Wavelength-Selective Sequential Polymer Network Formation Controlled with a Two-Color Responsive Initiation System

Fluorescence-readout as a powerful macromolecular characterisation tool

The last few decades have witnessed significant progress in synthetic macromolecular chemistry, which can provide access to diverse macromolecules with varying structural complexities, topology and functionalities, bringing us closer to the aim of controlling soft matter material properties with molecular precision. To reach this goal, the development of advanced analytical techniques, allowing for micro-, molecular level and real-time investigation, is essential. Due to their appealing features, including high sensitivity, large contrast, fast and real-time response, as well as non-invasive characteristics, fluorescence-based techniques have emerged as a powerful tool for macromolecular characterisation to provide detailed information and give new and deep insights beyond those offered by commonly applied analytical methods. Herein, we critically examine how fluorescence phenomena, principles and techniques can be effectively exploited to characterise macromolecules and soft matter materials and to further unravel their constitution, by highlighting representative examples of recent advances across major areas of polymer and materials science, ranging from polymer molecular weight and conversion, architecture, conformation to polymer self-assembly to surfaces, gels and 3D printing. Finally, we discuss the opportunities for fluorescence-readout to further advance the development of macromolecules, leading to the design of polymers and soft matter materials with pre-determined and adaptable properties.

Cooperative Network Formation via Two-Colour Light-Activated λ-Orthogonal Chromophores

Independently addressing photoreactive sites within one molecule with two colours of light is a formidable challenge. Here, we combine two sequence independent λ-orthogonal chromophores in one heterotelechelic dilinker molecule, to exploit their disparate reactivity utilizing the same reaction partner, a maleimide-containing polymer. We demonstrate that polymer network formation only proceeds if two colours of light are employed. Upon single colour irradiation, linker-decorated post-functionalized polymers are generated at either wavelength and in either sequence. Network formation, however, is only achieved by sequential or simultaneous two colour irradiation. The herein introduced photoreactive system demonstrates the power of wavelength orthogonal chemistry in macromolecular synthesis.

Digital Light Processing 3D Printing of Soft Semicrystalline Acrylates with Localized Shape Memory and Stiffness Control

Multimaterial three-dimensional (3D) printing of objects with spatially tunable thermomechanical properties and shape-memory behavior provides an attractive approach toward programmable "smart" plastics with applications in soft robotics and electronics. To date, digital light processing 3D printing has emerged as one of the fastest manufacturing methods that maintains high precision and resolution. Despite the common utility of semicrystalline polymers in stimuli-responsive materials, few reports exist whereby such polymers have been produced via digital light processing (DLP) 3D printing. Herein, two commodity long-alkyl chain acrylates (C₁₈, stearyl and C₁₂, lauryl) and mixtures therefrom are systematically examined as neat resin components for DLP 3D printing of semicrystalline polymer networks. Tailoring the stearyl/lauryl acrylate ratio results in a wide breadth of thermomechanical properties, including tensile stiffness spanning three orders of magnitude and temperatures from below room temperature (2 °C) to above body temperature (50 °C). This breadth is attributed primarily to changes in the degree of crystallinity. Favorably, the relationship between resin composition and the degree of crystallinity is quadratic, making the thermomechanical properties reproducible and easily programmable. Furthermore, the shape-memory behavior of 3D-printed objects upon thermal cycling is characterized, showing good fatigue resistance and work output. Finally, multimaterial 3D-printed structures with vertical gradation in composition are demonstrated where concomitant localization of thermomechanical properties enables multistage shape-memory and strain-selective behavior. The present platform represents a promising route toward customizable actuators for biomedical applications.

