Free-radical crosslinking

Table 1. Swelling of polyacrylamide hydrogels obtained by free-radical crosslinking polymerization...

For example, the molecular weight of unsaturated polyesters is controlled to less than 5000 g/mol. The low molecular weight of the unsaturated polyester allows solvation in vinyl monomers such as styrene to produce a low-viscosity resin. Unsaturated polyesters are made with monomers containing carbon-carbon double bonds able to undergo free-radical crosslinking reactions with styrene and other vinyl monomers. Crosslinking the resin by free-radical polymerization produces the mechanical properties needed in various applications. 

An alternative approach to stationary phase immobilization, and the most popular method in contemporary practice, is the free radical crosslinking of the polymer chains, using peroxides [201,202,245-248], azo-oospounds [202,249-251], ozone [252], or gamma radiation [253-255] as free radical generators. In this case, crosslinking occurs through the formation of (Si-C-C-Sl) bonds as shown below ... 

Matsumoto, A. Free-Radical Crosslinking Polymerization and Copolymerization of Multivinyl Compounds. Vol. 123, pp. 41-80. 

Recent work by Atalla(H) supports the idea that lignin is at least a semi-ordered substance in wood with the plane of the aromatic ring parallel to the cell wall surface. Woody plants synthesize lignin from trans-coniferyl alcohol (pines), trans-sinapyl alcohol 2 (deciduous), and trans-4-coumaryl alcohol 3 by free radical crosslinking initiated by enzymatic dehydrogenation(l2). Structures of these alcohols are given in Figure 1. 

Randomly - Crosslinked PDMS. The polydimethylsiloxane (PDMS) used to make random networks was obtained from General Electric. Membrane osmometry showed to be 430,000 g/g-mole. The polymer was mixed with various amounts of a free-radical crosslinking agent, dicumylperoxide (Di-Cup R, Hercules Chemical Co.). Samples were then pressed into sheets and crosslinking was effected by heating for 2 h at 150°C in a heated press. Mc values were calculated using equation 2, and are included in Table I. 

Microgel Formation in Solution by Free-Radical Crosslinking Copolymerization... 

The free-radical crosslinking polymerization can be regarded as a special example of specific diffusion control, in which the tendency to microgel formation and decrease of apparent reactivity of Internal double bonds depends on the size of the mlcrogel which in turn depends on the molecular weight of the primary chain. Polymerization of diallyl monomers exhibits much less of these features (W) because the degree of polymerization of their primary chains is extremely low due to degradative chain transfer. 

Fig. 7. Classification of the reactions of pendant double bonds during free radical crosslinking polymerizations...

Many thermosets are inhomogeneous. This may be the result of the polymerization mechanism (chemically induced inhomogeneities, such as those produced by the free-radical crosslinking polymerization of multifunctional monomers or by the step polymerization of three different monomers), or of the decrease of solubility of reaction products (thermodynamic driving force) or of both factors acting simultaneously, such as the case of several UP formulations. Inhomogeneities formed in the course of polymerization are fixed by the crosslinking reactions. 

For microelectronic production and for getting clinically reliable sensors the UV initiated free radical crosslinking of the polymer directly on the substrate is advantageous since it allows microfabrication and designing the physicochemical properties of the membrane [65]. 

Mashima, K., Nakayama, Y. and Nakamura, A. Recent Trends in Polymerization of a-Olefins Catalyzed by Organometallic Complexes of Early Transition Metals. Vol. 133, pp. 1-52. Matsumoto, A. Free-Radical Crosslinking Polymerization and Copolymerization of Multivinyl Compounds. Vol. 123, pp. 41-80. 

The antioxidant (a hindered phenol or, to a smaller extent, a phosphite) can retard this latter stage free radical crosslinking by any of several mechanisms, or by a combination of them. 

The antioxidant may retard the attack of oxygen at the original crosslinking site and thus preclude the sequence which leads ultimately to the polyene structures required for the later free radical crosslinking reaction. 

The antioxidant may retard the allylically propagated free radical dehydrochlorination along the chain. By so doing, it decreases the number of sites available for later free radical crosslinking. 

Free-Radical Crosslinking Polymerization and Copolymerization of Multivinyl Compounds... 

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