The free radical distribution in emulsion polymerization using oil-soluble initiators (original) (raw)
Related papers
Polymer, 1988
For emulsion polymerization of a 'zero-one' system, the method of moments is applied to the model proposed by Lichti et al. to describe the particle size distribution (PSD). Using the explicit expressions so obtained for the first four moments, the average number of free radicals par particle, ~, and kinetic parameters of the system involving the rate coefficients for adsorption and desorption of free radicals, p and k, and the propagation rate coefficient, kp, can be obtained easily by use of PSD data without involving a complicated curve fitting procedure as was required in their work. Detailed calculations on the styrene system data of Lichti et al. show that both p and k are proportional to particle surface area. The result for k is opposite to that proposed by those workers, in which k was considered to be inversely proportional to the particle surface area. After further manipulation and approximation of the expressions so obtained, explicit expressions for the number-average volume, 6n, the standard deviation, tr, and the skewness, u~, in terms of surfactant and initiator levels, temperature and reaction time are also obtained. The effects of the variations of these variables on the three characteristic parameters of the PSD can be determined.
Radical capture efficiencies in emulsion polymerization
Journal of Polymer Science: Polymer Chemistry Edition, 1981
A theoretical procedure is developed that allows the importance of bimolecular termination in the aqueous phase of an emulsion polymerization to be determined. This shows that with sparinglysoluble and slowly propagating monomers like styrene, significant termination occurs in the aqueous phase at high initiator concentrations, as found experimentally. With more water soluble monomers, aqueous phase termination is likely to be small.
Emulsion polymerization: State of the art in kinetics and mechanisms
Polymer, 2007
Over decades of carefully designed kinetic experiments and the development of complementary theory, a more or less complete picture of the mechanisms that govern emulsion polymerization systems has been established. This required means of determining the rate coefficients for the individual processes as functions of controllable variables such as initiator concentration and particle size, means of interpreting the data with a minimum of model-based assumptions, and the need to perform experiments that had the potential to actually refute a given mechanistic hypothesis. Significant advances have been made within the area of understanding interfacial processes such as radical entry and exit into and out of an emulsion polymerization particle, for electrostatic, steric and electrosteric stabilizers (the latter two being poorly understood until recently). The mechanism for radical exit is chain transfer to monomer within the particle interior to form a monomeric radical which can either diffuse into the water phase or propagate to form a more hydrophobic species which cannot exit. Entry is through aqueous-phase propagation of a radical derived directly from initiator, until a critical degree of polymerization z is reached; the value of z is such that the z-meric species is sufficiently surface active so that its only fate is to enter, whereas smaller aqueous-phase radical species can either be terminated in the aqueous phase or undergo further propagation. For both entry and exit, in the presence of (electro)steric stabilizers, two additional events are significant: transfer involving a labile hydrogen atom within the stabilizing layer to form a mid-chain radical which is slow to propagate and quick to terminate, and which may also undergo b-scission to form a water-soluble species. Proper consideration of the fates of the various aqueous-phase radicals is essential for understanding the overall kinetic behaviour. Intra-particle termination is explained in terms of diffusion-controlled chainlength-dependent events. A knowledge of the events controlling entry and exit, including the recent discoveries of the additional mechanisms operating with (electro)steric stabilizers, provides an extension to the micellar and homogeneous nucleation models which enables particle number to be predicted with acceptable reliability, and also quantifies the amount of secondary nucleation occurring during seeded growth. This knowledge provides tools to understand the kinetics of emulsion polymerization, in both conventional and controlled/living polymerization systems, and to optimize reaction conditions to synthesize better polymer products.
Radical entry mechanisms in redox-initiated emulsion polymerizations
Polymer, 2005
The mechanisms and kinetics of radical entry in emulsion polymerizations utilizing redox initiation are investigated using polymerization rate data obtained by reaction calorimetry and electrospray mass spectroscopy analysis of initiator-derived aqueous-phase products. These data have been used to evaluate an initiation scheme for redox-initiated emulsion polymerizations of common monomers such as styrene and methyl methacrylate based around the oxidant, tert-butyl hydroperoxide. Redox initiators are broadly classed by the solubility of their radical products: Hydrophilic radicals enter by propagating to a critical degree of polymerization to become surface-active whilst more hydrophobic radicals may enter particles directly. When direct entry is applicable (the hydrophobic case), initiation efficiency will always be very high. q
Free radical exit in emulsion polymerization. I. Theoretical model
Journal of Polymer Science Part A: Polymer Chemistry, 1994
The exit or desorption of free radicals from latex particles is an important kinetic process in an emulsion polymerization. This article unites a successful theory of radical absorption (i.e., initiator efficiency), based on propagation in the aqueous phase being the rate determining step for entry of charged free radicals, with a detailed model of radical desorption. The result is a kinetic scheme applicable to true "zero-one" systems (i.e., where entry of a radical into a latex particle already containing a radical results in instantaneous termination), which is still, with a number of generally applicable assumptions, relatively simple. Indeed, in many physically reasonable limits, the kinetic representation reduces to a single rate equation. Specific experimental techniques of particular significance and methods of analysis of kinetic data are detailed and discussed. A methodology for both assessing the applicability of the model and its more probable limits, via use of known rate coefficients and theoretical predictions, is outlined and then applied to the representative monomers, styrene and methyl methacrylate. A detailed application of the theory and illustration of the methodology of model discrimination via experiment is contained in the second article of this series.
