Diffusion-controlled grafting

In 1959, Hoffman et al. (9) reported the importance of the diffusion temperature in radiation-induced grafting of styrene under swelling conditions. They used the term diffusion-controlled grafting, and they noted that when styrene was grafted stepwise (first swelling, then grafting), only the first step was diffusion-controlled. Of course, since the medium inside the pellets is highly viscous, the termination reaction should be diffusion-controlled. This is discussed below. 

In an earlier paper (I) equations were developed to describe the grafting-dose process incorporating the two ideas of (a) a slow buildup of the free radical population during the grafting process, and (b) the gradual consumption of the monomer initially present in the fiber, thereby reducing the concentration to a smaller diffusion-controlled value (similar analyses could be made for films). 

The broad single peak distribution found with the mutual sample is not unexpected. It is known that with the film thickness and monomer-solvent mixture used the graft polymerization is diffusion controlled. The average chain length reached will be dependent on the distance of the growth site from the film surfaces resulting in the broad spectrum of chain lengths actually found. 

Finally, various attempts have been reported to interpret the kinetics of radiation grafting. The study by Mock, and Vanderkooi (123) concerned the mutual radiation grafting of styrene from the vapor phase to ethyl cellulose film at 50° C. The radical flux was determined separately using electron spin resonance. The kinetic scheme rigorously took account of the diffusion controlled nature of the reaction and the appropriate diffusion constants were separately determined (124). The value for the ratio of for the graft polymerization was determined as... 

The rate of dispersion (co)polymerization of PEO macromonomers passes through a maximum at a certain conversion. No constant rate interval was observed and it was attributed to the decreasing monomer concentration. At the beginning of polymerization, the abrupt increase in the rate was attributed to a certain compartmentalization of reaction loci, the diffusion controlled termination, gel effect, and pseudo-bulk kinetics. A dispersion copolymerization of PEO macromonomers in polar media is used to prepare monodisperse polymer particles in micron and submicron range as a result of the very short nucleation period, the high nucleation activity of macromonomer or its graft copolymer formed, and the location of surface active group of stabilizer at the particle surface (chemically bound at the particle surface). Under such conditions a small amount of stabilizer promotes the formation of stable and monodisperse polymer particles. 

From experiments conducted at the two dose rates investigated the dose-rate exponent is estimated to be a = 0.47. At the higher dose rate the reaction was diffusion controlled, heterogeneous films being obtained at elevated grafting ratios. 

Influence of Temperature. The reaction temperature necessarily plays a major role in grafting, where at least two reaction steps (chain propagation and termination) may become diffusion controlled. Indeed, grafting occurs faster at elevated temperatures, but here again each system exhibits a specific behavior. 

In a study, copper-catalyzed radical copolymerization of nBA and methacryloxy-capped poly(MMA) was compared with conventional radical copolymerization.267 431 The graft copolymers G-l obtained with copper catalysts are more homogeneous in terms of MWD (Mw/Mn 1.6 vs 3) and the number of side chains. This is attributed to diffusion control being less important in the metal-catalyzed radical polymerization, where the growing radical species is rapidly converted into the dormant covalent species. 

Mechanism of deterioration of polymers by auto-oxidization (B-3) Mechanism of free radical type polymerization by grafting initiators (B-4) Diffusion controlled reaction of free radical decay in solid polymers... 

The textural properties govern the diffusion process so that a well-defined structure may facilitate diffusion-controlled reactions. For example, the adsorption of heavy-metal ions (Hg + and Cu ) reveals a dramatic discrepancy between ordered and disordered mesoporous silica grafted with the aminopropyl or mercaptopropyl group [55]. The ordered mesostructure with a mean pore size of 6.5 nm brings about complete accessibOity of the functional groups. While... 

Substantial variations in the performance of grafted membranes with the same level and composition of grafting are frequent even with essentially noncrystalline substrates such as cellulose acetate and polyvinyl chloride. These are undoubtedly due to the variations in structure on at least two counts. Firstly, even with very thin films the grafting process is clearly diffusion controlled. The percent of grafting decreases through the half thickness of the membrane ie. is greater at the two surfaces. 

The quantitative effect of diffusion control on the rate of radiation-initiated graft polymerization has been studied theoretically for systems in which the diffusion-free reaction may show various dependences of rate on the concentration of monomer other than the usual first-order dependence. The study is very general and could be applied to systems involving a variety of modes of initiation and termination. 

The effects of diffusion control on the rate of radiation-initiated graft polymerization of cellulose have been studied theoretically. The effects of such variables as the initiator concentration, temperature, and monomer polymer ratio on the graft copolymerization of acrylamide and 0-methylcellulose have been investigated. The thermal properties and behaviour of graft copolymers of formaldehyde-cross-linked 0-cyanoethylcellulose and acrylates have been studied. Copolymerization of periodate-oxidized cellulose (aldehydocellulose) with glycidyl methacrylate in the presence of an enzyme e. peroxidase) afforded a means of immobilizing the enzyme without loss of activity. ... 

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