Living/Controlled Radical Polymerizations in Dispersed Phase Systems

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Prog. Polym. Sci. 27 *2002) 1039±1067 www.elsevier.com/locate/ppolysci

Living/controlled radical polymerizations in dispersed phase systems

Michael F. Cunningham*

Department of Chemical Engineering, Queen's University, Kingston, Ont., Canada K7L 3N6 Received 5 November 2001; revised 25 January 2002; accepted 28 January 2002

Abstract Living/controlled radical polymerization provides a route to synthesizing materials with designed microstructure and narrow molecular weight distributions. A variety of living radical systems have been developed in recent years, and are based on either reversible termination *SFRP, ATRP) or reversible transfer mechanisms *RAFT, degenerative transfer). Application of living radical polymerization to heterogeneous systems such as emulsion and miniemulsion polymerization may provide process and economic advantages over the traditional homogeneous bulk and solution polymerizations. However, adaptation of living radical chemistry to aqueous dispersions poses several challenges relating to maintaining effective control over the growth of living chains. These challenges originate from having two or even three phases in the reaction mixture, which can lead to issues related to phase partitioning of the controlling agent, transport of the controlling agent between phases, the role of aqueous phase kinetics, and the phenomena of particle nucleation and colloidal stability. This review examines recent progress in this area, with an emphasis on unresolved issues and future opportunities. q 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Living radical polymerization; Controlled radical polymerization; Stable free radical polymerization; Atom transfer radical polymerization; Reversible addition±fragmentation transfer; Degenerative transfer; Emulsion polymerization; Mini- emulsion polymerization

Contents 1. Introduction ...... 1040 1.1. Scope of the review ...... 1040 1.2. Aqueous dispersed phase polymerizations ...... 1040 1.2.1. Emulsion polymerization ...... 1040 1.2.2. Miniemulsion polymerization ...... 1042 2. Living radical polymerizations in dispersed aqueous systems ...... 1043

* Tel.: 11-613-533-2782; fax: 11-613-533-6637. E-mail address: [email protected] *M.F. Cunningham).

2.1. Overview of living radical polymerization ...... 1043 2.2. Aqueous dispersed systems with reversible termination ...... 1046 2.2.1. Stable free radical polymerization ...... 1046 2.2.2. Atom transfer radical polymerization ...... 1053 2.3. Aqueous dispersed systems with reversible transfer ...... 1057 2.3.1. Overview ...... 1057 2.3.2. Reversible addition±fragmentation transfer ...... 1058 2.3.3. Degenerative transfer using iodine exchange ...... 1061 3. Block copolymer synthesis ...... 1063 4. Concluding remarks ...... 1063 Acknowledgements ...... 1064 References ...... 1064

1. Introduction

`Living' or `controlled' radical polymerizations provide a novel and potentially inexpensive route to designing polymers with controlled microstructure. Extensive research has been conducted into homogeneous bulk and solution living radical polymerizations, but investigations into aqueous dispersed phase systems *emulsion and miniemulsion polymerization) have only recently appeared. There are a number of incentives to use emulsion/miniemulsion polymerization on a commercial scale, including ease of mixing and good heat transfer. Furthermore, there currently exists substantial investment in emulsion polymerization facilities throughout the world, and therefore there is considerable interest in adapting living radical polymerizations to emulsion-based polymerization systems.

1.1. Scope of the review

This review summarizes recent progress in living/controlled radical polymerizations conducted in dispersed aqueous systems *emulsion and miniemulsion polymerization), with the emphasis on those aspects of operating in a heterogeneous environment that in¯uence the polymerization rate, the molecular weight distribution and the livingness of the system. The important living radical chemistries are brie¯y reviewed but it is not within the scope of this review to provide a comprehensive summary of recent activity in those ®elds. More details of the various living radical systems can be found in the references of the papers referred to in this review. Related reviews of interest have been written by Qiu et al. [1] and Claverie and Kanagasabapathy [2].

