Aqueous-phase Propagation Research Articles

A Kinetic Investigation of Seeded Emulsion Polymerization of Styrene Using Reversible Addition−Fragmentation Chain Transfer (RAFT) Agents with a Low Transfer Constant

The mechanisms were investigated for the seeded emulsion polymerization of styrene mediated by xanthate-based reversible addition-fragmentation chain transfer (RAFT) agents with low transfer constants (ca. 0.7). gamma-Radiolysis relaxation measurements were used to determine the exit rate coefficient directly; this increased with the amount of RAFT, consistent with the chain-transfer characteristics of RAFT and the standard transfer/diffusion mechanism for exit. Combining the exit data with measurements of the steady-state rate with chemical initiation showed that the entry rate coefficients decreased with the amount of RAFT, while with gamma initiation the entry rate coefficients were unaffected, unless a very high RAFT concentration was used. This behavior is inconsistent with conventional assumptions about RAFT and the aqueous-phase propagation model for entry (which predicts the entry rate coefficient successfully for ordinary emulsion polymerization systems). It is postulated that the RAFT agents used in this work are surface active, which is not unexpected given the canonical forms of these RAFT agents. The dramatic decrease in entry rate coefficient for the chemically initiated system can be taken into account within the framework of the standard aqueous-phase model for entry if it is assumed that the surface activity gives an increased concentration of RAFT near the surface, which leads to increased transfer to radicals which desorb instead of entering, and whose subsequent reentry is frustrated by this event recurring, until they eventually undergo aqueous-phase termination.

Entry in Emulsion Polymerization: Effects of Initiator and Particle Surface Charge

The rate coefficient for radical entry into latex particles in emulsion polymerizations is measured for styrene systems in which the entering species are anionic (from persulfate) and cationic (from 2,2‘-azobis(2-methylpropionamidine) dihydrochloride, or V-50). These entry rate coefficients ρ are obtained by measuring rates in seeded emulsion polymerizations where the seeds have either cationic or anionic groups on the surface; “zero-one” conditions are employed, because these offer the advantage that particle size is sufficiently small (≈70 nm diameter) that intraparticle termination is not rate-determining. Data comprise steady-state rates with chemical initiator, combined with loss rates obtained using γ-radiolysis initiation and following the relaxation behavior after removal from the radiation source. Values for ρ as a function of initiator concentration can be meaningfully compared for different initiators through the dependence of initiator efficiency fentry on primary radical generation rate (radical flux). For the anionic latex, this dependence is seen to differ depending on the nature of the initiator used. This may be explained by the entry model [Maxwell, I. A.; Morrison, B. R.; Napper, D. H.; Gilbert, R. G. Macromolecules 1991, 24, 1629] wherein the rate-determining steps in entry are assumed to be only aqueous-phase propagation and termination to form surface-active z-meric oligomeric radicals; entry is solely by z-mers, for which actual entry into the particle is so fast as not to be rate-determining. The cationic latex shows a high rate of spontaneous initiation, which can be explained in terms of amidino radical chemistry; this can be reduced by heat treatment. Accurate fentry values are obtained using the heat-treated cationically stabilized latex for seeded studies. The fentry data are also consistent with the model for both cationic and anionic species entering the cationic latex. Values of z so inferred (≈2 for persulfate, ≈1 for amidinium) can be understood in terms of the hydrophobic free energy of these species. The results refute alternative models in the literature that suppose entry is controlled by double-layer (colloidal) interactions, surfactant displacement, or diffusion control.

Emulsion Polymerization of Vinyl neo-Decanoate, a “Water-Insoluble” Monomer

Rate and particle size data for the emulsion polymerization of vinyl neo-decanoate, where inhibitor/retarder effects are obviated through rigorous purification, can be quantitatively fitted by conventional mechanisms for particle formation and growth, even though the monomer has a very low water solubility. These mechanisms are (1) radical entry is by an oligomeric radical (z-mer) arising from aqueous-phase propagation at a critical degree of polymerization (z between I and 2 for this monomer) where the species becomes surface-active, (2) radical loss is by transfer to a species which desorbs into the aqueous phase and eventually terminates' (after reentry into another particle) rather than undergoing further propagation, and (3) particle formation is by a z-mer entering a micelle. These hypotheses are consistent with gamma relaxation data which give direct information on radical loss.

