Near‐Infrared‐Induced Cationic Polymerization Initiated by Using Upconverting Nanoparticles and Titanocene

Abstract

Although most of the technologically applied photochemical processes are based on free radical polymerization, after the discovery of onium salt photochemistry by Crivello, [11] photoinitiated cationic polymerization has gained interest due to their thermal stability, solubility in most of the cationically polymerizable monomers, insensitivity to oxygen and high quantum efficiency to generate reactive species. [3] The existing onium salt-based photoinitiating systems for cationic polymerization include iodonium, [11] sulfonium, [12] alkoxy pyridinium, [13] and phosphonium [14] salts. However, unless additional chromophores are incorporated [15] into the salt structure the absorption of these salts at above 300 nm is rather low. In order to match with the wavelength requirements for certain practical applications, the sensitivity range of onium salts was extended to higher wavelengths by using free radical photoinitiators, [16] charge transfer complexes, [17] and singlet and triplet photosensitizers. [18] It was also shown that nanoparticles [19] and highly conjugated molecules [20] can also sensitize cationic polymerization by photoinduced electron transfer reactions. Among all these strategies, so-called free radical promoted cationic polymerization outlined in Scheme 1 is most widely used methodology as many free radical photointiators acting at broad wavelength range are readily available.In this process, photochemically formed electron donor radicals are oxidized by onium salts. The cations thus generated are used as initiating species for cationic polymerizations. As a consequence of the redox reaction, aryl radicals are also generated that can undergo various subsequent reactions resulting in the chemical amplification of the photons absorbed by the system. Free radical photoinitiators including aromatic carbonyl compounds, [21] acyl phosphine oxides, [22] acyl germanes, [23] organotellurium, [24] manganese carbonyl compounds, [25] and silanes [26] were successfully used to promote cationic polymerizations Despite the fact that the above-mentioned indirect systems can successfully initiate cationic polymerization at the near UV and visible range, [3] it is highly desirable to extend the spectral sensitivity to lower energy irradiation, such as nonharmful near-infrared (NIR) region which also limits side reactions resulting in self-initiation of monomers and degradation of already-formed polymers and oligomers. However, until recently, reports describing NIR sensitized photopolymerizations are exceedingly scarce. Several examples reported in the literature are mostly based on cyanine dyes and polymethines. [27] Besides their use in conventional photopolymerization formulations, polymethines with zwitter ionic structure Photopolymerization A new near-infrared (NIR)-sensitized photoinitiating system for free-radicalpromoted cationic polymerization of oxirane and vinyl monomers such as cyclohexene oxide, and n-butyl vinyl ether (BVE), and N-vinyl carbazole (NVC) is described. A three-component photoinitiating ...

Photoinitiators of polymerization with reduced environmental impact: Nature as an unlimited and renewable source of dyes

“…[185] An enhancement of the TMPTA conversion in the TMPTA/EPOX (43%/57% w/w) blend compared to TMPTA alone was also demonstrated, resulting from the hydrogen-donating ability of EPOX, favoring the generation of additional radicals. [224][225] Ability of QA and QE as photoinitiators for thiol-ene reaction was also examined. If high monomer conversions could be obtained while using QA or QE (0.5 wt%) as a monocomponent photoinitiator, addition of Iod1 could further increase the TMPTA conversion, peaking at 98 and 93% respectively for the two-component QA/Iod and QE/Iod1 (0.5%/2.0%, wt%) systems after 800 s of irradiation at 405 nm (See Table 6).…”

Shining a Light on Bioorthogonal Photochemistry for Polymer Science

“…[42] Since then UCNPs have been used in a range of other polymer material applications, for example photochemical bond formation, [43] and photoinitiation. [44][45][46] While UCNPorganic hybrid materials offer great potential in translating common photochemical reactions into bioorthogonal systems, this review will focus on systems where the photochemical chromophore can be directly activated by long wavelength light.…”

3D printing of multi-scalable structures via high penetration near-infrared photopolymerization

“…Among the photopolymerization methodologies, near-infrared light (NIR) has a more salient role in rapid deep-curing for its remarkable penetration in various media by employing up-conversion strategies 24 – 27 . Recently, we have applied NIR light in free radical as well as cationic polymerization by utilizing up-conversion nanoparticle (UCNP), and successfully photocured resins for over 10 cm with relatively uniformed conversion 28 – 30 , which is promising to improve the curing manner and valuable to additive manufacturing.…”