1.2. Aqueous dispersed phase polymerizations

1.2.1. Emulsion polymerization Emulsion polymerization yields polymer particles in the 50±500 nm range, starting from an oil-in- water dispersion of monomer droplets in an aqueous surfactant solution. It is a widely used industrial process employed to make coatings, paints, adhesives and resins. Monomers used in emulsion polymerization are typically only sparingly soluble in water *e.g. styrene, acrylates, methacrylates) although a few percent of water-soluble comonomers such as acrylic/methacrylic acid are often added to enhance stability. Surfactants can be either ionic or nonionic. Anionic surfactants such as sodium dodecyl sulfate M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1041

Nomenclature AB degenerative transfer agent

ARi polymer molecule of length i terminated by a degenerative transfer fragment ARj polymer molecule of length j terminated by a degenerative transfer fragment Bz radical generated from degenerative transfer agent

Di dead polymer molecule of chain length i Di1j dead polymer molecule of chain length i 1 j FRj dithioester as a reversible addition±fragmentation chain transfer *RAFT) agent carrying a polymer molecule of chain length j I initiator M monomer M1z 1-mer radical generated by thermal initiation M2z dimer radical generated by thermal initiation Mtn/ligand transition metal complex for atom transfer reaction, without the halide R0z initiator radical Riz polymer radical of chain length i Ri11z polymer radical of chain length i 1 1 RiF z Rj RAFT agent in radical from, carrying two polymer molecules of chain length i and j, respectively Rjz polymer radical of chain length j Tz nitroxide radical TH hydroxylamine

TRi polymer molecule of chain length i capped by a nitroxide radical XRi alkyl halide of chain length i X-Mtn11/ligand transition metal complex for atom transfer reaction, with the halide

*SDS) or sodium dodecyl benzene sulfonate *SDBS) are most commonly used in research studies but nonionics are usually added to commercial formulations to provide greater colloidal stability. A typical emulsion polymer formulation contains an aqueous phase, consisting of surfactant above its critical micelle concentration *CMC), usually a water-soluble initiator such as potassium persulfate *KPS) although monomer-soluble initiators can also be used, and an organic phase consisting of monomers dispersed in 1±20 mm droplets. The aqueous phase may be buffered. Because the surfactant concentration is above the CMC, a high concentration of monomer-swollen micelles is also present in the aqueous phase. Upon heating, initiator decomposes to give aqueous phase radicals that propagate with small amounts of monomer dissolved in the aqueous phase *the aqueous phase is saturated with monomer). After adding a few monomer units, the aqueous oligoradicals become suf®ciently hydrophobic to enter micelles, thereby initiating particles. *For styrene, the aqueous phase radicals need to be 2±3 units long to enter micelles or particles [3].) Particle nucleation continues until all micelles have either been nucleated to form particles, or dispersed to stabilize the growing particle surface area. Particles can continue to be nucleated, albeit at a lower rate, by homogeneous nucleation as long as monomer droplets exist in the system [4±6]. Once the micelles have been depleted, the particles continue to grow. Monomer droplets function as reservoirs, with monomer diffusing through the 1042 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 aqueous phase to the particles. As long as monomer droplets are present, equilibrium swelling of the particles with monomer is maintained. Particles continue to grow until the monomer droplets are depleted, after which the remaining monomer in the particles is polymerized. During polymerization, small radicals can exit from the particles thereby lowering the number of radicals per particle and therefore the reaction rate. The probability of exit of a small radical depends on its water solubility and how it partitions between the particles and aqueous phase. Radicals formed from transfer to monomer can often exit but the probability of exit decreases rapidly as monomer units are added by propagation. A distinguishing feature of conventional emulsion polymerizations is `compartmentalization' of the propagating radicals, which profoundly affects both the reaction rate and molecular weight. When polymerizations are conducted in dispersed aqueous systems in which the particle size is relatively large such as suspension polymerization *20±1000 mm), the kinetics and molecular weight are very similar to bulk reactions. Essentially the particles act as miniature bulk reactors. However, as the particle size decreases below approximately 1 mm, the particle volume becomes suf®ciently small that the kinetics change. Propagating radicals are now isolated from each other, or compartmentalized, to such a degree that termination reactions between two growing chains becomes less likely. For many systems, including styrene under typical conditions, each particle contains either zero or one growing chain at any time, giving an average number of radicals per particle n of 0.5. The entry of an aqueous oligoradical into a particle with one propagating radical results in instantaneous termination. *As particle size increases, the average number of radicals per particle can increase.) The overall effect of compartmentalization is an increase in reaction rate and a much higher mean molecular weight as compared to bulk polymerization because of the impact of reducing the effective termination rate. The polymerization rate is directly proportional to n and the number of particles. Particle diameter therefore also in¯uences the rate. For a more detailed description of emulsion polymerization, the reader is referred to Refs. [3,7].

