyls such as acrylates. In such cases, both thiol−ene step growth and acrylate chain growth polymerization occur simultaneously, resulting in a difficultto-predict molecular architecture. Cramer and Bowman photocured resins with stoichiometric amounts of acrylate and thiol moieties and found acrylate conversion to be roughly twice that of thiol. This means that the kinetic rate constant of acrylate propagation is approximately 1.5 times that of thiol hydrogen abstraction. In the presence of air, reaction with thiol becomes preferential. Addition of multifunctional alkylthiols to diacrylates markedly improves final DBC. Degree of functionality was shown to have an effect both on cure time and on the modulus of the formed film. Higher functionality thiols are preferred since they increase reaction viscosity more quickly and hence slow diffusion of atmospheric oxygen. The gel point in thiol−ene polymerization, in both air and nitrogen, is greatly delayed relative to neat acrylate polymerization. While thiols tend to retard the rate of polymerization with both acrylates and methacrylates, the rate is increased in the case of vinyl esters and vinyl carbonates. In spite of the promise that thiols offer, odor (volatile thiols born from ester hydrolysis) and storage stability are two major obstacles to their widespread use. This second problem arises from the thermally induced addition to acrylates. Stabilization of thiol−ene formulations is addressed in multiple patents. In addition to small-molecule alkylthiols such as hexanedithiol (HDT) or pentaerythritol tetrakis-3-mercaptopropionate (TT), nonvolatile polymeric thiols such as homoand copolymers of mercaptopropylmethylsiloxane (MMSiO) have been investigated (Figure 7). Aromatic thiols have also been studied in photopolymerization due to their excellent hydrogen-donation capacity and better storage stability (Figure 7). Aromatic thiols have been used with type I photoinitiators and in place of amines with type II sensitizers. In the first case, addition of 0.1 wt % mercaptobenzoxazole (MBO) enhanced cure rate and mechanical properties of the film, while higher concentrations of thiol reduced cure rate. Since polymerizations were performed between KCl plates, the enhanced rate is explained by hydrogen transfer to early-stage peroxyl radicals formed with predissolved oxygen, while further hydrogen transfer retards propagating alkyl radicals. Type II triplet quenching of aryl thiols was found to be dependent on the sensitizer, viscosity, and hydrogen-donating capability of the monomer and on the presence of oxygen. With Scheme 13. Solvolysis of Acylphosphine Oxide in the Presence of an Amine Scheme 14. Main Reactions in Thiol−ene Copolymerization Figure 7. Commonly used thiols for reduced oxygen inhibition. Chemical Reviews Review dx.doi.org/10.1021/cr3005197 | Chem. Rev. XXXX, XXX, XXX−XXX O CQ as sensitizer, MBO and analogous mercaptobenzamidazole (MBI) allowed methacrylate polymerization rates in air comparable to those obtained with the commonly used amine DMAB. The polymerization rate achieved with mercaptobenzothiazole (MBT) was lower but could be increased by introduction of an electron-withdrawing nitro moiety to the aromatic ring. The thiazol dimer DBT increased polymerization rate in argon but not in air. MBO, MBI, and MBT were also tested in combination with ITX, where they were found to be effective in argon but inferior to DMAB in air. Poor reactivity with ITX is explained by the π, π* character of its lowest-lying triplet state, which purportedly abstracts hydrogen atoms less efficiently than the n, π* triplet of CQ. 3.4.3. Silanes. Organosilanes can play a number of different roles in radical polymerizations. On one hand, polysilanes with photocleavable Si−Si bonds can be utilized as photoinitiators, and on the other hand, silanes with Si−H bonds may be used as secondary additives to serve as hydrogen donors. In either case, a silane radical is formed that has very high reactivity toward molecular oxygen (kox ≈ 3 × 10 L·mol−1·s−1) to give a silylperoxyl radical (SiOO•). This silylperoxyl radical may rearrange to provide a new silyl radical that may then react with monomer or additional oxygen (Scheme 15). Polysilanes undergo efficient photocleavage to provide silane radicals and are reported to be less sensitive to oxygen inhibition. However, the efficiency of these macroradicals to initiate acrylate polymerization was found to be generally low. Small-molecule disilanes with pendant aromatic moieties to red-shift absorbance were synthesized by Laleveé et al. and found to have much better reactivity toward MMA. As expected, steric effects at the silane play an important role in initiation kinetics. Silanes with Si−H bonds may be utilized in type II systems and can even provide better results than amines, particularly when cured in air. Polysilanes with latent Si−H bonds along the backbone have also been investigated. These compounds can function in either of the above-mentioned roles and were shown to work very well as replacements for amines in visible light curing systems with CQ. A possible disadvantage of silanes is their sensitivity toward hydrolysis. 