Effect of Pickering stabilization on radical entry in emulsion polymerization
Aiche Journal, 2018
The production of latexes stabilized by solid particles, so-called Pickering stabilizers, has attracted considerable attention due to its benefits, including the enhanced mechanical properties of the polymer films. Clays for instance were found to enhance particle stabilization in emulsion polymerization, in a comparable way to conventional surfactants. Their concentration thus determines the polymer particles size and number, and consequently the reaction rate. In this work, we investigate the impact of the presence of such rigid and big platelets at the polymer particle's surface on radical exchange between the aqueous phase and the polymer particles. It was found for the system underhand, that the average number of radicals per particle () was independent of the stabilizer layer. Therefore, a radical capture model independent of the clay concentration could be used to simulate reactions involving different clay concentrations and predict the evolution of the monomer conversion, particle size, and .
The role of aqueous-phase kinetics in emulsion polymerizations
Progress in Polymer Science, 1993
The kinetics of species in the aqueous phase control many events in emulsion polymerization: the rate of entry of free radicals into particles (equivalent to initiator efficiency), the rate of exit (desorption) of free radicals from particles, the fate of desorbed free radicals and of free-radical species derived directly from aqueous-phase initiator. Aqueous-phase kinetics also dominate particle nucleation and re-seeding (secondary nucleation), and the in situ formation of surfactant. The mechanisms of each of these events are discussed, and it is shown how general methods can be constructed to deduce the ratedetermining events for each of these. The methodology is then applied extensively to styrene, which leads to the following conclusions. (a) The aqueous-phase events which govern entry (initiator efficiency) are propagation and termination, with entry occurring irreversibly when a critical degree of propagation z is reached so that the resulting species (a di-or tri-styrenesulfonate species in the case of styrene with persulfate initiator) is sufficiently surface-active that, once adsorbed onto the particle it does not desorb before it propagates; the actual adsorption event is sufficiently rapid so as not to be rate-determining except during nucleation. (b) Exit of free radicals is governed by transfer inside the particle to form a monomeric radical which may desorb and diffuse irreversibly away from the parent particle before it propagates therein. (c) The fate of desorbed free radicals in the wide range of styrene systems examined is to re-enter another particle and remain therein, rather than the other possible fates (aqueous-phase termination or re-exit until intra-particle termination eventually occurs). (d) Below the cmc, nucleation is by the homogeneous-coagulative mechanism, while above the cmc, nucleation is through a process which combines the essential features of both homogeneous-coagulative and micellar-entry models. (e) Analysis of the aqueous-phase products produced in an emulsion polymerization shows that the species involved in termination, entry and exit also undergo subsequent reactions: hydrolysis and reaction with persulfate.
Polymer-Plastics Technology and Engineering, 2005
An alternative approach to model emulsion polymerization is presented that is capable of rigorously solving both particle and radical kinetics for emulsion polymerization: the explicit radical-particle size distribution approach. The method is based on a direct solution of all population balances and fully covers the strong influence of compartmentalization on rates of reactions between macroradicals and, consequently, on chain length averages. An essential and new feature is the compartmentalization factor (D f), which accounts for compartmentalization in a transparent manner. The generic approach allows for studying the complete emulsion polymerization conversion range, including gel-effect, and the effect of various parameters on both chain length and particle size distribution. Well-known kinetic regimes for emulsion polymerization naturally arise as limiting cases from our model. The dynamic behavior of the model was studied by simulating several realistic seeded emulsion polymerization reactions for styrene. The model dealt with compartmentalization accurately and was able to correctly reproduce the dynamic behavior known to be typical for emulsion polymerization.
Polymer-Plastics Technology and Engineering, 2005
The objective of the present paper is to demonstrate that the explicit radical-particle size distribution approach correctly predicts the effect of compartmentalization on the overall reaction rates and therefore chain length averages. Modeling results for the seeded emulsion polymerization of styrene were compared with experimental results. Several experiments were carried out with systematically varied compartmentalization of radicals by varying seed latex particle numbers and the amount of initiator. The overall polymerization rate was measured using reaction calorimetry and the final particle size distribution was measured using Transmission Electron Microscopy. The results demonstrated that the model is able to predict successfully the rate of polymerization and particle size distributions as a function of time for all recipes. This proves that the model deals correctly with the effect of compartmentalization on overall reaction rates and thus on chain length averages. The work described in this paper demonstrates that the explicit radical particle size distribution approach is a powerful method for predicting emulsion polymerization kinetics and product properties, such as particle size distributions and chain length distributions.
Model Discrimination of Radical Exit in Emulsion Polymerisation
2011
Analysis of published experimental data on monomeric radical diffusion in the emulsion polymerisation of styrene shows that it can be quantitatively described equally well by non-equilibrium diffusion from particles, where all parameters are derived from properties of the discrete phase, or by steady-state diffusion where all parameters are derived from properties of the continuous phase. The non-equilibrium model better describes an observed experimental trend to a reduced desorption rate coefficient at higher weight fraction of polymer in the particles. The theoretical upper bound of the non-equilibrium model is also higher than the theoretical upper bound of the steady-state model allowing fits to experimental data which must be discarded as anomalous in the continuous phase model.