1.2.2. Miniemulsion polymerization Miniemulsion polymerization *Chapter 20in Ref. [7]) shares many similarities with emulsion polymerization, including compartmentalization. The ®nal product is a latex with the same particle size as emulsion polymerization but the particle nucleation mechanism is quite different. In miniemulsion polymerization, a costabilizer *usually highly water-insoluble hydrophobes such as long chain alkanes) is added to the monomer, and the initial reaction mixture is subjected to very high shear, creating monomer droplets of 50±500 nm. The presence of the hydrophobe stabilizes the monomer droplets against diffusional degradation *Ostwald ripening). The aqueous phase, as in emulsion polymerization, contains surfactant and initiator. Surfactant concentration is maintained below the CMC to avoid micellar nucleation. When the miniemulsion is heated and initiator thermally decomposes, the monomer droplets are nucleated by entering aqueous oligoradicals. Ideally, all monomer droplets become particles and no new particles are nucleated by homogeneous nucleation, although some homogeneous nucleation usually occurs and not all droplets become particles. Droplet nucleation leads to simpler kinetic behavior by largely eliminating the particle nucleation step. Miniemulsion polymerization, unlike emulsion polymerization, can be used to produce composite particles since additives such as dyes, pigments and other solids or water-insoluble materials can be added to the monomer prior to dispersion. A disadvantage of miniemulsions is the possible need to remove low molecular weight hydrophobe M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1043 from the ®nal product, but it has been shown that polymers can also perform satisfactorily as costabilizers [8].

2. Living radical polymerizations in dispersed aqueous systems

2.1. Overview of living radical polymerization

Living radical polymerizations can be classi®ed as having the chain growth controlled by either reversible termination or reversible transfer. Reversible termination mechanisms, such as nitroxide- mediated polymerizations *stable free radical polymerization *SFRP) [9] *Scheme 1) and atom transfer radical polymerization *ATRP) [10] *Scheme 2)) use a controlling agent that reacts reversibly with a propagating polymeric radical to yield a dormant chain. The equilibrium is shifted strongly toward the dormant species so that the active *propagating) radical concentration is lower than conventional radical polymerizations. Because propagation is ®rst order with respect to radical concentration, while irreversible biradical termination is second order, the lower radical concentration results in a signi®cantly reduced termination rate that preserves the living character of the chains. Some irreversible biradical termination inevitably occurs, leading to dead chains and a broadening of the molecular weight distribution. In addition, irreversible termination also results in an increase in the concentration of the controlling agent, driving the equilibrium toward the dormant state, and thereby lowering the radical concentration and reaction rate. In SFRP and ATRP, conditions are chosen so that all the chains are initiated within a few minutes at the start of the reaction, although the generation of new chains during polymerization by thermal initiation may be important, especially with styrene. Reversible transfer mechanisms such as reversible addition±fragmentation transfer *RAFT) [11] *Scheme 3) and degenerative transfer *Scheme 4) employ a chain transfer agent that reacts with a

Scheme 1. Stable free radical polymerization. 1044 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067

