Free radical vinyl polymerization comparison

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate. By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially this includes the formation of polyesters and polyamides (nylons). Thus, there exists a large overlap between the terms stepwise kinetics and condensation polymers, and chainwise kinetics and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1. 

Because of the unique growth mechanism of material formation, the monomer for plasma polymerization (luminous chemical vapor deposition, LCVD) does not require specific chemical structure. The monomer for the free radical chain growth polymerization, e.g., vinyl polymerization, requires an olefinic double bond or a triple bond. For instance, styrene is a monomer but ethylbenzene is not. In LCVD, both styrene and ethylbenzene polymerize, and their deposition rates are by and large the same. Table 7.1 shows the comparison of deposition rate of vinyl compounds and corresponding saturated vinyl compounds. 

Case Study 2 Comparison of Mathematical Models FOR Free Radical Homopolymerization of Vinyl Monomers in scCOj In this case study, a comparison of performance of the different kinetic models proposed in the literature for dispersion polymerization of styrene and MMA in SCCO2 is presented. The models used by Quintero-Ortega et al. [43] (models 1 and 2) and those presented by the groups of Kiparissides [47] (model 3) and Morbidelli... 

In theory, any asymmetric olefmic, as well as acrylic, styre-nic, and vinylic, monomer can have tactic chain microstructures. However, it depends on the coordination power of catalysts. Free-radical polymerization processes do not have any coordination power and thus produce atactic polymers. Ionic polymerization processes have certain levels of coordination power, contributed by counterions. The counterion at the vicinity of a propagating center can coordinate the insertion of monomer molecules. Unfortunately, this coordination power is weak and yields a low degree of tacticity. As a result of the equilibrium between ion pairs and free ions, individual chains can have some tactic segments generated from the former with others atactic from the latter. In comparison, Ziegler-Natta catalysts possess strong coordination power for tactic polymer production. 

In addition, Cui-N-propyl-2-pyridylmethanimine mediated "living" radical polymerization of vinyl monomers by use of l-butyl-3-methylimidazolium hexafluorophosphate as solvent has been reported (Carmichael et al., 2000). It has been pointed out that the rate of polymerization was enhanced in comparison to other polar/coordinating solvents. Moreover, the polymerization product was made copper free by a simple solvent wash, which avoids the contamination of the polymer product by the catalyst. Other atom transfer radical polymerizations in ionic liquids have recently been reported (Sarbu Matyjaszewski, 2001 Biendron Kubisa, 2001). 

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