Free radical polymerization termination phase

Gramain and Frere [82] observed that the free radical polymerization of co-meth-acryloyl terminated PEO macromonomers in the aqueous phase deviates from the solution polymerization. Polymerizations initiated by KPS in water were much faster than those that proceeded in the solution. Low molecular weight polymers were formed in the aqueous systems (ca. up to 20 macromonomer units were incorporated into polymer molecules). 

Free-Radical Polymerization in Emulsion. In suspension polymerization, the particle size is fixed by the size of the monomer droplet which contains the initiator. Emulsion polymerization differs from suspension polymerization in that the initiator is dissolved in the aqueous phase and the polymer particle grows during polymerization. Free radicals are generated in the water and diffuse to the monomer-water interface. The length of the polymer chain formed, or equivalently the molecular weight, depends on the rate of free radical arrival and termination. S. Katz,... 

The use of stable free-radical polymerization techniques in CO2 represents an emerging new area of research. Odell and Hammer have demonstrated the use of reversibly terminating free radicals generated by systems such as benzoyl peroxide or AIBN and 2,2,6,6,-tetramethyl-l-piperidinyloxy free radical (TEMPO) to polymerize styrene at a temperature of 125°C and pressures of 245-280 bar in CO2 [92]. At low monomer concentrations (10% by volume), the polymerization resulted in low conversions of PS with an of about 3000 g/mol and a narrow molecular weight distribution (PDI <1.3). NMR analysis of the resulting polymer confirmed that the precipitated polystyrene chains are predominantly end-capped with TEMPO. Additionally, the polymer could be isolated and later extended by the addition of more monomer under an inert argon blanket. It was also determined that the precipitated PS could be extended while still in the CO2 continuous phase simply by increasing monomer concentration in the reactor. 

Polymerization of vinyl or methacrylic monomers (especially in conjunction with crosslinking monomers) within the wood often results in an autoacceleration during the latter phase of the polymerization this phenomenon is known as the Trommsdorff or gel effect in homopolymerization reactions (Duran and Meyer, 1972 Trommsdorff et a/., 1948). The gel effect arises from a decrease in the termination rate of the free radical polymerization, caused in turn by the effect of the local viscosity on the diffusion rates of the growing polymer chains. Since the heat of polymerization cannot be removed rapidly enough to maintain isothermal conditions, autoacceleration is characterized by a strong exotherm the intensity of the exotherm depends on the catalyst level, as illustrated in Figures 11.4 and 11.5 (Siau et al., 1968). 

To approach truly living conditions in free radical polymerization, the extent of the termination reactions has to be reduced as much as possible. The natural way to establish such conditions is the reduction of the concentration of active chains, which, in turn, results in the decrease of polymer productivity. To contrast such decrease, the segregation of the radical chains, which is present in heterogeneous processes such as emulsion polymerization, can be exploited. This allows reducing terminations while preserving the overall concentration of radical chains. However, the appUcation of RAFT mediated polymerizations to emulsion systems has also been rather problematic, mainly with respect to the transport of RAFT agent through the aqueous phase. Attempts to carry out RAFT polymerizations in ab-initio emulsion processes... 

Free radical polymerization kinetics has received much attention and many aspects of the process are well understood (see Chapter 4). Most academic investigations have been carried out in idealized conditions where the extent of monomer conversion is low. The classical expression for the rate of polymerization (Rp), in a single-phase reaction, is Eq. (13), where kp is the propagation rate coefficient, Cm is monomer concentration, J i is the initiation rate and kt is the termination rate coefficient [63]. 

Strictly speaking, any model based on the time-independent thermodynamics cannot be used to adequately predict the concentration of monomer in latex particles during Smith-Ewart Interval II. This is because the free radical polymerization of monomer in the discrete latex particles is governed by the simultaneous kinetic events such as the generation of free radicals in the continuous aqueous phase, the absorption of free radicals by the particles, the propagation of free radicals with monomer molecules in the particles, the bimolecular termination of free radicals in the particles, and the desorption of free radicals out of the particles. The equilibrium (or saturation) concentration of monomer in the growing latex particles may not be achieved if the rate of consumption of monomer in the major reaction loci is much faster than that of diffusion of monomer molecules from the monomer droplets to the reaction loci. Therefore, the equilibrium concentration of monomer in the latex particles represents an upper limit that is ultimately attainable in the course of polymerization. Nevertheless, the general... 

Phase transfer catalysis has been employed by Akashi to graft polyacrylamide (with a terminal carboxylic acid function, and obtained by free radical polymerization in the presence of mercap-topropionic acid) onto a partially chloromethylated polystyrene backbone, in the presence of tetrabutylammonium hydrosulfate. The grafting yield is satisfactory. 

Monomers may be polymerized using a water-soluble initiator while dispersed, by agitation, in a concentrated soap solution. In this emulsion system initiation occurs in the aqueous phase and propagation occurs in the soap micelles. Since the growing macroradicals are not terminated until a new free radical enters the micelle, high molecular weight products are rapidly obtained. The rate of polymerization and DP is proportional to the number of activated micelles. 

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