Scheme 2. Atom transfer radical polymerization. propagating macroradical. The reversible reaction is between a dormant chain and an active radical, in which an end group originating from the transfer agent is exchanged between the two chains. In degenerative transfer, there is direct exchange involving, for example, an iodine atom *DT1). With RAFT, an addition±fragmentation process is used to exchange a moiety such as a dithioester between the two chains. Macromolecular design via interchange of xanthate *MADIX) [12] is similar to RAFT but uses the exchange of xanthates. With reversible transfer mechanisms, a conventional initiator is used to initiate the chains. The transfer agent is then consumed by the radicals originating from initiator decomposition. The total number of chains in the system is the sum of transfer agent and primary radical molecules. To maximize livingness, there should be a large excess of transfer agent to initiator. A highly active transfer agent is rapidly consumed *within a few percent monomer conversion), while a less active transfer agent may take most of the polymerization to be consumed. If consumption is rapid, there will

Scheme 3. Reversible addition±fragmentation transfer polymerization. M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1045

Scheme 4. Degenerative transfer. be few dead chains resulting from irreversible termination, resulting in a narrower molecular weight distribution. If transfer agent consumption is slow, a broader distribution and more dead chains result. Because a conventional initiator is used, new chains are continually being created as long as initiator remains. Higher temperatures lead to faster initiator decomposition so that all chains are created within a narrower time frame, yielding a narrower molecular weight distribution. The average degree of polymerization is given by Eq. *1) [13] M 2 M DP 0 1 AB0 2 AB 1 1 I0 2 I where DP is the degree of polymerization; [M]0, the initial monomer concentration; [M], the monomer concentration at any given time; [AB]0, the initial transfer agent concentration; [AB], the concentration of transfer agent at any given time; [I]0, the initial concentration of initiator; [I], the initiator concentration at any given time; 1 is the initiator ef®ciency. As with reversible termination mechanisms, some irreversible termination inevitably occurs, leading to a broadening of the molecular weight distribution. However, unlike reversible termination, the rate is not consequently suppressed. Because the reversible step is transfer, and not termination, the concentration of radicals is not affected compared to a conventional free radical polymerization. The fundamental differences between reversible termination and reversible transfer mechanisms assume a central role in how each type of polymerization behaves in emulsion/miniemulsion polymerization. The most important difference relates to compartmentalization. With reversible termination, the radical concentration is lower than in bulk. In addition, the deactivation step of a growing radical is very fast *close to diffusion controlled) and is therefore comparable to termination rate coef®cients. However, the higher concentration of deactivating species compared to radicals means deactivation strongly dominates over irreversible termination. A consequence of this reduced importance of irreversible termination is that reducing the particle size no longer leads to compartmentalization effects as it does in conventional free radical polymerization. Therefore, many of the advantages of emulsion/miniemulsion polymerization *increased rate and higher molecular weight) are not expected to be realized in 1046 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 reversible termination systems because compartmentalization is not important. However in reversible transfer, the radical concentration is not affected, and therefore compartmentalization is maintained. Butte et al. [14] have presented a quantitative analysis of this issue.

Although the linearity of the relationship between Mn and conversion is often interpreted as evidence that a system is living, all it really requires is that the total number of chains is unchanged and that some of the chains *and possibly only a single chain) are living. Plots of Mn versus conversion should be examined together with the polydispersity, although polydispersity is not very sensitive to the formation of small numbers of dead chains.

2.2. Aqueous dispersed systems with reversible termination

SFRP and ATRP, although similar in many aspects, behave differently in emulsion/miniemulsion polymerization because of differences in the reactions occurring in the aqueous phase. Accordingly recent progress in these areas will be reviewed separately.