Wavelength Selective Multi-Material 3D Printing of Soft Active Devices Using Orthogonal Photoreactions

Orthogonal photoreactions provide a unique way to locally and independently control (thermo)mechanical properties and functionality of polymer networks simply by choice of the wavelength. Herein, a library of acrylate functional coumarin monomers is synthesized, which are cured by sequence-dependent wavelength orthogonality. In the presence of a long wavelength absorbing photoinitiator, the monomers undergo rapid curing by visible light induced radical chain growth polymerization. Subsequent irradiation with light in the UV-A region selectively initiates the [2+2] photocycloaddition of the coumarin chromophores, which is confirmed by FTIR and UV-vis experiments. Through a well-targeted design, acrylate-based and thiol-acrylate resin formulations are prepared, whose fast curing rate, low viscosity, and prolonged storage stability enable the one-step fabrication of multi-material structures by digital light processing (DLP) 3D printing. By using a dual-wavelength printer, which operates at two different wavelengths (405 and 365 nm), objects comprising soft (ε = 22%, σ = 7.5 MPa) and stiff (ε = 2%, σ = 8.3 MPa) domains are printed with a single resin vat. Along with tensile properties, the wavelength selective change in the network structure features a local control of the glass transition temperature (ΔT_g  = 17 °C) in the 3D-printed objects. Soft active devices are fabricated by dual-wavelength DLP 3D printing, with distinct domains having a higher T_g and the local programming of multi shapes is demonstrated.

A Review of Multi-Material 3D Printing of Functional Materials via Vat Photopolymerization

Additive manufacturing or 3D printing of materials is a prominent process technology which involves the fabrication of materials layer-by-layer or point-by-point in a subsequent manner. With recent advancements in additive manufacturing, the technology has excited a great potential for extension of simple designs to complex multi-material geometries. Vat photopolymerization is a subdivision of additive manufacturing which possesses many attractive features, including excellent printing resolution, high dimensional accuracy, low-cost manufacturing, and the ability to spatially control the material properties. However, the technology is currently limited by design strategies, material chemistries, and equipment limitations. This review aims to provide readers with a comprehensive comparison of different additive manufacturing technologies along with detailed knowledge on advances in multi-material vat photopolymerization technologies. Furthermore, we describe popular material chemistries both from the past and more recently, along with future prospects to address the material-related limitations of vat photopolymerization. Examples of the impressive multi-material capabilities inspired by nature which are applicable today in multiple areas of life are briefly presented in the applications section. Finally, we describe our point of view on the future prospects of 3D printed multi-material structures as well as on the way forward towards promising further advancements in vat photopolymerization.

Continuous Additive Manufacturing using Olefin Metathesis

The development of chemistry is reported to implement selective dual-wavelength olefin metathesis polymerization for continuous additive manufacturing (AM). A resin formulation based on dicyclopentadiene is produced using a latent olefin metathesis catalyst, various photosensitizers (PSs) and photobase generators (PBGs) to achieve efficient initiation at one wavelength (e.g., blue light) and fast catalyst decomposition and polymerization deactivation at a second (e.g., UV-light). This process enables 2D stereolithographic (SLA) printing, either using photomasks or patterned, collimated light. Importantly, the same process is readily adapted for 3D continuous AM, with printing rates of 36 mm h^-1 for patterned light and up to 180 mm h^-1 using un-patterned, high intensity light.

Photostationary State in Dynamic Covalent Networks

We explore a cross-linked polymer network based on a visible light photodynamic [2 + 2] cycloaddition driven by styrylpyrene chemistry. Based on a polymer backbone with pendent styrylpyrene units, the network can be formed by using λ = 450 nm irradiation. Upon irradiation with λ = 340 nm, a photostationary state is generated within the network with ∼17% of the styrylpyrene units open compared to close to 2% in the visible light cured state. The limited fraction of open [2 + 2] couples is caused by their proximity and is in sharp contrast to solution experiments on the photoreactive moiety. Thus, the polymer network retains its mechanical properties even at the photostationary point. We hypothesize that the application of an additional stimulus could serve as a second gate for inducing network disintegration by spacing the [2 + 2] units during ultraviolet irradiation.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

Sequence-independent activation of photocycloadditions using two colours of light

We exploit two reactive chromophores to establish sequence-independent photochemical activation, employing ortho-methyl benzaldehyde (oMBA) and N,N-(dimethylamino)pyrene aryl tetrazole (APAT) with N-(2-hydroxy)ethyl maleimide (NHEM), without any additives. Critically, the order of the irradiation sequence is irrelevant, as the shorter wavelength does not activate the higher wavelength activated species. Therefore, full sequence-independent λ-orthogonality is achieved through differences in both the reaction quantum yields (Φ r,oMBA and Φ r,APAT) and wavelength-dependent reactivity profiles of the employed chromophores.

