Immobilized chains, free radical

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 ... 

Polypropylene (PP) is a stereospecific polymer prepared by polymerization using organo-metallic catalyst system. Commercial PPs have up to 95% isotactic content, which means that pendant methyl groups are almost all on the same side of the chain. When PP is exposed to ionizing radiation, free radicals are formed that cause chemical changes. Because PP is highly crystalline, these radicals are relatively immobile and consequently may not be available for reaction for long periods of time.96... 

Free Radical Polymerization Kinetics of Immobilized Chains... 

When the viscosity of a sample is high, the reactive chain ends, which are attached to fairly large molecules, diffuse relatively slowly compared to the monomer and growth occurs primarily by diffusion of the latter to react with the relatively immobile free radicals. It is frequently desirable to form images in samples which are somewhat viscous and depleted of inhibitors. 

The term free radical is often used in the context of a reactive intermediate, as in the case of polymerization of vinyl monomers, but the same structure (unpaired electron) can and does exist in a kind of immobilized environment. For example, a bulk-polymerized (monomer and initiator only in the polymerization system) poly(methyl methacrylate) (PMMA) contains an appreciable number of free radicals that can be detected by electron spin resonance (ESR) [1]. When the polymerization system becomes highly viscous toward the end of the bulk polymerization, gel formation occurs and immobilizes the growing end of free radical chain growth polymerization, preventing recombination of two free radical ends of growing chains. 

Bachman and coworkers (44), using ESR techniques, monitored the lifetime of free radicals in dry gelatin irradiated to approximately 50 kGy and stored in air at 20°C. Unlike the radicals in the gel, these are relatively immobile, and required several days to decay appreciably. Some were still detectable after about a month. The work and Friedberg and coworkers (45) indicates that the formation of these gelatin radicals in the dry state involves rupture of peptide bonds and that their decay does not involve combination of long chains. They found a decrease in the viscosity of solutions made from dry gelatin irradiated to a dose of 155 kGy. 

Generally, there are two approaches to grafting a polymer chain onto a surface , (a) via fi ee radical copolymerization of an immobilized double bond by using a single coupling agent such as methacrylpropylsiloxane or vinyl trimethoxylsilane, and (b) via initiation by an immobilized free radical intiator Using these two methods,... 

In this limit, termination within the particle is rate determining. An important implication of termination being diffusion controlled (and hence chain-length dependent) is that, in conventional free-radical polymerisation, termination events are dominated by termination between a mobile short chain (formed in an emulsion polymerisation by entry of a z-mer, by re-entry of an exited radical or by transfer to monomer) and a long, relatively immobile one. This is known as short-long termination . 

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