2.2.1. Stable free radical polymerization

2.2.1.1. Overview. While relatively little has been published about SFRP systems in emulsion, the following general behaviors are expected since they are established principles in emulsion polymerization and/or SFRP systems. In an SFRP emulsion polymerization, nitroxide distributes between the monomer droplets, aqueous phase and micelles. As new particles are nucleated, there is a thermodynamic driving force for the nitroxide to diffuse from the droplets through the aqueous phase to the particles. Upon initiator decomposition in the aqueous phase, oligomeric radicals grow in the aqueous phase until they reach suf®cient length to enter a micelle or particle, or they react with the nitroxide to yield a dormant chain that either remains water-soluble or enters a micelle or particle. An oligoradical that enters a micelle or particle can propagate, become deactivated by reaction with nitroxide or terminate with another radical. The water-solubility of the nitroxide, or more accurately the partition coef®cient and aqueous phase diffusivity, will determine if it can diffuse to the particles suf®ciently quickly to maintain its concentrations among the three phases *droplets, particles, aqueous phase) at thermodynamic equilibrium. For most nitroxides, diffusion rates are likely fast enough that equilibrium is attained, however a greater concern is nitroxide partitioning between the phases. SFRP reactions are quite sensitive to the free nitroxide concentration, and loss of a portion of the nitroxide into the aqueous phase from the reaction locus *particles) can seriously affect the course of the polymerization. Free nitroxide in a particle remains capable of exiting that particle unless it reacts with a growing macroradical of suf®cient length to render it insoluble in the aqueous phase. However, when the chain is activated and the nitroxide is released, it is again capable of exiting into the aqueous phase. Depending on the water-solubility of the monomer, a nitroxide-capped radical may be capable of exit if it is ®xed to a short oligomeric radical. The fate of living but dormant oligomeric radicals in the aqueous phase is more complex. If the monomer or living oligomer concentration in the aqueous phase is high enough, propagation of the dormant chain can continue in the aqueous phase until the macroradical becomes hydrophobic and enters a particle. If however aqueous monomer concentrations are low, propagation may be slow with the result that those chains remain as oligomers in the aqueous phase *they do not enter particles) thereby reducing the apparent number of chains in the system. Aqueous oligoradicals may also undergo irreversible termination with other aqueous radicals. M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1047 A miniemulsion polymerization will not have the issues related to particle nucleation *assuming homogenous nucleation is not a concern) nor transport of the nitroxide through the aqueous phase. Partitioning of the nitroxide between the aqueous and organic phases, and the phenomena of entry and exit remain as potential issues, however. An excellent theoretical discussion of nitroxide-mediated polymerizations in miniemulsion is presented by Charleux [15]. It should be noted that aspects of the discussion are relevant only for systems for which thermal initiation is not important.

2.2.1.2. SFRP in emulsion polymerization. Heterogeneous SFRP reactions have been reported in suspension [9,16], emulsion [17±20,55,56] and miniemulsion [20±34,57,58]. Gabaston et al. [35] conducted dispersion polymerization of styrene using nitroxides. Bon et al. [17] reported the ®rst attempt to conduct a nitroxide-mediated polymerization in emulsion. Using seeded styrene emulsion polymerization, they were able to achieve near complete conversion using an alkoxyamine *1-tert-butoxy-2-phenyl-2-*1-oxy- 2,2,6,6-tetramethylpiperidinyl)ethane) at 125 8C, albeit at a low rate *36 h reaction time). As conversion progressed, their molecular weight distributions displayed broadening to lower molecular weights. This evidence of bimolecular termination was attributed to system heterogeneity effects *e.g. partitioning, compartmentalization) and possibly the additional radical generation caused by an enhanced Diels±Alder *thermal initiation) reaction. Experimental molecular weights were lower than theoretically expected values. Polydispersities ranged from 1.41 to 1.54, but this included the seed polymer and therefore the molecular weight distribution re¯ected the combined contributions from the seed polymer and the polymer produced in the presence of nitroxide. Bon et al. also observed that molecular weight distribution broadening occurred after complete conversion if the reaction time was extended, indicating bimolecular termination due to instability of the alkoxyamine bond. Marestin et al. [18] reported the ®rst ab initio nitroxide-mediated styrene emulsion polymerization. They experienced considerable dif®culty in ®nding conditions that yielded a stable latex and reasonable conversions. They noted that most common surfactants decompose under harsh conditions *prolonged reaction times at high temperatures). Using KPS as initiator and SDS as surfactant at 130 8C, they found the addition of hexadecanol improved latex stability. Hexadecanol addition likely gives this system some features of a miniemulsion polymerization. Experiments were attempted with several different nitroxides but most gave a coagulated latex and/or very low molecular weight and conversion. The best results were obtained with 4-amino-2,2,6,6-tetramethyl-1-piperidinyloxy *4-amino-TEMPO), where

