Photo-initiated radical polymerization

Hyperbranched poly(ethyl methacrylate)s prepared by the photo-initiated radical polymerization of the inimer 13 were characterized by GPC with a lightscattering detector [51]. The hydrodynamic volume and radius of gyration (i g) of the resulting hyperbranched polymers were determined by DLS and SAXS, respectively. The ratios of Rg/R are in the range of 0.75-0.84, which are comparable to the value of hard spheres (0.775) and significantly lower than that of the linear unperturbed polymer coils (1.25-1.37). The compact nature of the hyperbranched poly(ethyl methacrylate)s is demonstrated by solution properties which are different from those of the linear analogs. 

In addition to catalysis of small molecule transformations and biocatalysis, non-functionalized LLC phases used as reaction media have also been found to accelerate polymerization reactions as well. For example, the L and Hi phases of the sodium dodecylsulfate/n-pentanol/sulfuric acid system have been found to lower the electric potential needed to electropolymerize aniline to form the conducting polymer, polyaniline [110]. In this system, it was also found that the catalytic efficiency of the L phase was superior to that of the Hi phase. In addition to this work, the Ii, Hi, Qi, and L phases of non-charged Brij surfactants (i.e., oligo(ethylene oxide)-alkyl ether surfactants) have been observed to accelerate the rate of photo-initiated radical polymerization of acrylate monomers dissolved in the hydrophobic domains [111, 112]. The extent of polymerization rate acceleration was found to depend on the geometry of the LLC phase in these systems. Collectively, this body of work on catalysis with non-functionalized LLC phases indicates that LLC phase geometry and system composition have a large influence on reaction rate. 

A 9-line spectrum shown in Fig. 6 was obtained in a photo-initiated radical polymerization of MMA in its polymer at room temperature The 9 lines consist of strong 5 lines, which are characterized by a hyperfine splitting constant of 23.4 G and whose intensity distribution is 1 4 6 4 1, and weak 4 lines, each appearing between the 5 lines. The intensity distribution of these 9 lines seemed abnormal for a single species. Similar ESR spectra were independently obtained for the x-ray or y-ray decomposition ofpolyMMA (see Fig. 7 a), and various interpretations were proposed for them. 

An additional class of photo initiated radical polymerizations in wide use is composed of a combination of a styrenic monomer together with a polyester oligomer bearing unsaturated double bonds arrayed along the backbone. Such systems are, in fact, photoinitiated copolymerizations that have a strong tendency toward alternation. A graphical representation of the mechanism involved in these chemistry systems is Shown in Scheme 12. 

For SCVP of styrenic inimers, the mechanism includes cationic (14 [18], 19 [29]), atom transfer radical (15 [22, 27]), nitroxide-mediated radical (16 [21]), anionic (20 [19]), photo-initiated radical (17 [2], 18 [52-55]), and ruthenium-catalyzed coordinative (21 [56]) polymerization systems. Another example in-... 

UV spectroscopic measurements revealed that the polymerization reaction initiated with CD-complexed photo initiator is faster and, thus, ends up with higher yields than the polymerization reaction initiated with the same molar concentration of uncomplexed photo initiator. Radical photo polymerization is achieved by the homolytic fragmentation of carbon-carbon bonds of a photo-excited molecule as illustrated in Fig. 21. 

YOS 10a] Yoshida E., Photo-living radical polymerization of methyl methacrylate using alkoxyamine as an initiator . Colloid and Polymer Science, vol. 288, pp. 7-13,2010. 

The chemistry of UPRs involves the synthesis of unsaturated polyesters (UPs) by polyesterification or step-by step ionic copolymerization. The thus synthesized UP is then dissolved in an unsaturated monomer and crosslinked applying the radical polymerization approach. Thus, the chemistry of UPRs involves the polycondensation or ionic polymerization methods and crosslinking by peroxide or photo chemically initiated radical polymerization. 

These addition polymerizations occur by means of a chain reaction mechanism involving the propagation of an active center by interaction with the monomer. The active center consists of reactive species, typically, a radical, a cation, or an anion. In the simplest case, a photo initiated addition polymerization can be represented as shown in eqns [16]-[18] of Scheme 8. 

As an example stereolithography is a 3-D rapid process that produces automatically simple to very complex shaped models in plastic. Basically it is a method of building successive layers across sections of pho-topolymerized plastics on top of each other until all the thin printed layers can be joined together to form a whole product. The chemical key to the process, photopolymerization, is a well established technology in which a photo initiator absorbs UV energy to form free radicals that then initiate the polymerization of the liquid monomers. The degree... 

Because of their fast addition onto alkenes, phosphonyl radicals have found wide use as initiating radicals in photo-polymerizations. Several groups [31]... 

Fig. 3 Photo-initiators releasing phosphonyl radicals for photo-initiated free radical polymerizations...

The various morphological variants available in bead form can be repHcated in thin films ( 2 cmx8cmx50-100 pm) produced simply by photo-initiated free radical polymerization of comonomer mixtures introduced by capillary action into an appropriate mold formed with microscope sHdes [48]. With appropriate choice of comonomers, and porogen in the case of macroporous films, reasonably mechanically robust self-supporting films can be removed from the mold for further exploitation (Fig. 1.9). 

Common thermosets are cured by a free radical addition mechanism. These types of composites are cured by heat initiators, such as peroxides, or by photo initiators, such as a-diketones. A characteristic of cured acrylates is large shrinkage in the course of polymerization, which is undesirable for many uses. Another undesirable characteristic of acrylates is the formation of an oxygen-inhibited layer on the surface upon curing. 

Detailed studies on the decomposition of organic peroxides are of fundamental interest and of high importance in polymerization reactions. The time-scales of intermediate radical formation and of their subsequent decomposition determine process parameters such as the initiator efficiency in radical polymerizations. An improved understanding of the mechanism and dynamics of photo-induced fragmentation is achieved by systematic investigations in which quantum-chemical calculations are carried out in conjunction with highly time-resolved experiments. 

The rotating-sector method was applied to determine the individual rate constants of chain propagation and chain termination of the radical polymerization of ethylene [23,24]. The photo-initiator was diphenyldisulfide. First, the overall rate of polymerization was measured under steady illumination at pressures of 50 - 175 MPa and 132 - 199°C (Fig. 3.3-9). It increases first steeply and then less steeply with increasing pressure. At 175 MPa the rate of polymerization is ten times higher than at the low pressure of 50 MPa. 

In the photopolymer systems studied by Chen (17) it is likely that the a-amino radical produced in the quenching process is the species which initiates chain polymerization. Dye photo-bleaching then occurs by a disproportionation process. 

When 14C-benzoin (19) or its methyl ether (20) is used as photosensitizer for polymerizations, more of the sensitizer is incorporated in polymer than can possibly be accounted for by the initiation process. The reactions have the characteristics of mono-radical polymerizations and separate experiments with thermal initiators have shown that transfer to the carbonyl compound is of little importance. It appears that photo-excited states of these compounds, but not the ground states, can engage... 

Physical entrapment or chemical coupling is a well-established procedure for MIP preparation. First, a complex is formed between a functional monomer and template in an appropriate solvent solution. Then the complex is immobilized by polymerization in excess of a cross-linker. Predominantly, free-radical polymerization thermally launched with a 2,2-azobis(isobutyronitrile) (AIBN) initiator, is performed. In the case of photo-radical polymerization, a benzophenone or acetopho-none derivative is also used as the initiator [101]. Next, the template is extracted by rinsing the resulting MIP block with a suitably selected solvent solution. The bulk... 

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