Maximum Achievable Particle Size in Emulsion Polymerization: Modeling of Large Particle Sizes

A simplified model for particle formation in emulsion polymerization (comprising aqueous-phase propagation to degrees of polymerization which may enter a pre-existing particle and/or form new particles by homogeneous or micellar nucleation, coupled with the aqueous-phase and intra-particle kinetics of oligomeric radicals) is formulated to provide a model suitable for the simulation of systems containing large-sized particles. The model is particularly useful to explore conditions for growth of large particles while avoiding secondary particle formation. Applied to the Interval II emulsion polymerization of styrene with persulfate initiator at 50°C, it is found that there is an effective maximum particle size that can be achieved if the formation of new particles is to be avoided. The parameter space of initiator concentration, particle number concentration and particle radius is mapped to show a “catastrophe” surface at the onset of new nucleation. Advanced visualization techniques are used to interpret the large number of simulations in the series, showing a maximum achievable particle diameter of around 5 μm.

Effects of Poly(acrylic acid) Electrosteric Stabilizer on Entry and Exit in Emulsion Polymerization

A methodology developed to obtain rate coefficients for entry and exit (desorption) in emulsion polymerizations was applied to systems stabilized electrosterically by a copolymer of acrylic acid and styrene embedded in a styrene seed particle. This was grown as a second-stage procedure, by adding styrene and acrylic acid to a styrene seed and then polymerizing. Rate coefficients for entry (ρ) and exit (k ) for subsequent homopolymerization of the resulting latices with styrene were obtained from the time dependence of the approach to steady state using both chemical and γ-radiolytic initiation; the latter was used in relaxation mode, which measures k directly. Compared to the same latices with an electrostatic stabilizer, at pH 7 the electrosteric stabilizer greatly reduced both ρ and k. When ionic strength was increased, ρ increased relative to that found for electrosterically stabilized latex in the absence of added electrolyte. For electrostatically-stabilized latices, entry is supposed to occur by aqueous-phase propagation to a critical degree of polymerization z which then undergoes irreversible entry; the present data for electrostatically-stabilized latices support this model, including its prediction that ρ be independent of particle size, all other things being equal. The decrease in ρ in the electrosterically-stabilized latices is ascribed to a layer through which diffusion of a z-mer is slow, so that it may be terminated prior to actual entry. For electrostatically-stabilized latices, exit is supposed to occur by transfer, resulting in a monomeric radical which exits by diffusing through the aqueous phase, this event competing with intraparticle propagation; the decrease k in the electrosterically-stabilized latices (also seen in other polymerically-stabilized systems) can be interpreted by assuming that aqueous-phase diffusion slower in the hairy layer.

Chain-Length-Dependent Termination Rate Processes in Free-Radical Polymerizations. 3. Styrene Polymerizations with and without Added Inert Diluent as an Experimental Test of Model

Experiments were performed to test a model for the kinetics of free-radical polymerization systems, including the dependence of the termination rate coefficients on the lengths of both chains involved. The model has few adjustable parameters, the values of which are moreover confined within fairly narrow limits. The data comprised the rate of polymerization in a seeded emulsion polymerization of styrene, with and without benzene as diluent, with initiation by persulfate and by gamma-radiolysis. The latter can be switched off instantly, providing relaxation data which are sensitive to termination kinetics. Data from a single relaxation at a fixed weight-fraction polymer (omega(p)) were fitted to fur the unknown parameters, of which the only significant one is the probability p of reaction between two radicals upon encounter, incorporating the effect of spin multiplicity; this must lie between 0.25 and 1. Modeling using the value so obtained then successfully fitted (a) relaxation data at the same omega(p) but with 15 mol % benzene diluent, (b) relaxation data with and without diluent over the range 0.5 less than or equal to omega(p) less than or equal to 0.8, and (c) chemically initiated data over the same omega(p) range. This provides convincing evidence for the correctness of the termination model, which calculates the termination rate coefficients between two chains from the Smoluchowski equation, incorporating p, with diffusion coefficients (as a function of chain length and of omega(p)) obtained from a ''universal'' scaling law inferred from NMR data, and where the interaction distance for termination is the van der Waals radius of a monomeric unit; contributions from ''reaction diffusion'' (whereby a chain end moves by propagating) are also important at high conversion. The data also support a model for initiator efficiency in emulsion polymerization, this model being based on competition between aqueous-phase propagation (to a sufficient degree of polymerization for surface activity) and termination.

