Photoinitiation chain reaction

Figure 8, Chain reaction photoinitiation. The efficiency of photoinitiation can be increased by chain reactions. Hydrogen abstraction by triplet excited benzophenone forms a THF free radical. Subsequent oxidation by the aryliodonium salt produces the THF cation capable of initiating polymerization, and a phenyl radical. Hydrogen abstraction by the phenyl radical produces the THF free radical completing the...

One of the first methods of polymerizing vinyl monomers was to expose the monomer to sunlight. In 1845, Blyth and Hoffman [7] obtained by this means a clear glassy polymeric product from styrene. Berthelot and Gaudechon [8] were the first to polymerize ethylene to a solid form and they used ultraviolet (UV) light for this purpose. The first demonstration of the chain reaction nature of photoinitiation of vinyl polymerization was done by Ostromislenski in 1912 [9]. He showed that the amount of poly(vinyl bromide) produced was considerably in excess of that produced for an ordinary chemical reaction. 

A chain reaction polymerization of vinyl monomer, which is usually carried out by a photoinitiator to produce a primary radical (R ), which can interact with a monomer molecule (M) in a propagating process to form a polymer chain composed of a large number of monomer units (see Eq. [2] and reaction Scheme [3]. 

Photoinitiated free radical polymerization is a typical chain reaction. Oster and Nang (8) and Ledwith (9) have described the kinetics and the mechanisms for such photopolymerization reactions. The rate of polymerization depends on the intensity of incident light (/ ), the quantum yield for production of radicals ( ), the molar extinction coefficient of the initiator at the wavelength employed ( ), the initiator concentration [5], and the path length (/) of the light through the sample. Assuming the usual radical termination processes at steady state, the rate of photopolymerization is often approximated by... 

A variety of processes exist in which photographic images are produced as a result of a photoinitiated chain reaction. Such reactions, which are characterized by three fundamental steps (initiation, propagation and termination), provide the basis for certain processes in which photopolymerization, photocrosslinking or colour formation or destruction represents the main image forming stage. 

In recent years visible photoinitiators for the formation of polymers via a radical chain reaction have also been developed. These absorb light which is blue, green, or red and also cause the polymerization of polyolacrylates, in some instances, such as encapsulated systems, with speed which is near photographic. Some of these photoinitiators provide the photochemical backbone of the nonsilver, near-photographic speed, imaging processes such as the Cycolor processes invented by the Mead Corporation. Cycolor initiators are cyanine dye, borate ion salts (4)—so-called ( +, —) ion pair... 

These photoinitiation processes which depend on the formation of free radicals in some photochemical reaction lead to chain reactions, since each molecule of initiator can promote the addition of many monomer units to a polymer chain. The quantum yield of monomer addition can therefore be much larger than unity, but it cannot be controlled since the growth of a polymer chain is then limited by termination reactions in which two free radicals react to produce closed-shell molecules. 

J. Hutchison, "Photoinitiated Free Radical Chain Reaction," Vol. 11, Energy Research Abstracts, Abstract No. 51555, Electricity Council Research Centre, 1986. 

In fact, alkylated succinamides were isolated in some cases, though in very poor yields, and result from radical combination, which is a chain termination step. The experimental observations, i.e. the formation of (a) 1 1 adducts, (b) telomeric products, (c) alkylated succinamides, and (d) oxamide (when an olefin is absent), are consistent with a free radical mechanism. The telomeric products obtained support the assumption that we deal here with a chain reaction, because they are characteristic products of this type of reaction. Another proof for the chain reaction mechanism is the fact that when benzophenone is used as a photoinitiator (vide infra), the amount of benzpinacol formed is smaller than the amount of the 1 1 addition product of formamide and olefin (16). Quantum yield determinations will supply extra evidence for the validity of a chain reaction mechanism for this photoaddition reaction. 

Figure 6.14 Example of polymerization chain reaction ethylene polymerization initiated by the [M(CO)s] photoinitiator (generated from its [Mn2(CO)uJ precursor by photodissociation) and propagated by its successive chain carriers...

Photoinitiation Photoproduction of a free radical or ion capable of initiating a chain reaction such as a polymerization. 

In all three of these designs, chemical amplification is the result of photoinitiated chain or catalytic reactions where irradiation is used only to initiate a chain reaction or to generate a catalyst within localised areas of a resist film. 