Determining Michael Acceptor Reactivity from Kinetic, Mechanistic, and Computational Analysis for the Base-catalyzed Thiol-Michael Reaction

A combined experimental and computational study of the reactivities of seven commonly used Michael acceptors paired with two thiols within the framework of photobase-catalyzed thiol-Michael reactions is reported. The rate coefficients of the propagation (kP), reverse propagation (k-P), chain-transfer (kCT), and overall reaction (koverall) were experimentally determined and compared with the well-accepted electrophilicity parameters of Mayr and Parr, and DFT-calculated energetics. Both Mayr's and Parr's electrophilicity parameters predict the reactivities of these structurally varying vinyl functional groups well, covering a range of overall reaction rate coefficients from 0.5 to 6.2 s-1. To gain insight into the individual steps, the relative energies have been calculated using DFT for each of the stationary points along this step-growth reaction between ethanethiol and the seven alkenes. The free energies of the individual steps reveal the underlying factors that control the reaction barriers for propagation and chain transfer. Both the propagation and chain transfer steps are under kinetic control. These results serve as a useful guide for Michael acceptor selection to design and predict thiol-Michael-based materials with appropriate kinetic and material properties.

Wavelength-Selective Photopolymerization of Hybrid Acrylate-Oxetane Liquid Crystals

We report on the wavelength-selective photopolymerization of a hybrid acrylate-oxetane cholesteric liquid crystal monomer mixture. By controlling the sequence and rate of the orthogonal free-radical and cationic photopolymerization reactions, it is possible to control the degree of phase separation in the resulting liquid crystal interpenetrating networks. We show that this can be used to tune the reflective color of the structurally colored coatings produced. Conversely, the structural color can be used to monitor the degree of phase separation. Our new photopolymerization procedure allows for structuring liquid crystal networks in three dimensions, which has great potential for fabricating liquid crystal polymer materials with programmable functional properties.

Expanding the Thiol-X Toolbox: Photoinitiation and Materials Application of the Acid-Catalyzed Thiol-ene (ACT) Reaction

The acid-catalyzed thiol-ene reaction (ACT) is a unique thiol-X conjugation strategy that produces S,X-acetal conjugates. Unlike the well-known radical-mediated thiol-ene and anion-mediated thiol-Michael reactions that produce static thioether bonds, acetals provide unique function for various fields such as drug delivery and protecting group chemistries; however, this reaction is relatively underutilized for creating new and unique materials owing to the unexplored reactivity over a broad set of substrates and potential side reactions. Solution-phase studies using a range of thiol and alkene substrates were conducted to evaluate the ACT reaction as a conjugation strategy. Substrates that efficiently undergo cationic polymerizations, such as those containing vinyl functional groups, were found to be highly reactive to thiols in the presence of catalytic amounts of acid. Additionally, sequential initiation of three separate thiol-X reactions (thiol-Michael, ACT, and thiol-ene) was achieved in a one-pot scheme simply by the addition of the appropriate catalyst demonstrating substrate selectivity. Furthermore, photoinitiation of the ACT reaction was achieved for the first time under 470 nm blue light using a novel photochromic photoacid. Finally, using multifunctional monomers, solid-state polymer networks were formed using the ACT reaction producing acetal crosslinks. The presence of S,X-acetal bonds results in an increased glass transition temperature of 20 °C as compared with the same polymeric film polymerized through the radical thiol-ene mechanism. This investigation demonstrates the broad impact of the ACT reaction and expands upon the diverse thiol-X library of conjugation strategies towards the development of novel materials systems.

