Copolymerization diffusion control models

Values of 0 required to fit the rate of copolymerization by the chemical control model were typically in the range 5-50 though values <1 are also known. In the case of S-MMA copolymerization, the model requires 0 to be in the range 5-14 depending on the monomer feed ratio. This "chemical control" model generally fell from favor wfith the recognition that chain diffusion should be the rate determining step in termination. 

More complex models for diffusion-controlled termination in copolymerization have appeared.1 tM7j Russo and Munari171 still assumed a terminal model for propagation but introduced a penultimate model to describe termination. There are ten termination reactions to consider (Scheme 7.1 1). The model was based on the hypothesis that the type of penultimate unit defined the segmental motion of the chain ends and their rate of diffusion. 

The rate of copolymerization, unlike the copolymer composition, depends on the initiation and termination steps as well as on the propagation steps. In the usual case both monomers combine efficiently with the initiator radicals and the initiation rate is independent of the feed composition. Two different models, based on whether termination is diffusion-controlled, have been used to derive expressions for the rate of copolymerization. The chemical-controlled termination model assumed that termination proceeds with chemical control, that is, termination is not diffusion-controlled [Walling, 1949]. But this model is of only historical interest since it is well established that termination in radical polymerization is generally diffusion-controlled [Atherton and North, 1962 Barb, 1953 Braun and Czerwinski, 1987 North, 1963 O Driscoll et al., 1967 Prochazka and Kratochvil, 1983] (Sec. 3-10b). 

Problem 7.16 Bulk polymerization of styrene in the presence of 1 g/L of AIBN initiator at 60°C gave a measured polymerization rate of 5.92 mol/L-s. Predict the rate of copolymerization at 60°C of a mixture of styrene (Mi) and methyl methacrylate (M2) with 0.579 mole fraction styrene and the same initial concentration of the initiator as in the homopolymerization case. Compare the rates predicted from chemical control, diffusion control, and combined models with the experimental value of 4.8x10 mol/L-s [25]. Use relevant kp and kt values for homopolymerization from Table 6.7 and assume 0 = 15. [Other data ri = 0.52, T2 = 0.46 monomer density = 0.90 g/cm .]... 

Tip 17 Termination in homopolymerization and copolymerization, initiation rate in homopolymerization, and copolymerization. Cope with the statement Termination reactions are almost always diffusion-controlled right from the outset of polymerization. Food for thought (and additional investigation). How important is the chain length dependence of the termination rate constant in polymer reactor modeling ... 

The kinetics of diffusion-controlled termination in copolymerization is difficult to study. The original interpretation of low-conversion rate data was based on a chemically controlled model utilizing a cross-termination factor ... 

Condensation of TEOS could be controlled by the reaction rate and/or the diffusion of water, while copolymerization could be controlled solely by the diffusion rate of PDMS. Proposed structural models of ormosils based on the reaction mechanisms before gelation are shown in Figure 14. The TEOS/PDMS ratio of the ormosils was 1/0.082. Immediately after mixing, the self-condensation of TEOS(I) was predominant over copolymerization between PDMS and TEOS. As the reaction time increased, copolymerization between PDMS and TEOS(II) was promoted. At this time, the PDMS chains were broken into shorter chains and/or cyclic D4C tetramers. As copolymerization and condensation reactions of TEOS proceeded, the solution gelled (III). After gelation, syneresis (IV) occurred and nonbridging PDMS chains and cyclic D4C tetramers were released from the gel. 

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