(255am) Modeling of Reverse Atom Transfer Radical Polymerization in Miniemulsion Initiated By a Water-Soluble Radical Initiator

Controlled Radical Polymerization (CRP) provides an efficient method for the synthesis of polymers with predetermined molecular weight, low polydispersity and controlled molecular architecture in mild reaction conditions. The latter feature has been one of the keys to its success with respect to the anionic polymerization, which requires stringent conditions.

One of the most employed CRP techniques is atom transfer radical polymerization (ATRP). Among its advantages one may mention the commercial availability of reactants, the simple production of polymers with specific tailored functionalities and the wide range of monomers and temperatures that may be used.^[1] These positive characteristics are the reason behind the intensive research that has recently been produced both in academia and industry. This body of work has led to the development of several procedures for initiating an ATRP.^{[2, 3]}

One of the procedures developed is reverse ATRP. It has emerged as a response to the handling problem in direct ATRP caused by the high air sensitivity of Cu^I. Reverse ATRP employs more stable Cu^II complexes in the initiation step and a conventional free radical initiator, making the system easier to handle and more compatible with industrial scale processes.^[4] Additionally, due to these features, reverse ATRP has been widely employed in dispersed systems.^[3]

Several studies on CRP have been conducted in homogeneous bulk and solution systems. Nevertheless, the need for making this technique more suitable for commercial use has led to investigate the feasibility of CRP processes in aqueous dispersed systems, such as emulsion and miniemulsion.^[4] Aqueous dispersions are a good alternative for large-scale production, since they provide several benefits, for instance excellent heat transfer, process flexibility and ease of mixing and handling of the final product. However, employing aqueous dispersed systems presents some complications, such as partitioning of species between aqueous and organic phases, exit of radicals from particles and poor colloidal stability, among others. Although the first attempts were with emulsion polymerization, many of them have failed, leading to the use of miniemulsions. They have demonstrated to be robust for the different CRP techniques,^[3] because droplet nucleation minimizes the requirement of mass transport of the control agent to the polymer particle.^[4]

Miniemulsion polymerization offers unique advantages over emulsion polymerization, since it allows the polymerization of highly water insoluble monomers and it is suitable for making particles containing additives (such as pigments or dyes). Nevertheless, a disadvantage of miniemulsions is the possible need to remove a low molecular weight hydrophobe from the final latex. This compound stabilizes the monomer droplets against diffusional degradation.^[3]

In this work, a mathematical model was developed for a reverse ATRP in miniemulsion using a water-soluble initiator. This model is able to predict average molecular properties, such as number and weight molecular weights, as well as the full molecular weight distribution (MWD).

The model is derived from the deterministic balance equations of the reacting species. Average properties are predicted using the well-known method of moments and the MWD is modeled using the probability generating function (pgf) technique. This technique has proven to be capable of predicting the MWD for several systems accurately and efficiently in terms of computational time, without making any simplifying assumptions or having any a priori information of the distribution shape.^[5-7]

The model was formulated and solved in gPROMS (Process Systems Enterprise, Ltd.). It takes into account the chemical reactions in the aqueous and organic phases, the entry of oligoradicals into the polymer particles as well as the partition of the catalyst and monomer in both phases. For the latter phenomenon, two different approaches have been used with similar results, showing that both are valid for representing the mass transfer across the interface in the system.

The results were validated using experimental information taken from the literature.^[4] Most of the kinetic and diffusion parameters were taken from the literature, with the exception of the ATRP activation and deactivation kinetic constants which were estimated from experimental data.^[4]Our results show that the predicted average molecular properties as well as the MWD agree well with the experimental information.

[1] W.A. Braunecker, K. Matyjaszewski, Prog. Polym. Sci.,32, 93 (2007).

[2] K. Matyjaszewski, Macromolecules, 45, 4015 (2012).

[3] M.F. Cunningham, Prog. Polym. Sci., 33, 365 (2008).

[4] M. Li, K. Matyjaszewski, Macromolecules, 36, 6028 (2003).

[5] M. Asteasuain, C. Sarmoria and A. Brandolin, Polymer, 43, 2513 (2002).

[6] C. Fortunatti, C. Sarmoria, A. Brandolin, M. Asteasuain, Comput. Chem. Eng., 66, 214 (2014).

[7] C. Fortunatti, C. Sarmoria, A. Brandolin, M. Asteasuain, Macromol. React. Eng., 8, 260 (2014).