Phosphonium Tetraphenylborate: A Photocatalyst for Visible-Light-Induced, Nucleophile-Initiated Thiol-Michael Addition Photopolymerization

A photoinitiation system that utilizes phosphonium tetraphenylborate as the key component was developed for the visible light-triggered nucleophile-catalyzed thiol-Michael addition reaction. This highly reactive catalyst was composed of a photocaged phosphine (methyldiphenylphosphonium tetraphenylborate, MDPP·HBPh₄), a photosensitizer (isopropylthioxanthone, ITX), and a radical scavenger (TEMPO). Unlike the prevailing photobase catalysts, this photoactivatable phosphine system triggers the thiol-Michael addition polymerization by a nucleophile-catalyzed mechanism and provides a controlled stoichiometric reaction between the thiol and the vinyl precursors. This approach enables the formation of homogeneous polymer networks upon low-energy visible light exposure and, thus, broadens its potential applications in bulk polymer materials synthesis and UV-sensitive bioscaffold formation.

Combining Photodeprotection and Ligation into a Dual-Color Gated Reaction System

We report a photochemical reaction system which requires activation by two colors of light. Specifically, a dual wavelength gated system is established by fusing the visible light mediated deprotection of a dithioacetal with the UV light activated Diels–Alder reaction of an o‐methylbenzaldehyde with n‐ethylmaleimide. Critically, both light sources are required to achieve the Diels–Alder adduct, irradiation with visible or UV light alone does not lead to the target product. The introduced dual gated photochemical system is particularly interesting for application in light driven 3D printing, where two color wavelength activated photoresists may become reality.

Rapid High-Resolution Visible Light 3D Printing

Light-driven 3D printing to convert liquid resins into solid objects (i.e., photocuring) has traditionally been dominated by engineering disciplines, yielding the fastest build speeds and highest resolution of any additive manufacturing process. However, the reliance on high-energy UV/violet light limits the materials scope due to degradation and attenuation (e.g., absorption and/or scattering). Chemical innovation to shift the spectrum into more mild and tunable visible wavelengths promises to improve compatibility and expand the repertoire of accessible objects, including those containing biological compounds, nanocomposites, and multimaterial structures. Photochemistry at these longer wavelengths currently suffers from slow reaction times precluding its utility. Herein, novel panchromatic photopolymer resins were developed and applied for the first time to realize rapid high-resolution visible light 3D printing. The combination of electron-deficient and electron-rich coinitiators was critical to overcoming the speed-limited photocuring with visible light. Furthermore, azo-dyes were identified as vital resin components to confine curing to irradiation zones, improving spatial resolution. A unique screening method was used to streamline optimization (e.g., exposure time and azo-dye loading) and correlate resin composition to resolution, cure rate, and mechanical performance. Ultimately, a versatile and general visible-light-based printing method was shown to afford (1) stiff and soft objects with feature sizes <100 μm, (2) build speeds up to 45 mm/h, and (3) mechanical isotropy, rivaling modern UV-based 3D printing technology and providing a foundation from which bio- and composite-printing can emerge.

Photocontrolled Synthesis of n-Type Conjugated Polymers

Current approaches to synthesize π-conjugated polymers (CPs) are dominated by thermally driven, transition-metal-mediated reactions. Herein we show that electron-deficient Grignard monomers readily polymerize under visible-light irradiation at room temperature in the absence of a catalyst. The product distribution can be tuned by the wavelength of irradiation based on the absorption of the polymer. Conversion studies are consistent with an uncontrolled chain-growth process; correspondingly, chain extension produces all-conjugated n-type block copolymers. Preliminary results demonstrate that the polymerization can be expanded to donor-acceptor alternating copolymers. We anticipate that this method can serve as a platform to access new architectures of n-type CPs without the need for transition-metal catalysis.

Light-Controlled Orthogonal Covalent Bond Formation at Two Different Wavelengths

We report light-induced reactions in a two-chromophore system capable of sequence-independent λ-orthogonal reactivity relying solely on the choice of wavelength and solvent. In a solution of water and acetonitrile, LED irradiation at λ_max =285 nm leads to full conversion of 2,5-diphenyltetrazoles with N-ethylmaleimide to the pyrazoline ligation products. Simultaneously present o-methylbenzaldehyde thioethers are retained. Conversely, LED irradiation at λ_max =382 nm is used to induce ligation of the o-methylbenzaldehydes in acetonitrile with N-ethylmaleimide via o-quinodimethanes, while 2,5-diphenyltetrazoles also present are retained. This unprecedented photochemical selectivity is achieved through control of the number and wavelength of incident photons as well as favorable optical properties and quantum yields of the reactants in their environment.

