Kinetics polymer formation

Charge transfer kinetics for electronically conducting polymer formation, 583 Charge transport in polymers, 567 Chemical breakdown model for passivity, 236... 

Polymer formation icont.) diffusion components and, 421 diffusion control of oxidation in, 389 electrochemical responses, 400 influence of concentration, 397 and kinetic equations, 381 nucleation and, 379 oxidized area, 387... 

Potential sweep kinetics, with polymer formation, 416... 

The formation of polyesters from a dialcohol (diol) and a dicarboxylic acid (diacid) is used to illustrate the stepwise kinetic process. Polymer formation begins with one diol molecule reacting with one diacid, forming one repeat unit of the eventual polyester (structure 4.3) ... 

Thus, the reactants are consumed with few long chains formed until the reaction progresses toward total reaction of the chains with themselves. Thus, polymer formation occurs one step at a time, hence the name stepwise kinetics. 

Compared with free radical polymerizations, the kinetics of ionic polymerizations are not well defined. Reactions can use heterogeneous initiators and they are usually quite sensitive to the presence of impurities. Thus, kinetic studies are difficult and the results sensitive to the particular reaction conditions. Further, the rates of polymer formation are more rapid. 

The number of attempts to model the kinetics of plasma-polymerization has been limited thus far. Nevertheless, these efforts have been useful in demonstrating the role of different processes in initiating polymerization and the manner in which the physical characteristics of the plasma affect the polymerization rate. It is anticipated that future modeling efforts will provide more detailed descriptions of the polymer deposition kinetics and thereby aid the development of a better understanding of the interactions between the physical characteristics of a plasma and the chemistry associated with polymer formation. 

The kinetics of template polymerization depends, in the first place, on the type of polyreaction involved in polymer formation. The polycondensation process description is based on the Flory s assumptions which lead to a simple (in most cases of the second order), classic equation. The kinetics of addition polymerization is based on a well known scheme, in which classical rate equations are applied to the elementary processes (initiation, propagation, and termination), according to the general concept of chain reactions. 

We have described some of the general characteristics of polymers, and how they can be grouped according to structure, but we have not addressed any of the more quantitative aspects of polymer structures. For instance, we have stated that a polymer is made up of many monomer (repeat) units, but how many of these repeat units do we typically find in a polymer Do all polymer chains have the same number of repeat units These topics are addressed in this section on polymer molecular weight. Again, the kinetics of polymer formation are not discussed until Chapter 3—we merely assume here that the polymer chains have been formed and that we can count the number of repeat units in each chain. 

Breitenbach and Frank (5) showed that with styrene-divinylbenz-ene, no further additives (such as peroxides) are necessary for popcorn polymer formation. Breitenbach and Fally (6) found, in methyl acrylate polymerization, the possibility of crosslinking in the polymerization of a monovinyl compound. Miller and coworkers (7) developed the kinetics of the process Pravednikow and Medvedev (8) studied the chain scission, and assumed radical formation by that process as an important step. 

Reaction of live polymers with protic agents (e.g., HsO, ROH, and RNH2) results in simple protonation of the live ends and cessation of formation of the kinetic polymer chain. On the other hand, reaction of live polymers with some electrophiles can produce end-group substitution, as well as chain termination, as shown. 

There is evidence for isomerization of chemisorbed propylene oxide to acrolein on silver and for surface polymer formation on metal oxide catalysts (11,12). Formation of a surface polymeric structure has also been observed during propylene oxidation on silver (13). It appears likely that the rate oscillations are related to the ability of chemisorbed propylene oxide to form relatively stable polymeric structures. Thus chemisorbed monomer could account for the steady state kinetics discussed above whereas the superimposed fluctuations on the rate could originate from periodic formation and combustion of surface polymeric residues. 

In this section we discuss unique MWDs formed via Hnear emulsion polymerization, while the kinetics of branched and crossUnked polymer formation are considered in Sect. 4.2. 

Assuming that classical chemical kinetics are valid and that the crosslinking reaction rate is proportional to the concentrations of polymer radicals and pendant double bonds, it was shown theoretically that the crosslinked polymer formation in emulsion polymerization differs significantly from that in corresponding bulk systems [270,316]. To simplify the discussion, it is assumed here that the comonomer composition in the polymer particles is the same as the overall composition in the reactor, and that the weight fraction of polymer in the polymer particle is constant as long as the monomer droplets exist. These conditions may be considered a reasonable approximation to many systems, as shown both theoretically [316] and experimentally [271, 317]. First, consider Flory s simplifying assumptions for vinyl/divinyl copolymerization [318] that (1) the reactivities of all types of double bonds are equal, (2) all double bonds... 

Combining these trends, we can postulate the mechanism of polymer powder formation as follows. When a relatively high concentration (pressure) of monomer vapor meets with the luminous gas phase of a carrier gas in a relatively small volume at sufficiently high pressure, the formation occurs quickly in a relatively small-volume element. Because a sufficient quantity of reactive species are created in the small-volume element, the polymer formation steps approach a critical level above which particles cannot stay in the gas phase without the reactive species diffusing out of the volume element. In other words, the kinetic pathlength is very short under such conditions, as proved by the fact, shown in Table 8.1, that most powders are soluble in solvent. 

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