Photo-Induced Controlled Polymerization

Dual Photo/Thermal Initiation Studies. A series of studies were performed using reactive formulations containing both a photoinitiator and a thermal initiator dissolved in the Derakane resin. The objective of these studies was to investigate a dual cure strategy in which the heat liberated by the photo-induced polymerization leads to the production of additional active centers by the dissociation of a thermal initiator. In this way, the dual cure strategy could offer both the temporal control of the start of the reaction afforded by the photopolymerization, as well as enhanced reaction rate and completeness of cure provided by the thermal initiation. 

Photo-induced GASP (Figure 4.3) of vinyl monomers MMA and St was carried out on solid surfaces precoated with a photoiniferter, 2-cyanoprop-2-yl-N,N -dimethyldithiocarbamate, under UV-irradiation, resulting in the formation of polymers on the irradiated parts of substrate surfaces [16]. Obtained polymers showed a proportional relation between M and polymer yield. Consecutive copolymerization of MMA and St led to the formation of block copolymers. These results demonstrate that photo-controlled radical polymerization of vaporized monomers occurred on solid surfaces. 

In addition to the aforementioned transformations, the photo-induced Bergman cyclization was utilized by Zhu and coworkers to form intramolecular polymeric nanoparticles.The appropriate reactive diyne unit was incorporated into the polymer using both direct controlled radical polymerization and aHqme protection strategies. The resulting linear polymers were subjected to photochemical Bergman cyclization conditions using a continuous addition strategy, similar to the case previously discussed. ... 

Near-infrared (NIR) is routinely used to qualify monomers prior to polymerization reactions. It is used to measure the kinetics of polymer onset and can be used to detect end-point completion and initiator compound levels in polymerization reactions. NIR spectro- scopy can also be used to sort polymers and to control the quality of incoming raw monomers and finished polymeric materials. Molecular spectroscopy using NIR and IR measurement techniques is often used for competitive analysis and to determine thermal or photo-induced oxidation or degradation reactions in polymers. In general, NIR spectroscopy is valuable for polymer identification, characterization, and quantitation. NIR spectroscopy can be completed for in situ process applications where no sample preparation is a requirement, and where rugged optical systems are a necessity. Some of the earliest work in applying IR and NIR spectroscopy to polymer characterization is found in References 1 to 11. 

Hie system of Koumura et al. based on the photo-induced polymerization in the presence of Mn2(CO)io and ethyl-2-iodoisobutyrate at 40 °C was tested for the bulk polymerization of VAc (Scheme 18d). The addition of n-Bu3N accelerated the polymerization while maintaining a rather good control of the pol5mierization. For instance, poly(VAc) of = 18 000 and... 

By a technique which interposed a rotating sector between a source of UV radiation and a dilatometer bearing a monomer, variations in the rotational speed of the sector and size of the opening, controlled bursts of radiation strike the monomer and induce polymerization. From the frequency of exposure and the effect on the polymerization many of the kinetic constants were evaluated. In this connection it should be noted that vinyl acetate exhibits virtually no absorption of UV radiation at 290-300 nm. On the other hand, acetaldehyde has an extinction coefficient of 14 at 290 nm and an extinction coefficient of 15 at 300 nm. Therefore acetaldehyde can act as a photo-sensitizer for the polymerization of vinyl acetate at wavelengths above 299.8 nm [23]. At 366 nm, 2,2 -azo-bisisobutyronitrile has been used as a sensitizer [29,180]. Azobicyclohexane carbonitrile is a UV sensitizer suitable for use with a 124 watt mercury arc at 25°C which does not produce a dark reaction in rotating-sector experiments. Its absorption peak is at 350 mn with an extinction coefficient of 16 [181]. 

Recently, semirigid-rod diacrylate compounds were investigated by Hikmet et al. in order to obtain densely crosslinked LC network materials [57] as well as loosely crosslinked anisotropic gel [58-60]. Copolymerization of an LC monoacrylate having a chiral group and LC diacrylate led to a ferroelectric LC network. In a series of papers by Broer et al. [48,49,61,62], highly crosslinked LC networks were prepared by the photopolymerization of diacrylate LC monomers. Photo-polymerization allowed control of the initiation of polymerization and, therefore, the temperature of polymerization. Orientation was induced using surface treatment technique as applied in liquid crystal display, such as a rubbed polyimide film. The resulting ordered networks showed anisotropic behavior in a number of physical properties, such as coefficient of thermal expansion, modulus of elasticity, and refractive index. 

---