Seeing the Light: Advancing Materials Chemistry through Photopolymerization

The application of photochemistry to polymer and material science has led to the development of complex yet efficient systems for polymerization, polymer post-functionalization, and advanced materials production. Using light to activate chemical reaction pathways in these systems not only leads to exquisite control over reaction dynamics, but also allows complex synthetic protocols to be easily achieved. Compared to polymerization systems mediated by thermal, chemical, or electrochemical means, photoinduced polymerization systems can potentially offer more versatile methods for macromolecular synthesis. We highlight the utility of light as an energy source for mediating photopolymerization, and present some promising examples of systems which are advancing materials production through their exploitation of photochemistry.

Access to Disparate Soft Matter Materials by Curing with Two Colors of Light

A platform technology for multimaterial photoresists that can be orthogonally cured by disparate colors of light is introduced. The resist's photochemistry is designed such that one wavelength selectively activates the crosslinking of one set of macromolecules, while a different wavelength initiates network formation of a different set of chains. Each wavelength is thus highly selective towards a specific photoligation reaction within the resist. Critically, the shorter wavelength does not induce ligation of the longer wavelength selective species within the same resist mixture, defined as "wavelength orthogonality." Uniquely, this dual-color addressable resist system allows generating spatially resolved soft matter materials by simply selecting the curing wavelength, thus constituting a wavelength-orthogonal multimaterial resist with applications ranging from coatings to 3D additive manufacturing of multimaterial architectures.

Modeling the Optimal Conditions for Improved Efficacy and Crosslink Depth of Photo-Initiated Polymerization

Optimal conditions for maximum efficacy of photoinitiated polymerization are theoretically presented. Analytic formulas are shown for the crosslink time, crosslink depth, and efficacy function. The roles of photoinitiator (PI) concentration, diffusion depth, and light intensity on the polymerization spatial and temporal profiles are presented for both uniform and non-uniform cases. For the type I mechanism, higher intensity may accelerate the polymer action process, but it suffers a lower steady-state efficacy. This may be overcome by a controlled re-supply of PI concentration during the light exposure. In challenging the conventional Beer⁻Lambert law (BLL), a generalized, time-dependent BLL (a Lin-law) is derived. This study, for the first time, presents analytic formulas for curing depth and crosslink time without the assumption of thin-film or spatial average. Various optimal conditions are developed for maximum efficacy based on a numerically-fit A-factor. Experimental data are analyzed for the role of PI concentration and light intensity on the gelation (crosslink) time and efficacy.

A substituentpara-to-orthopositioning effect drives the photoreactivity of a dibenzothiophene-based oxalate series used as LED-excitable free radical photoinitiators

Dibenzothiophene-based oxalate derivatives were synthesized as type I photoinitiators, and their photoinitiation properties depend on the substituentpara-to-orthopositioning effect of the oxalates.

Exploiting Wavelength Orthogonality for Successive Photoinduced Polymerization-Induced Self-Assembly and Photo-Crosslinking