69% conversion was obtained in 36 h *Mn 6000; polydispersity 1.7). Use of an amino alkoxyamine instead of 4-amino-TEMPO, gave 69% conversion in the same time but yielded a higher molecular weight to Mn 42 000 *polydispersity 1.7). These polydispersities are much higher than typical values for bulk SFRP polymerizations *1.1±1.3). Using four differently substituted TEMPO derivatives *all in the four position), Cao et al. [19] were able to achieve reasonably well-controlled reactions in styrene emulsion polymerization. By varying the substituent, the water-solubility of the nitroxide was also varied, although no values for partition coef®cients were measured. Using either KPS or azobisisobutyronitrile *AIBN) at 120 8C with SDS, signi®cant differences were observed for the different nitroxides. Very low water solubility of the nitroxide resulted in an uncontrolled polymerization, while too high water solubility resulted in slow aqueous phase initiation that hindered the polymerization rate. Using 4-acetoxy-2,2,6,6-tetramethyl-1- piperidinyloxy *ATEMPO), 81% conversion was reached in 12 h, giving a polymer with Mn 18 000 and polydispersity ,1.3. The latex was stable, and particle sizes were typically ,100 nm. However, it is important to note that complete particle size distributions were not reported, and therefore it is premature 1048 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 to conclude that emulsion polymerization was successfully conducted. It is interesting to note that a small portion *,2±10%) of high molecular weight polymer was also produced. The proportion of the high molecular weight was higher at low conversions, and was probably caused by some particles being nucleated without nitroxide. However, as time progressed evidently there was enough mobility of the nitroxide between particles to reduce or eliminate the uncontrolled propagation. At present, a successful SFRP emulsion polymerization system has not yet been demonstrated. While living polymers have been made in some cases, unsolved problems related to colloidal stability remain as major issues. It will be seen in Section 2.2.1.3 that much greater progress has been made with SFRP using miniemulsions.

2.2.1.3. SFRP in miniemulsion polymerization. Prodpran et al. [21,24] and Macleod et al. [22] were the ®rst to report nitroxide-mediated styrene polymerization in miniemulsion. Prodpran et al. used benzoyl peroxide *BPO) and TEMPO with hexadecane as a costabilizer at 125 8C, and were able to produce stable latexes with over 90% conversion in 12 h. Molecular weights reached as high as Mn , 40 000, while polydispersities ranged from ,1.15 to 1.6. Camphorsulfonic acid was added to some runs to accelerate the polymerization rate. A polymerization done in bulk *with the same amounts of BPO and TEMPO) was observed to proceed much faster than in miniemulsion, which the authors attributed to monomer segregation in droplets and diffusion of active species to the aqueous phase. The molecular weight varied linearly with conversion, signifying they did have a living polymerization. They also noted the dif®culty of producing a colloidally stable latex in this temperature range. MacLeod et al. used the water-soluble initiator KPS and TEMPO in their studies. They were able to achieve up to 87% conversion within 6 h without using rate accelerants such as CSA. The polydispersities they reported were consistently in the range 1.1±1.2, as low as is usually achieved in bulk. A long chain alkoxyamine, or TEMPO-terminated oligomeric polystyrene *TTOPS) was used in the miniemulsion polymerization of styrene by Pan et al. [25]. The use of nitroxide-terminated oligomers

*Mn 7050) allowed the number of initial chains to be controlled. Experiments were run at 125 8C, using hexadecane as costabilizer and the anionic surfactant Dowfax 8390. With either 5 or 20% TTOPS, approximately 70% conversion was reached in 12 h *Fig. 1). The nitroxide-mediated runs were signi®- cantly slower than a conventional miniemulsion experiment run under the same conditions. Theoretical molecular weights were found to be always higher than experimental values *Fig. 2), and the difference

Fig. 1. Conversion versus time behavior of styrene miniemulsions with varying levels of TTOPS. Temperature 125 8C. From Ref. [25]. Reproduced with permission of American Chemical Society. M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1049