Photoinitiated polymerization uses the energy of light for the rapid conversion of monomeric liquids to solid polymeric products. The term photopolymerization implies that the initiation step of a radical, cationic, or anionic chain reaction producing a macromolecule requires the absorption of a photon. Since the absorption of one photon may start the reaction of up to 10 monomeric units, photoinitiated polymerization is, in practice, one of the most powerful chemical amplification techniques. 

In 1983, radiation curable 100 % solvent-free silicone acrylates were introduced into the market [4], This system provide the opportunity to be cured by either ultraviolet light (UV) or electron beam (EB). Similar in concept to peroxide initiation, silicone acrylate systems employ photoinitiators to generate free radicals and initiate cure, which is based on the polymerization of the acrylic C=C double bond via a radical chain reaction. 

Considered are reactions catalyzed by light which do not show a chain mechanism. They differ from photoinitiated radical chain reactions of the SrnI type to be discussed subsequently. Results not included in reviews52,53 are examined. 

Not only radical ions, generated via electron transfer from a photoinitiator (Scheme 6.275), but also photoproduced ions or radicals can be the key intermediates in the photoinitiation processes. Today, the term photoinitiator is mostly connected with photoinitiated (chain) polymerization reactions,1134,1470,1471 in which the reactive intermediates are generated from relatively small amounts of an excited initiator (initiation step) to start a chain reaction (propagation steps). Initiation of polymerization by light, rather than by heat, has several advantages, such as high reaction rates at ambient temperature and spatial control of the process.1492... 

There are special features in photochemical reactions where side reactions become the major track and make a well planned synthesis unsuccessful. Sometimes the photoactive compound in the reaction mixture which could be applied in stoichiometric or catalytic (photosensitizer) amounts may also be the photoinitiator of a radical chain reaction. Again, the role of the solvent has to be considered carefully. If hydrogen abstraction reactions can be induced (e.g. when reactive free radicals are produced), solvents labile to this process must be excluded. Many saturated hydrocarbons, ethers, and alcohols (which normally are useful solvents because of their optical properties) have to be avoided in these... 

In each system, the primary photo-event is dissociation of the cationic photoinitiator to produce an acid. This reaction proceeds with a quantum efficiency that is characteristic of the particular initiator. The photogenerated acid then interacts with a carefully chosen polymer matrix to initiate a chain reaction, or acts as a catalyst, such that a single molecule of photogenerated acid serves to initiate a cascade of bond making or breaking reactions. The effective quantum efficiency of the overall process is the product of the photolysis reaction efficiency times the length of the chain reaction (or the catalytic chain length). This multiplicative response constitutes... 

Photoinitiated polymerization is typically a process that transforms a monomer into polymer by a chain reaction initiated by reactive species (free radicals or ions), which are generated from photosensitive compounds, namely, photoinitiators and/or... 

Easic Principles Practical Photochemistry General Considerations Carbonyl Compounds a-Cleavage Carbonyl Compounds Hydrogren Abstraction Steroids Carbonyl Compounds Cycloaddition Enone and Dienone Rearrangements Alkenes Isomerisation and Rearrangement Alkenes Cycloaddition Alkenes Photo-Cxidation Terpenoids Aromatic Compounds Isomerisation and Cycloaddition Practical Photochemistry Scale-up Aromatic Compounds Substitution and Cydisation Alkaloids Photoinitiated Free-radical Chain Reactions. 

The ring-opening polymerization of the EPI epoxy groups was followed by IR spectroscopy and shown to proceed rapidly upon UV exposure. The photoinitiated chain reaction can be formally written as follows hv... 

In a general sense, photoinitiated polymerizations are chain reaction mechanisms that include initiation, propagation, and termination steps during which a liquid solution is commonly converted into a solid polymer or highly cross-linked struaure when multifunctional monomers or macromers are used (Scheme 1). 

Oxygen inhibits free radical polymerization through two pathways. First, an oxygen molecule can quench the active triplet state of the photoinitiator and be excited to singlet state itself The seeond path is scavenging of the photoinitiator and polymer radicals through oxidation with the yield of peroxy radicals. The overall chain reactions of photopolymerization with the presence of O2 is schematically shown as follows... 

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