Photoinitiated polymerization is usually referred to as a chain process that is initiated by light and both the initiating species and the growing chain ends are radicals and cations, and in some cases, anions or weak bases. The use of photoinitiated polymerization is continuously growing in industry as reflected by the large number of applications in not only conventional areas such as coatings, inks, and adhesives but also high-tech domains, optoelectronics, laser imaging, stereolithography, and nanotechnology.
a) Photoinitiated Free Radical Polymerization
When polymerizations are initiated by light and both the initiating species and the growing chain ends are radicals, we speak of radical photopolymerization.
In far the most cases of photoinduced polymerization, initiators are used to generate radicals. One has to distinguish between two different types of photoinitiators.
See animation of the photopolymerization of the different type monomers including monofunctional (producing linear polymer) and difunctional (producing cross-linking polymer) monomers.
Free radical process
Type I photoinitiators
Type II photoinitiators
b) Photoinitiated Cross-linking Polymerization
The photoinitiated polymerization of multifunctional monomers, or UV-radiation curing, has found a large number of applications in various industrial sectors. UV curing is typically a process that transforms a multifunctional monomer into a cross-linked polymer by a chain reaction initiated by reactive species (free radicals or ions), which are generated by UV irradiation. There are two major classes of UV-curable resins, which differ basically by their polymerization mechanism:
Photoinitiated radical polymerization of multifunctional monomers, mainly acrylates or unsaturated polyesters;
Photoinitiated cationic polymerization of multifunctional epoxides and vinyl ethers
This technology is now commonly utilized to perform the ultrafast drying of protective coatings, varnishes, printing inks and adhesives, and to produce the high-definition images required in the manufacture of microcircuits and printing plates. Besides its great speed and spatial resolution, radiation curing presents a number of other advantages, in particular ambient temperature operation, solvent-free formulations, low energy consumption and the production of polymer materials having tailor-made properties.
c) Type I Photoinitiators
Photoinitiators termed unimolecular are so designated because the initiation system involves only one molecular species interacting with the light and producing free-radical active centers. Initiating radicals, formed by direct photofragmentation process (α or less common β cleavage) of Type I photoinitiators upon absorption of light, are capable of inducing polymerization. The photoinitiator forms an excited singlet state, which then undergoes rapid intersystem crossing to form a triplet state. In the triplet state, two radicals (benzoyl and benzyl radicals) are generated by α-cleavage fragmentation. The benzoyl radical is the major initiating species, while, in some cases, the benzyl radical may also contribute to the initiation.
The majority of Type I photoinitiators are aromatic carbonyl compounds with appropriate substituents. Benzoin ether derivatives, benzil ketals, hydroxylalkylphenones, α-aminoketones and acylphosphine oxides are the most efficient ones.
d) Type II Photoinitiators
Bimolecular photoinitiators are so-called because two molecular species are needed to form the propagating radical: a photoinitiator that absorbs the light and a co-initiator that serves as a hydrogen or electron donor. These photoinitiators do not undergo Type I reactions because their excitation energy is not high enough for fragmentation, i.e., their excitation energy is lower than the bond dissociation energy. The excited molecule can, however, react with co-initiator to produce initiating radicals.
In these systems, photons are absorbed in the near UV and visible wavelengths. Free radical active centers are generated by hydrogen abstraction or photo-induced electron transfer process aforementioned.
Photosensitizers of Type II system are benzophenones, thioxanthones, camphorquinones, benzyls, and ketocoumarins. Whereas, an amine, ether, thiol or alcohol with an abstractable α-hydrogen are also known as co-initiators in Type II system.