Light initiated polymerization

Lee and coworkers report on a photo-initiated polymerization for the preparation of poly(butyl methylacrylate-co-ethylene dimethylacrylate) within fused silica capillaries. UV light-initiated polymerization is well suited to monolith formation in restricted spaces since polymer forms only in those areas that are exposed to irradiation. This leads to far greater control over the length and size of the monolithic column formed. The resulting columns are robnst since their separation ability does not deteriorate with time or number of injections and their reprodncibility is excellent. ... 

The present high level of industrial activity in the field of light initiated polymerizations has stimulated research In basic polymerization phenomena including polymerization klnetics.(1-4) Unfortunately, previously used methods of determining monomer conversion such as dilatometry or measurements of unreacted monomers are not easily adapted to thin coating films. In addition, the presence of multifunctional monomers yielding networks at low conversion in photopolymerizable formulations also complicates analyses. 

A typical light-initiated polymerization Involved weighing 7-9 mg of acrylate with initiator into the standard DSC aluminum cup. The cup was placed in the DSC sample holder and an empty aluminum cup was placed in the reference holder. The usual nitrogen flow within the cell enclosure was established with the UV lamp, filter, and shutter assembly positioned over the holders. 

Sulfonium Salts. Other onium salts beside the diazonium and halonium salts will, on exposure to light, initiate polymerization of epoxides and other monomers capable of undergoing cationic polymerization. The use of onium salts of the elements of Groups Va and Via was first described in 1975 (41, ) and 1976 ( ). 

In analogy to the n = 2 template 14, template 24 was prepared according to Scheme 8-8. This compound was prone to light initiated polymerization when left concentrated and therefore was stored as a stock solution typically on the order of 1.0 M in benzene. The n = 4 base was synthesized starting from protocatechuic acid. Bis-ester formation was... 

Figure2.30 UV-light-initiated polymerization of an urethane-acrylate system (a) and an acrylate/epoxide blend (1 1, w/w) ...

Vinylpyridinium ions, 1, like other vinyl monomers are subject to thermal and light initiated polymerization. A recent study in our laboratory has confirmed that dilute ( < 0.20 M) aqueous solutions of 1 (R = CH3, C2H5) and sonicated suspensions of 1 and water are stable at temperatures below 70 °C in normal laboratory light. Suspensions of water-insol monomers, e.g. 1(R = C12H25), polymerize rapidly at temperatures above 80 °C. Dilute aqueous solutions of the water-soluble monomer 1 (R = CH3 or C2H5) resist polymerization at 80-100 °C but polymerize when subjected to UV radiation (300 nm). 

Photopolymerization. In many cases polymerization is initiated by ittadiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directiy to form active free radicals, or it may first undergo a bimoleculat electron-transfer reaction, the products of which initiate polymerization (14). TriphenylaLkylborate salts of polymethines such as (23) ate photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

Chemical Reactivity - Reactivity with Water No reaction Reactivity with Common Materials No reaction Stability During Transport Stable Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Heat may cause an explosive polymerization. Strong ultraviolet light can also initiate polymerization Inhibitor of Polymerization Hydroquinone and its methyl ether, in presence of air. ... 

One of the most interesting recent developments has been the advent of the resin glass polyalkenoate cements (Antonucci, McKinney Stansbury, 1988 Mitra, 1989 Wilson, 1989, 1990 Mathis Ferracane, 1989 Minnesota Mining Manufacturing Company, 1989 Albers, 1990). They are dual-cure hybrids that set by a combination of acid-base and polymerization reactions, and there are several types. Polymerization is effected by either chemical or light initiation. 

Photopolymer technology, which encompasses the action of light to form polymers and light initiated reactions in polymeric materials, is an immense topic. Previous papers in this symposium have described some of basic chemistry utilized in photopolymer technology. The primary objectives of this paper are a) to develop the connections between basic photopolymer chemistry and practical uses of the technology and b) to provide an overview of the wide variety of photopolymer applications that have been developed since the 1950 s. Every attempt has been made to make this review as inclusive as possible, but because of the extensive nature of this topic, there are many applications of photopolymer chemistry that have not been included. In addition, only limited representative references are provided since the patent and open literature for this technology are quite vast (7). 

Another very important visible light-initiated reaction of alkyl aluminum porphyrins is their 1,4-addition to alkyl methacrylates to produce ester enolate species [Eq. (4)]. This enolate then acts as the active species in the subsequent polymerization of the acrylate monomer. For example, Al(TPP)Me acts as a photocatalyst to produce polymethylmethacrylate with a narrow molecular weight distribution in a living polymerization process [Eq. (4)]. Visible light is essential for both the initiation step (addition of methylmethacrylate to Al(TPP)Me) and the propagation... 

For a purely photochemical polymerization, the initiation step is temperature-independent (Ed = 0) since the energy for initiator decomposition is supplied by light quanta. The overall activation for photochemical polymerization is then only about 20 kJ mol-1. This low value of Er indicates the Rp for photochemical polymerizations will be relatively insensitive to temperature compared to other polymerizations. The effect of temperature on photochemical polymerizations is complicated, however, since most photochemical initiators can also decompose thermally. At higher temperatures the initiators may undergo appreciable thermal decomposition in addition to the photochemical decomposition. In such cases, one must take into account both the thermal and photochemical initiations. The initiation and overall activation energies for a purely thermal self-initiated polymerization are approximately the same as for initiation by the thermal decomposition of an initiator. For the thermal, self-initiated polymerization of styrene the activation energy for initiation is 121 kJ mol-1 and Er is 86 kJ mol-1 [Barr et al., 1978 Hui and Hamielec, 1972]. However, purely thermal polymerizations proceed at very slow rates because of the low probability of the initiation process due to the very low values f 1 (l4 IO6) of the frequency factor. 

Fuel performance problems initiated by light are not common. However, fuel quality can be affected. The primary concern of light exposure is fuel color darkening and the possible formation of high-molecular-weight deposits due to free-radical-initiated polymerization of fuel components. 

---