Rate of radical polymerization

Exercise 29-21 The rate of radical polymerization of ethenylbenzene, induced by benzoyl peroxide in mixtures of tetrachloromethane and benzene, is independent of the concentration of tetrachloromethane. At high concentrations of tetrachloromethane, the average molecular weight of the polymer is greatly reduced and chlorine is found in the polymer. Explain. 

Only a few measurements of the effect of pressure on the rates of radical polymerizations have been reported, but the general principles are well enough established that some information about mechanism can be obtained. The main experimental facts of many radical polymerizations can be accounted for in terms of the following reaction scheme (for a recent account see Bamford et al., 1958). 

The rate of polymerization increases in this series of metals Mg(II) < Sr(II) < Ba(II) < Ca(II), The nature of the cation is likely to have a significant effect on the kinetics of the polymerization of salts of unsaturated acids in ionizing environments [68-70]. These differences are attributed to a different charge density at the macroradical anion, which influences the rate of interaction in the propagating macroradical-monomeric anion system. In comparable conditions the rate of radical polymerization of transition metal acrylates is lower than that of acrylic acid (AA) and decreases in the series (Fig. 4-7) [71] AA > Co(II) > Ni(II) > Fe(III) > Cu(II) (see Experiment 4-1, Section 4.6). The resulting metallopolymers are insoluble in any organic solvent, which indicates... 

The steady-state rate of radical polymerization (RP) is given by... 

Kataoka [30,31] also studied the polymerization of acrylic acid derivatives using potassium persulfate as the initiator and chitosan as template. The rate of radical polymerization reached a maximum at a MA-Na-chi-... 

During Stages II and III the average concentration of radicals within the particle determines the rate of polymerization. To solve for n, the fate of a given radical was balanced across the possible adsorption, desorption, and termination events. Initially a solution was provided for three physically limiting cases. Subsequentiy, n was solved for expHcitiy without limitation using a generating function to solve the Smith-Ewart recursion formula (29). This analysis for the case of very slow rates of radical desorption was improved on (30), and later radical readsorption was accounted for and the Smith-Ewart recursion formula solved via the method of continuous fractions (31). 

If the reaction with monomer is not the rate determining step, the rate of radical generation in pholoinitiated polymerization is given by eq. 9... 

Most values of / have been measured at zero or low conversions. During polymerization the viscosity of the medium increases and the concentration of monomer decreases dramatically as conversion increases (i.e. as the volume fraction of polymer increases). The value of / is anticipated to drop accordingly. 32, u 9j % For example, with S polymerization in 50% (v/v) toluene at 70 °C initialed by 0.1 M AIBN the instantaneous" / w as determined to vary from 76% at low conversion to <20% at 90-95% conversion (Figure 3.3).32 The assumption that the rate of initiation (kAf) is invariant with conversion (common to most pre 1990s and many recent kinetic studies of radical polymerization) cannot be supported. 

Pulsed laser photolysis (PLP) has emerged as the most reliable method for extracting absolute rate constants for the propagation step of radical polymerizations,343 The method can be traced to the work of Aleksandrov el al.370 PLP in its present form owes its existence to the extensive work of Olaj and eoworkers 71 and the efforts of an 1UPAC working party/45"351 The method has now been successfully applied to establish rate constants, /rp(overall), for many polymerizations and copolymerizations. 

Stable radicals can show selectivity for particular radicals. For example, nitroxides do not trap oxygcn-ecntcrcd radicals yet react with carbon-ccntcrcd radicals by coupling at or near diffusion controlled rates.179,184 This capability was utilized by Rizzardo and Solomon181 to develop a technique for characterizing radical reactions and has been extensively used in the examination of initiation of radical polymerization (Section 3.5.2.4). In contrast DPPH, w hile an efficient... 

Solvent effects on radical polymerization have been reviewed by Coote and Davis,59 Coote et. Barton and Borsig,71 Gromov,72 and Kamachi" 1 A summary of kinetic data is also included in Beuennann and Buback s review.74 Most literature on solvent effects on the propagation step of radical polymerization deals with influences of the medium on rate of polymerization. 

Polymerization Rate and Radical Occlusion. In modeling the rate of precipitation polymerization, the reaction can occur at three different loci in the diluent, at the surface of the particles, or in the interior of the particles. 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

Rate of radical generation in the aqueous phase during emulsion polymerization (Chap. V). 

Successful application of radical polymerization requires the appropriate choice of the specific initiator to achieve the desired initiation rate at the desired reaction temperature and the realization that higher polymerization rates achieved by increasing the initiation rate (either by increasing [I] or kmolecular weights. Higher radical concentrations result in more propagating chains but each propagates for a shorter time. 

---