We report a facile benchtop process for the synthesis of cross-linked polymeric nanoparticles by exploiting wavelength-selective photochemistry to perform orthogonal photoinduced polymerization-induced self-assembly (Photo-PISA) and photo-crosslinking processes. We first established that the water-soluble photocatalyst, zinc meso-tetra(N-methyl-4-pyridyl) porphine tetrachloride (ZnTMPyP) could activate the aqueous PET-RAFT dispersion polymerization of hydroxypropyl methacrylate (HPMA). This photo-PISA process could be conducted under low energy red light (λ_max = 595 nm, 10.2 mW/cm²) and without deoxygenation due to the action of the singlet oxygen quencher, biotin (vitamin B₇), which allowed for the synthesis of a range of nanoparticle morphologies (spheres, worms, and vesicles) directly in 96-well plates. To perform wavelength selective nanoparticle cross-linking, we added the photoresponsive monomer, 7-[4-(trifluoromethyl)coumarin] methacrylamide (TCMAm) as a comonomer without inhibiting the evolution of the nanoparticle morphology. Importantly, under red light, exclusive activation of the photo-PISA process occurs, with no evidence of TCMAm dimerization under these conditions. Subsequent switching to a UV source (λ_max = 365 nm, 10.2 mW/cm²) resulted in rapid cross-linking of the polymer chains, allowing for retention of the nanoparticle morphology in organic solvents. This facile synthesis of cross-linked spheres, worms, and vesicles demonstrates the utility of orthogonal light-mediated chemistry for performing decoupled wavelength selective chemical processes.

Implementation of two distinct wavelengths to induce multistage polymerization in shape memory materials and nanoimprint lithography

Here, a process is introduced for forming dual stage thiol-Michael/acrylate hybrid networks photocured by two different wavelengths, demonstrating its use in nanoimprint lithography (NIL) and shape memory materials. Initiated with a visible light sensitive photobase and a UV-sensitive radical initiator, thiol-Michael-acrylate hybrid polymerizations were programmed to proceed sequentially and orthogonally, with base-catalyzed thiol-Michael photopolymerization as the first stage and radical mediated acrylate photopolymerization as the second stage. By regulating the photopolymerization formulations, i.e. thiol-to-acrylate ratios, initiator loadings and irradiation conditions, a series of materials with highly tunable mechanical performance was achieved, with ultimate Tg values ranging from 23 to 70 °C. With a photopatternable first stage and a readily reconfigurable second stage, its implementation in nanoimprint lithography (NIL) enabled surface features on the scale of 10 nm to be formed on a photopatterned substrate. Additionally, the dual stage polymer results in a relatively homogenous polymer network with a narrow glass transition temperature (Tg), which enables rapid response in applications as shape memory materials, with shape-fixity values above 95% and shaperecovery values above 99%. With its unique photocuring process and programmable mechanical properties, the two color light controlled photopolymerization can be exploited as a useful tool in a wide range of materials science applications.

Solution Mask Liquid Lithography (SMaLL) for One-Step, Multimaterial 3D Printing

A novel methodology for printing 3D objects with spatially resolved mechanical and chemical properties is reported. Photochromic molecules are used to control polymerization through coherent bleaching fronts, providing large depths of cure and rapid build rates without the need for moving parts. The coupling of these photoswitches with resin mixtures containing orthogonal photo-crosslinking systems allows simultaneous and selective curing of multiple networks, providing access to 3D objects with chemically and mechanically distinct domains. The power of this approach is showcased through the one-step fabrication of bioinspired soft joints and mechanically reinforced "brick-and-mortar" structures.

o-Nitrobenzyl-Based Photobase Generators: Efficient Photoinitiators for Visible-Light Induced Thiol-Michael Addition Photopolymerization

In this contribution, three o-nitrobenzyl-based photobase systems were synthesized and evaluated for visible light initiated thiol-Michael addition polymerizations. With a modified structure, the (3,4-methylenedioxy-6-nitrophenyl)-propyloxycarbonyl (MNPPOC) protected base performance exceeds that of the nonsubstituted 2-(2-nitrophenyl)-propyloxycarbonyl (NPPOC) protected base and an ITX sensitized photobase system, with respect to both long-wavelength light sensitivity and photolytic efficiency. In material synthesis, MNPPOC-TMG is capable of initiating photo thiol-Michael polymerization efficiently and orthogonally with only limited visible light exposure and generating a highly homogeneous cross-linked polymer network. This approach enables the thiol-Michael "click" reaction to be conducted with a low-energy, visible light irradiation and, thus, expands its applications in biocompatible and UV sensitive materials.

Installing lactone chain termini during photoinduced polymerization

We exploit the Thorpe–Ingold effect as a spontaneous end group transformation method during photo-induced polymerization of methacrylates using the functional (2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropio-phenone) species as radical photoinitiator.