3.4.4. Other Hydrogen Donors. In addition to amines, thiols, and silanes, a number of hydrogen donors based on other elements have been tested as type II co-initiators and as peroxyl radical scavengers. While not the only parameter, bond dissociation energy (BDE) of the donor−H bond is a very useful metric for gauging the ability of a reagent to function as a hydrogen donor. Triphenyl derivatives of group 14 elements silicon, germanium and tin were tested for their ability to quench the excited state of benzophenone and for hydrogen donation to peroxyl radicals (Table 3). The quenching rate is highest for stannane, which is to be expected from the dissociation energy. Similar to thiols, hydrogen donors based on other chalcogens (selenium and tellurium) have been investigated in radical photopolymerization. Arylselenols are highly reactive hydrogen donors but tend to be avoided as they are also reactive vesicants. Organic derivatives of tellurium may be photodecomposed to provide initiating radicals; however, oxygen stability is limited. Thiols and selenols can both oxidize to provide dimers with photolabile S−S or Se−Se bonds. Although such compounds can be used as initiators, the ability to counter oxygen inhibition is compromised. While low-oxidation-state compounds such as phosphane tend not be appropriately stable, phosphites with a P−H bond have also been studied for oxygen inhibition. Schmitt et al. used didecylphenyl phosphite to reduce the photopolymerization time of methacrylates in open air. This phosphite may react directly with oxygen to form a phosphate or alternatively donate hydrogen to an alkylperoxyl radical. The phosphite radical formed from this latter reaction can be oxidized by a second peroxyl radical to provide a stable phosphate radical and an alkoxyl radical, which may reinitiate polymerization (Scheme 16). A variety of borane and metal hydride hydrogen donors that undergo similar reactions will be discussed in the next section. 3.5. Other Reducing Agents While hydrogen donors may also be classified as reducing agents, the term is used here to refer specifically to additives that react with either alkylperoxyl radicals or molecular oxygen by a reduction that does not involve hydrogen. The two most important classes of additives that serve this role are organic molecules based on either boron or phosphorus, where the oxidation state of boron or phosphorus is raised during the course of the reaction. A number of metallic-based additives are suggested for use as peroxyl radical reducing agents. One example is provided by Courtecuisse et al., demonstrating the use of zirconium complex Zr(TEA)4 to combat oxygen inhibition (Figure 8). Purportedly zirconium is oxidized by peroxyl radicals releasing an amine ligand as an initiating radical. Hydrolytic stability of the zirconium complex may, however, be a concern. 3.5.1. Boranes. Organoboranes are an interesting class of radical initiators that have also found some application in photocuring. An interesting feature of borane radical initiators is that most actually require oxygen as a co-initiator. While this may at first sound like a simple solution to avoid oxygen inhibition, the problem in this case is that formulations are not stable and react prematurely. The mechanisms given by Bhanu and Kishore for oxidation of alkylborane gives a number of radicals that can potentially initiate polymerization (Scheme 17). Although this scheme presents the reaction with a trialkylborane, the same radical interchange reaction has been observed with boronic esters, where Ingold measured a rate constant of 4.8 × 10 L·mol−1·s−1. Scheme 15. Reaction of Silyl Radical with Oxygen Table 3. Bond Dissociation Energy of Group 14 Triaryl Hydrides and Rate Constant for Hydrogen Donation to Benzophenone and tert-Butylperoxyl Radical hydrogen donor BDE (kcal·mol−1) kH→BP × 10 7 (L·mol−1·s−1) kH→tBuOO (L·mol−1·s−1)