Controlled radical polymerization miniemulsion systems

A review article by Qiu et al. [212] and references herein [217-226] covers NMCRP in miniemulsions up to 2001. Cunningham wrote a related review in 2002, also covering controlled radical polymerization in dispersed phase systems [227]. Here, the main results reported in the Qiu review will be summarized, and new developments in the field since then will be reviewed. 

Controlled radical polymerization techniques are suitable for synthesizing polymers with a high level of architectural control. Notably, they not only allow a copolymerization with functional monomers (as shown previously for free-radical polymerization), but also a simple functionalization of the chain end by the initiator. Miniemulsion systems were found suitable for conducting controlled radical polymerizations [58-61], including atom transfer radical polymerization (ATRP), RAFT, degenerative iodine transfer [58], and nitroxide-mediated polymerization (NMP). Recently, the details of ATRP in miniemulsion were described in several reviews [62, 63], while the kinetics of RAFT polymerization in miniemulsion was discussed by Tobita [64]. Consequently, no detailed descriptions of the process wiU be provided at this point. 

Living free-radical polymerization represents a promising technique to produce polymers with highly controlled structures. Different possible systems known from bulk polymerizations have been used in miniemulsions. The living free radical polymerization of, e.g., styrene via the miniemulsion approach allows one to eliminate the drawback of the bulk system where an increase in polydis-persity was found at high conversions due to the very high viscosity of the reaction medium [90]. 

Claverie et al. [325] have polymerized norbornene via ROMP using a conventional emulsion polymerization route. In this case the catalyst was water-soluble. Particle nucleation was found to be primarily via homogenous nuclea-tion, and each particle in the final latex was made up of an agglomeration of smaller particles. This is probably due to the fact that, unlike in free radical polymerization with water-soluble initiators, the catalyst never entered the polymer particle. Homogeneous nucleation can lead to a less controllable process than droplet nucleation (miniemulsion polymerization). This system would not work for less strained monomers, and so, in order to use a more active (and strongly hydrophobic) catalyst, Claverie employed a modified miniemulsion process. The hydrophobic catalyst was dissolved in toluene, and subsequently, a miniemulsion was created. Monomer was added to swell the toluene droplets. Reaction rates and monomer conversion were low, presumably because of the proximity of the catalyst to the aqueous phase due to the small droplet size. 

Living free radical polymerization is a unique technique used to prepare a variety of polymers with well-controlled molecular structures such as polymers with narrow molecular weight distribution, multiblock copolymers, and star polymers, which cannot be achieved by conventional free radical polymerization [77, 78]. This polymerization technique was originally investigated in homogeneous bulk and solution polymerization systems, and it has been successfully applied to heterogeneous miniemulsion polymerization system in the last decade [62,63]. 

Reversible atom transfer free radical polymerization of n-butyl acrylate was conducted in miniemulsion systems using the water-soluble initiator 2,2 -azobis(2-amidinopropane) dihydrochloride (V-50) and the hydrophobic ligand 4,4 -di(5-nonyl)-4,4 -bipyridine to form a complex with the copper ions [67, 80]. The resultant Cu(II) complex has a relatively large solubility in the continuous aqueous phase, but this should not impair its capability of controlling the free radical polymerization. This is because the rapid transport of the Cu(II) complex between the dispersed organic phase and the continuous aqueous phase assures an adequate concentration of the free radical deactivator. As a consequence, the controlled free radical polymerization within the homogenized monomer droplets can be achieved. 

RAFT in emulsion polymerization The development of RAFT in true emulsion polymerization processes was more challenging than in miniemulsion. A general difficulty of RAFT in aqueous dispersed systems, and particularly emulsion polymerization, is related to the need for a radical initiator in conjunction with the RAFT agent. Consequendy, it is not always easy to control the locus where reversible transfer will take place, and this may have important and sometimes deleterious consequences on the control over molar mass and MMD. Again, the most important parameters to consider are both the water solubility and the reactivity of the chain transfer agent. 

As discussed, miniemulsions are characterized by a rather large surface area, sufficient to alter the principal mechanism of nucleation. In miniemulsion polymerization, nucleation occurs predominantly by radical entry into monomer in the interior of the miniemulsion droplets. In addition, because of the improved physical stability of the miniemulsion droplets, it is possible to adjust the total surfactant concentration so as to limit the total number of micelles in solution, thereby limiting aqueous phase nucleation. For example, Fontenot and Schork studied the batch polymerization of methyl methacrylate with SDS as the surfactant and hexadecane as the secondary disperse phase component. The mechanism of particle nucleation could be altered from droplet control to micellar control as the concentration of SDS was increased. Relative to micelles, droplets can have higher radical numbers (due to their larger size), and can, therefore, significantly enhance the early stages of the polymerization process. The rate of polymerization per particle is faster, and the systems are converted faster. Following nucleation, the reaction proceeds with polymerization of the monomer in the miniemulsion droplets. Thus, there is no distinct Interval II. Droplet nucleations have been found... 

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