Rate-Determining Kinetic Mechanisms in the Seeded Emulsion Copolymerization of Styrene and Methyl Acrylate

A kinetic study of the seeded emulsion copolymerization of styrene (S) and the relatively water-soluble methyl acrylate (MA) was carried out so as to investigate the mechanisms of radical entry into latex particles, radical exit from the latex particles, and the fate of radical species in the aqueous phase. Three seed latices of differing size were used, two of poly(S-co-MA) latices and one poly(S) latex. This way of determining kinetic parameters of emulsion copolymerization on a homopolymer seed (heteroseeded polymerization) is possible because the techniques used involve only a relative small change in conversion. This means that phase separation will not be a problem at the high monomer concentrations used and that composition drift is negligible. Conditions were such that the system followed zero-one kinetics (Smith-Ewart cases 1 and 2), which means that no latex particles contain more than one growing chain. Data were obtained for the steady-state rate of polymerization with persulfate as initiator and for the non-steady-state relaxation kinetics in γ-radiolysis initiation experiments. The latter kinetics are dominated by radical loss mechanisms such as radical exit and termination. It was possible to obtain a value for the chain transfer constant of styrene-terminated radicals to methyl acrylate from the molecular weight distributions. It was found that this copolymer system is retarded at very low conversions, possibly by oxygen. The data can be explained with the following mechanisms. Radical exit occurs via the same transfer - diffusion mechanism as found in a number of other systems, i.e., transfer of the radical activity to monomer(s) and subsequent desorption into the aqueous phase. These desorbed monomeric radicals re-enter the particles and either terminate or propagate therein. The entry efficiency of persulfate is low, with 50-95% of the persulfate-derived radicals undergoing termination in the aqueous phase. This is in accord with the mechanism for entry that states that the rate-determining events are aqueous-phase propagation and termination; radicals of a critical degree of polymerization enter a particle irreversibly and instantaneously. At high fractions of S in the S-MA system, although MA polymerizes quickly (high propagation rate constant), the occasional addition of S in the aqueous phase slows the propagation rate down considerably. Thereby it also decreases the probability of the attainment of a sufficiently high degree of polymerization for surface activity, perhaps 10-20 monomer units with the relatively water-soluble MA.

Polymer-Fraction Dependence of Entry Rate Coefficients in Emulsion Polymerization

Data on the rate of approach to steady state in a series of studies of the seeded emulsion polymerization of styrene yield the dependence of the rate coefficient for entry of free radicals into latex particles as a function of the weight-fraction polymer and hence of monomer concentration in the aqueous phase. The results are in accord with a model for the entry process (i.e., for the initiator efficiency) based on aqueous-phase propagation and termination being the rate-controlling events.

Entry of free radicals into latex particles in emulsion polymerization

A model, together with a comparison with previously published experimental data, is presented for initiator efficiency in seeded styrene emulsion polymerization systems in the absence of secondary particle formation. The data had shown that a number of previous models are inapplicable, viz., those assuming that the rate-determining step for free-radical entry into a particle is either diffusional capture, surfactant displacement, or colloidal entry. The data support the supposition that the rate-determining step for free-radical capture by latex particles is aqueous-phase propagation to a critical degree of polymerization, whereupon capture (irreversible adsorption) of the resulting oligomeric free radical by a particle is essentially instantaneous. Mutual aqueous-phase termination of smaller species also occurs. When account is taken of the fact that the rate coefficients for (a) the first aqueous-phase propagation step and (b) aqueous-phase termination are both in the diffusion limit, this model is in qualitative and quantitative accord with the experimental dependences of the entry rate coefficient on the concentrations of initiator, of surfactant, of aqueous-phase monomer, and of latex particles as well as on particle size and on ionic strength. For styrene emulsion polymerization initiated by persulfate, the critical oligomer size for entry was found to be dimeric.