Polymerization kinetics, initiation problems

Description of polymerization kinetics in heterogeneous systems is complicated, even more so given that the structure of complex formed is not very well defined. In template polymerization we can expect that local concentration of the monomer (and/or initiator) can be different when compared with polymerization in the blank system. Specific sorption of the monomer by macromolecular coil leads to the increase in the concentration of the monomer inside the coil, changing the rate of polymerization. It is a problem of definition as to whether we can call such a polymerization a template reaction, if monomer is randomly distributed in the coil on the molecular level but not ordered by the template. 

The aforesaid complexities make it virtually impossible to write explicit general equations for the rate of polymerization, kinetic chain length, average degree of polymerization as has been done above for a completely dissociated ionic initiator. With the exception of those simple cases discussed above, each system in anionic polymerization represents a kinetically unique problem and must be solved separately. 

When a mixture of water, monomer, surfectants, initiator, and property control agents forms microdroplets of monomer in water with dimensions of 1 pm, an emulsion has been formed (8). If polymerization occurs in the 1 pm droplets by migration of an initiator active site from the aqueous phase to the microdroplet to completely react the droplet, the process is emulsion polymerization. A major problem in emulsion polymerization is developing a mixture which remains a stable emulsion throughout the polymerization. Polymerization kinetics is controlled by the number of microdroplets in the reaction mixture and the polymerization rate of the monomer. Polymer can be recovered as fine particles fiom the completed reaction but is most often used as the emulsion of polymerized product. Emulsion polymerization is commonly used to synthesize two polymer types covered in this book, polyvinyl chloride... 

One of the main differences between the polymerization kinetics with coordination catalysts and free-radical initiators is that the former depends on the characteristics of the active site as well as on monomer type, while the latter is almost exclusively regulated by monomer type. As we will see, even though this may not constitute a problem for establishing an operative mechanism for coordination polymerization, it creates a significant challenge for model parameter estimation. 

Homogeneous polymerization systems are used mainly for laboratory preparations rather than for industrial synthesis. The choice of the method for preparation depends to a large extent upon the amount of polymer which is to be produced and the physical condition in which it is required. Bulk polymerization can be used for studies of polymerization kinetics and the production of small amounts of polymer. In this case no solvent is used and the initiator, monomer and polymer are mixed together in one phase. Care must be taken to remove heat from the system during polymerization otherwise serious problems can arise. The viscosity of the mixture increases as polymer is formed and the reactions are often stopped at about... 

The Instantaneous values for the initiator efficiencies and the rate constants associated with the suspension polymerization of styrene using benzoyl peroxide have been determined from explicit equations based on the instantaneous polymer properties. The explicit equations for the rate parameters have been derived based on accepted reaction schemes and the standard kinetic assumptions (SSH and LCA). The instantaneous polymer properties have been obtained from the cummulative experimental values by proposing empirical models for the instantaneous properties and then fitting them to the cummulative experimental values. This has circumvented some of the problems associated with differenciating experimental data. The results obtained show that ... 

Addition of a core compound even in a batch wise polymerization makes the distribution narrower. This problem can simply be handled by TBP. If one starts with a core and the monomer is gradually added, the distribution can be narrowed even more [51, 83-87]. This follows from kinetic simulations. However, the monomer addition method has certain limits. It is effective for low molecular weights, so that B groups are almost absent in all initial larger molecules. Unless the monomer addition is infinitesimally slow, which is impractical, condensates by reactions between the monomers are formed containing B... 

It is essential to characterize the reactant species in solution. One of the problems, for example, in interpreting the rate law for oxidation by Ce(IV) or Co(III) arises from the difficulties in characterizing these species in aqueous solution, particularly the extent of formation of hydroxy or polymeric species. We used the catalyzed decomposition of HjOj by an Fe(III) macrocycle as an example of the initial rate approach (Sec. 1.2.1). With certain conditions, the iron complex dimerizes and this would have to be allowed for, since it transpires that the dimer is catalytically inactive. In a different approach, the problems of limited solubility, dimerization and aging of iron(III) and (Il)-hemin in aqueous solution can be avoided by intercalating the porphyrin in a micelle. Kinetic study is then eased. 

