Free-radical Polymerization Homogeneous

Z. Guan, Homogeneous free radical polymerization in supercritical carbon dioxide, University of... 

Eastmond, G. C., Kinetic Data for Homogeneous Free Radical Polymerizations of Various Monomers, Chap. 3 in Comprehensive Chemical Kinetics, Vol. 14A, C. H. Bamford and C. F. H. Tipper, eds., American Elsevier, New York, 1976b. 

Guan, Z. Combes, J. R. Menceloglu, Y. Z. DeSimone, J. M. Homogeneous Free Radical Polymerizations in Supercritical Carbon Dioxide 2. Thermal Decomposition of 2,2 -Azobis(isobutyronitrile), Macromolecules 1993, 26, 2663-2669. 

Although CO2 dissolves many small molecules readily, it is a very poor solvent for most high molecular weight polymers. Currently, only amorphous or low melting fluoropolymers and silicone polymers are known to be very soluble in CO2 (T < 100 °C, P < 400 bar), or C02-philic, while many industrially important polymers are relatively insoluble. In 1992, we reported the successful homogenous free radical polymerization of a C02-philic fluorinated acrylate, 1,1-dihydroperfluorooctyl acrylate (FOA)2. Homogenous polymer synthesis in CO2 is fundamentally limited however, by the extremely low solubility of most polymers at readily accessible conditions. 

Acrylonitrile (AN) is a particular case among vinjd monmners reprding the solvent dependence of the propagation rate constant (kp) in homogeneous free radical polymerization. Whereas in dimethylformamide and dimethylsulfoxide kp has values which might be expected from a comparison with other monomers and from the reactivity of the growing radical (400 and 1910 1 mol s , respectively, at 25 the propagation rate constant is considerably hig ier in water... 

Rates of radiation induced polymerizations are normally determined by dilatometric [85] or gravimetric [84] experiments. Some of the first quantitative results from cyclopentadiene [86] and a-methylstyrene [87] were obtained by competitive kinetic methods, based on the retarding effect of ammonia and amines. This approach tends to yield maximum values for Rp. More recently, however, a procedure combining stationary state kinetic and conductance measurements has been described [88, 89], and further refined [85]. Because the ions generated by 7-ray irradiation have a transient existence, the kinetic treatment leads to expressions which are very similar to those derived for homogeneous free radical polymerizations [90]. A simplified version of the kinetic scheme is as follows ... 

In homogeneous free radical polymerization, water is often employed as solvent for water-soluble monomers and polymers with more polar functional substituents such as hydroxyl, amino, oxyethylene, ammonium, and carboxylate groups, along with emulsion, suspension, and dispersion processes. This is also the case for metal-catalyzed living radical polymerization. 

In general, a polymerization process model consists of material balances (component rate equations), energy balances, and additional set of equations to calculate polymer properties (e.g., molecular weight moment equations). The kinetic equations for a typical linear addition polymerization process include initiation or catalytic site activation, chain propagation, chain termination, and chain transfer reactions. The typical reactions that occur in a homogeneous free radical polymerization of vinyl monomers and coordination polymerization of olefins are illustrated in Table 2. 

The data in Table 2.4 provide evidence that the slow rates and low molecular weights obtained in homogeneous free radical polymerization of these dienes are not due to a low rate constant for propagation but rather must be caused by a high rate constant for termination (as indicated in Table 2.1) (Matheson et al., 1949,1951 Morton and Gibbs, 1963). Hence, under the special conditions of emulsion polymerizations, where the termination rate is controlled by the rate of entry of radicals into particles, it becomes possible to attain both faster rates and higher molecular weights. It is this phenomenon which led to the rise of the emulsion polymerization system for the production of diene-based synthetic rubbers. 

Beuermann, S., Buback, M., Isemer, C. and Wahl, A. (1998) Homogeneous phase free-radical polymerization of styrene in supercritical carbon dioxide. Paper presented at theM TO ASI on Supercritical Fluids, Kemer, Antalya, Turkey, July 1998. (b) Sabine, B., Buback, M., Isemer, C., and Wahl, A. (1999) Homogeneous free-radical polymerization of styrene in supercritical COiMacromol. Rapid. Commun. 20, 26-32. 

S. Beuermann, M. Buback, C. Isemer and A. Wahl, Homogeneous free-radical polymerization of styrene in supercritical CO2... 

Stereo control can also be achieved in the homogeneous free-radical polymerization of vinyl monomers by varying the polymerization temperature. Some typical monomers that behave in this manner include methyl methacrylate,(39) vinyl acetate,(41) vinyl chloride,(42) isopropyl and cyclohexyl acrylates.(40) As the polymer temperature is lowered the crystallizabiUty of the polymers becomes more discernible. (46) This observation can be attributed to the fact that as the temperature is lowered there is a preference for units in the same configuration to be added to the growing chain. It has been found that in general there is a preference for syndiotactic sequences to develop as the temperature is lowered. As an example, the observed melting temperature of poly(vinyl chloride) increases from 285 °C to 310 °C as the polymerization temperature is lowered from —15 °C to —75°C, with a concomitant increase in the syndiotacticity.(47)... 

Many commercial polymers, including polystyrene, PMMA, and LDPE, are synthesized via homogeneous free-radical polymerization of a single monomer. Homopolymer properties are controlled by average chain length and chain-length distribution as well as, in some cases, structural characteristics such as branching level. 

Table 4.1 lists the range of concentration and rate coefficient values typically encountered in homogeneous free-radical polymerization systems at low conversion. These can be combined with Eqs. (9)-(12) to illustrate the tradeoffs involved be-... 

Equation 9.13 is the classical rate expression for a homogeneous, free-radical polymerization. It has been an extremely usefiil approximation over the years, particularly in fairly dilute solutions, but deviations from it are sometimes observed. It is now evident that many deviations are due to the fact that the termination reaction is diffusion controlled, so kt is not really a constant but decreases in time as the polymerization proceeds. In certain... 

Cmd Chemicals is producing a polymer with minimum polydispersity by homogeneous, free-radical polymerization in a CSTR. The polymer terminates inherently by disproportionation. To maintain and control the desired average chain length, they... 

---