Free radical exit in emulsion polymerization. II. Model discrimination via experiment

Abstract

SYNOPSISIn emulsion polymerizations, desorption (exit) from latex particles of monomeric radical species that arise from transfer can be a n important determinant of the overall kinetics. An examination of various methodologies for the testing of postulated free radical exit mechanisms is made. These utilize the model descriptions for the exit process presented in the accompanying article of Casey et al., employing data consisting of conversion as a function of time for the approach to steady state polymerization conditions. Experimental data are presented on the exit rate coefficients as a function of such experimental parameters as: particle size, monomer concentration, and aqueous-phase free-radical concentration for a series of styrene polymerizations a t 50°C, where the average number of free radicals per particle ( i i ) never exceeds 0.5. It is demonstrated for these systems that while the conversion/ time dependence from a single run, under conditions sensitive to exit, is insensitive to mechanistic assumptions as to the fate of desorbed free radicals, the variation of the exit rate coefficient with particle size so obtained suggests a second order dependence on 6, implying complete re-entry of desorbed free radicals under all conditions studied. Once the monomeric radicals have re-entered, they are more likely to remain inside the particle where they will either propagate or undergo termination rather than re-escape. The article also presents a n estimate for the rate coefficient at 50°C of the first propagation step of the monomeric radical subsequent to transfer. The conclusions drawn here for seeded systems should prove useful for study of particle nucleation mechanisms, when exit is particularly likely in small, newly formed, particles.

Radical entry mechanisms in redox-initiated emulsion polymerizations

“…The results presented in the previous section showed an interesting feature: After the redox initiator feed was switched off, the concavity of the rate data was initially the opposite to that expected (as mechanistically predicted for a sudden stoppage in initiation and as observed in g relaxation data [49]) if initiation had ceased, such as observed in Fig. 4.…”

“…Using the feed rate and aqueous termination rate coefficient given previously together with the saturated water phase concentration for styrene (C sat W Z 1 !10 K3 M [30]), the established value for z for styrene and persulfate initiator (zZ2-this assumes that the ionic species has a single change and is hydrophilic, like SO K 4 ; note that the value of z is different for more hydrophobic initiating entities [5]), the aqueous phase propagation rate coefficient (k p,aq Z4k p M K1 s K1 [7,49]) and a typical experimental value for N p (5!10 16 dm K3 ), the initiator efficiency is calculated to be approximately 10%. It is fortuitous to note for the purposes of this study that the choices for parameters such as k p,aq are not overly critical as reasonable changes to these numbers do not drastically affect either the efficiency of the diffusion-limited entry of hydrophobic radicals or the comparison of their entry rates with the hydrophilic radicals.…”

The influence of RAFT on the rates and molecular weight distributions of styrene in seeded emulsion polymerizations

“…8 The more water-soluble the leaving group (3) is, the greater the extent of exit is. 16 The leaving radicals from cumyl RAFT (2-phenylprop-2-yl dithiobenzoate) and EMA RAFT [2-(ethoxycarbonyl)propyl-2-yl dithiobenzoate] are cumyl radical (3a) and 2-(ethoxycarbonyl)prop-2-yl radical (3b), respectively. The coefficient for desorption of a monomeric free radical (exit) from a particle with swollen radius r s is given by the following: 12…”

“…The size of the seed must be as small as possible to explore the effects of exit because exit is proportional to the inverse of the particle radius squared (see eq 1). 16,18 However, the seed must have a minimum size; otherwise, the concentration of monomer will change drastically during particle growth. This, to a good approximation, is determined by the Morton equation 19,20 and shows that for particles with unswollen radii greater than 20 nm, the monomer concentrations inside the particles remain relatively constant during interval II.…”

A critical evaluation of reaction calorimetry for the study of emulsion polymerization systems: thermodynamic and kinetic aspects

“…Standard models for entry and exit [19,36,37] require a knowledge of the following rate parameters: k p (it is assumed here that this has the same value in particle and water phases), k d (the initiator dissociation rate coefficient), z (the critical degree of polymerization for entry), k 1 p (the propagation rate coefficient of a monomeric radical inside the particle), k t,aq (the rate coefficient for termination of oligomeric radicals in the water phase), C sat W (the water solubility of monomer), k tr (the rate coefficient for transfer to monomer), r spont (the rate coefficient for spontaneous, or 'thermal', entry of radicals into the particles), and D w (the diffusion coefficient of a monomeric radical in water). The values of n calculated using the parameters in the text and in Table 4 are given in Table 3.…”