The simplest, but least accurate, method of assaying DPO activity is to record the final color yield when the enzyme is incubated with a suitable chromogenic substrate such as catechol, DOPA, or 4-methylcatechol. DOPA is the most frequently used substrate in colorimetric assays because it yields a dark brown/black end-product. In this reaction, catecholase catalyzes the conversion of DOPA to dopaquinone and then to the red dopachrome, which subsequently polymerizes to yield dark brown melanin-type pigments. Unfortunately, this simple procedure has serious limitations, as it measures the end-product of a sequence of reactions rather than the true initial reaction rate. Furthermore, because different substrates yield different final colors, valid kinetic comparisons between substrates are not possible. Nevertheless, this simple assay technique has proved adequate for useful comparative studies of the levels of enzymic browning in different fruit varieties and similar problems (Vamos-Vigyazo, 1981 Machiex et al., 1990). 

The thermal and kinetic models discussed above are the basis for determining the processing conditions for reactive processing by ionic polymerization,29 addition polymerization, vulcanization of rubbers and radical polymerization, although in the latter case additional assumptions of a constant initiation rate and a quasi-stationary concentration of radicals are made.89 These models can also be used to solve optimization problems to improve the performance and properties of end-products. 

The mechanism can be best understood within the framework of the conventional theory of radical chain kinetics, provided that certain of the usual simplifying assumptions are omitted. A solution is given to the problem of steady-state polymerization rate as a function of monomer and initiator concentration, taking into account termination reactions of primary radicals and recombination of geminate chains arising from the same initiation event. This model is shown to account for the kinetic data reported herein. With appropriate rate constants it should be generally applicable to radical polymerizations. 

Formaldehyde polymerizes because the two resulting C-O o bonds are very slightly more stable than its C=0 k bond, but the balance is quite fine. Alkenes are different two C-C o bonds are always considerably more stable than an alkene, so thermodynamics is very much on the side of alkene polymerization. However, there is a kinetic problem. Formaldehyde polymerizes without our intervention, but alkenes do not. We will discuss four quite distinct mechanisms by which alkene polymerization can be initiated—two ionic, one organometallic, and one radical. 

All the data discussed above are only an approximation or analogy of the real initiation of radical polymerization. The detailed investigation of the kinetics and mechanism of actual initiation processes is often our future task. Only rarely it is possible to obtain quantitative data on initiation, for example the value of the initiation rate constant, without simplifying assumptions. [133]. Some further information on this problem will be presented in Sect. 8.1 and in Chap. 5, Sect. 4.2 and in Chap. 8, Sect. 1.1. 

For ideal radical polymerization to occur, three prerequisites must be fulfilled for both macro- and primary radicals, a stationary state must exist primary radicals have to be for initiation only and termination of macroradicals only occur by their mutual combination or disproportionation. The rate equation for an ideal polymerization is simple (see Chap. 8, Sect. 1.2) it reflects the simple course of this chain reaction. When the primary radicals are deactivated either mutually or with macroradicals, kinetic complications arise. Deviations from ideality are logically expected to be larger the higher the concentration of initiator and the lower the concentration of monomer. Today termination by primary radicals is an exclusively kinetic problem. Almost nothing has been published on the mechanism of radical liberation from the aggregation of other initiator fragments and from the cage of the... 

Despite these problems useful polymers are made from allyl monomers. High initiator concentrations are needed because the kinetic chains terminate at low degrees of polymerization and multifunctional monomers are used to produce cross-linked structures even at low conversions. 

Cationic polymerizations differ from free-radical and homogeneous anionic syntheses of high polymers in that the cationic systems have not so far been fitted into a generally useful kinetic framework involving fundamental reactions like initiation, propagation, and so on. To explain the reasons for the peculiar problems with cationic polymerizations we will, however, postulate a conventional polymerization reaction scheme and show where its inherent assumptions are questionable in cationic systems. 

Such a situation was considered by Azad et of. (1980) in their discussion of the effect of additives to the monomer phase on the degree of swelling and on the kinetics of polymerization. A more relevant approach to this problem might be to consider the effect of a given amount of Z2 or a given initial volume fraction of Z2, 4> b> if monomer phase on (I2 = 0) (i e., tbe ratio of the volume fractions of monomer in the particles with and without addition of Zj to the monomer phase] for various values of the initial ratio of monomer to polymer. 

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