Initiators

Photo polymerization initiators are specialized compounds that activate polymerization upon exposure to light, typically UV light. They decompose to generate free radicals, cations, or anions when exposed to specific wavelengths of light. The photoinitiators are generally divided into Type I photoinitiators and Type II photoinitiators. These initiators are used in offset inks, screen inks, adhesives, white lacquers, photo resists, pigmented coatings and in coatings of paper, metal, wood and plastic.

Type I Photoinitiators

Type 1 photoinitiators undergo a unimolecular bond cleavage upon irradiation, leading to the direct formation of free radicals. Commonly used Type 1 photoinitiators include Benzoin methyl ether, 4,4′-Dimethylbenzil, 2,2-Dimethoxy-2-phenylacetophenone, and 2-Hydroxy-2-methylpropiophenone.

These initiators are known for their fast curing speed and high efficiency, eliminating the need for any co-initiator. However, they do have some limitations: they exhibit limited solubility, are susceptible to oxygen inhibition, and, while some Type I initiators are less pigmented than Type II initiators, they can still contribute to yellowing over time, particularly in high-concentration systems.

Type II Photoinitiators

Type II photoinitiators undergo a bimolecular reaction, where the excited state of the photoinitiator interacts with a second molecule (a co-initiator) to generate free radicals. Examples of commonly used Type 2 photoinitiators include 4,4′-Bis(diethylamino)benzophenone and 2,4-Diethyl-9H-thioxanthen-9-one.

These initiators absorb visible light and can provide improved film properties, such as enhanced adhesion and reduced yellowing. However, they have a slower curing speed and require a co-initiator, which may pose risks of migration and toxicity. Due to their lower efficiency and the necessity for a co-initiator, Type II systems often require higher concentrations to achieve the desired level of cure.

Biocompatible Photoinitiators

Irgacure 2959 and Lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) are commonly used photoinitiators in the preparation of gelatin methacrylate for 3D bioprinting of vascular-like constructs.

Biocompatible initiators play a crucial role in various applications, including photodynamic therapy, antibacterial coatings, 3D bioprinting, and the preparation of hydrogels (such as gelatin-methacrylate and dextran methacrylate hydrogels) utilized in tissue engineering. These applications include cartilage and bone tissue engineering, as well as dental materials like dentin primers. Among these, Lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) is the most widely used initiator.

LAP is a photoinitiator that is both water-soluble and cytocompatible, making it particularly suitable for biological applications. It is favored over Irgacure 2959 due to its excellent water solubility and effective polymerization rates within the visible light spectrum. Additionally, LAP demonstrates reduced toxicity and enhanced cell viability, further supporting its use in biocompatible applications.

Radical Initiators

Radical initiators are used for the polymerization of vinyl and acrylic monomers. Azo polymerization initiators are widely used in acryl resins for paints, water-absorbent resins, polymer coagulants, adhesives, and paper finishing agents. Radical polymerization is initiated by the formation of free radicals, which can be generated by thermal energy, light, or radioactivity. These radical initiators are also used in controlled radical polymerization.

Controlled Radical Polymerization Initiators

Controlled Radical polymerization initiators are used for polymerization of vinyl monomers. There are three types of polymerizations including atom transfer radical polymerization (ATRP), Reversible Addition/Fragmentation Chain Transfer Polymerization (RAFT) and Nitroxide-mediated Polymerization (NMP).

Atom Transfer Radical Polymerization (ATRP) Initiators

Atom Transfer Radical Polymerization (ATRP) is a controlled radical polymerization technique that allows for the synthesis of well-defined polymers with precise molecular weights and narrow polydispersity. ATRP initiators are typically halogenated compounds, such as bromo- or chloro-alkanes. For example, 2-Hydroxyethyl 2-bromoisobutyrate is commonly used to form hydroxy functionalized telechelic polymers, which are applied to modify carboxylate- or isocyanate- modified surfaces, particles, or biomolecules. Pentaerythritol tetrakis (2-bromoisobutyrate) is used in the preparation of tetrafunctional polymers.

Reversible Addition/Fragmentation Chain Transfer Polymerization (RAFT) Initiators

RAFT polymerization is used to prepare homopolymers and block copolymers of vinyl monomers. Common initiators include 1,1′-Azobis(cyclohexanecarbonitrile) or 2,2′-Azobis(2-methylpropionitrile). Block copolymers synthesized through RAFT are widely used in drug delivery, bio-mineralization, bio-compatibilization, and hydrogel applications.

Nitroxide-Mediated Polymerization (NMP) Initiators

NMP process involves the usage of radical initiator, a monomer, and a nitroxide radical to trap intermediate radical species, enabling the formation of homo and block polymers. Initiators such as N-tert-Butyl-N-(2-methyl-1-phenylpropyl)-O-(1-phenylethyl)hydroxylamine and N-tert-Butyl-O-[1-[4-(chloromethyl) phenyl]ethyl]-N-(2-methyl-1- phenylpropyl)hydroxylamine are used in the synthesis of styrene and acrylate polymers and co-polymers. TEMPO, a stable nitroxide radical, is essential for controlling living radical polymerizations. Block copolymers generated from NMP are used in pigment dispersion, memory devices and in composite manufacturing.

Initiators Based on Solubility

Water-soluble Photoinitiators

Water-soluble initiators are compounds that can initiate polymerization in aqueous environments. When water-insoluble initiators are used with water-soluble monomers, they must first be dissolved in a non-aqueous solvent before being mixed with the monomer to form an emulsion in the aqueous phase. This process can limit their effectiveness in promoting photopolymerization when exposed to light. In contrast, photopolymerizable occurs more efficiently when the photopolymerizable compounds are in the same solvent phase as the initiator.

The rise of hydrophilic polymerization conditions, particularly in hydrogel systems, has created a demand for a new class of water-soluble photoinitiators. We offer lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) and water-dispersible photoinitiator nanoparticles of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) as effective initiators for aqueous photopolymerization.

Oil-soluble Photoinitiators

Oil-soluble photoinitiators are compounds that dissolve in oils or other organic solvents and are used to initiate polymerization reactions in oil-based systems. The resultant polymers are commonly utilized in coatings for wood, inks, and adhesives. The most frequently used initiators in this category include benzophenones, thioxanthones, and camphorquinone, such as 4-hydroxybenzophenone and camphorquinone.