Fig. 2. Experimental and theoretical number average molecular weight as a function of conversion for styrene miniemulsions with varying levels of TTOPS. Temperature 125 8C. From Ref. [25]. Reproduced with permission of American Chemical Society. increased with conversion. It was theorized this could be due to an increasing number of chains arising from thermal initiation, and/or the formation of low molecular weight polymer from irreversible termination. Thermal initiation and irreversible termination were also believed to be the causes of the observed increase in polydispersity during polymerization. For 5% TTOPS, the polydispersity increased from 1.24 at low conversions to 1.76 at 76% conversion, while for 20% TTOPS polydispersity increased from 1.24 to 1.86 at 77% conversion. However, the 20% TTOPS run exhibited lower polydispersity for most of the run. This was attributed to better control arising from a higher free nitroxide concentration that would occur at higher initial loadings of the alkoxyamine. The average number of radicals per particle was calculated as ,0.003±0.004, well below the typical value of ,0.5 for conventional styrene emulsion/miniemulsion polymerization. Lansalot et al. [20] conducted batch emulsion and miniemulsion styrene polymerizations at temperatures below 100 8C using the acyclic phosphonylated nitroxide SG1 *N-tert-butyl-N-*1-diethylphos- phono-2,2-dimethylpropyl)). SG1 has a larger equilibrium constant than most other nitroxides, including TEMPO, and is therefore suitable for polymerization temperatures as low as ,90 8C *SG1: K 1.9 £ 1028 mol dm23 at 125 8C; TEMPO: K 2.1 £ 10211 mol dm23 at 125 8C). Experiments were run with the oil-soluble initiator AIBN and with an aqueous redox initiating system *K2S2O8/Na2S2O5). Runs with AIBN gave fair polydispersities *1.6) but reached a limiting conversion at ,60% conversion, probably because at 90 8C the rate of thermal polymerization is negligible and thus excess nitroxide accumulates in the system, leading to rate suppression. When the redox system was used, the polymerization rates were much greater but the polydispersity was .2, suggesting nonliving behavior.

A more detailed study of styrene miniemulsion polymerization was done using SG1 with the Na2S2O5/ K2S2O8 redox initiating system [23]. Using a nitroxide/K2S2O8 ratio of 1.2, several factors were studied, including initiator concentration, pH, and the monomer/water ratio. At this nitroxide/initiator ratio, there is a de®ciency of nitroxide with respect to the number of primary radicals that can be produced from 1050 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 initiator decomposition. Most experiments showed relatively high polydispersities *.1.5) that were attributed to nitroxide partitioning into the aqueous phase, thereby leaving a low nitroxide concentration in the particles. When the monomer/water ratio was increased, polydispersities improved signi®cantly due to more favorable partitioning. The effect of pH was also examined in a series of runs [23]. Stability of the SG1 nitroxide is poor at low pH, and reactions conditions that resulted in low pH led to degradative redox reactions that likely partially consumed the nitroxide and the initiating system. The results were a faster reaction rate and higher polydispersity, signifying loss of control over the reaction.

Stability of the SG1 was improved signi®cantly by buffering the aqueous phase, with a K2CO3 buffer being more effective than a NaHCO3 buffer. Reaction rates were generally high, with ,50±100% conversion achieved in 8 h, but the broad polydispersities indicate poor control for many of the reactions. Farcet et al. attributed these observations to nitroxide partitioning into the aqueous phase. The calculated average number of radicals per particle was ,0.13±0.23, much higher than values reported by Cunningham et al. [28] and Pan et al. [25], and consistent with low SG1 concentrations in the organic phase. Extensive discussion on the effects of initiator concentration was also reported. Because of the heterogeneous nature of the system, these effects are complex and dif®cult to generalize. Cunningham et al. [28] studied the effects of initiator concentration and the TEMPO/initiator ratio in KPS-initiated styrene miniemulsion polymerizations run at 135 8C. A strong interaction effect was observed between these two experimental variables. Using TEMPO/KPS ratios of 1.7 and 4.0, the initiator concentration was varied threefold. After 6 h reaction time, conversion varied from ,35 to 50% with induction periods observed at the higher TEMPO/KPS ratio. Final polydispersities were consistently low, ranging from 1.15 to 1.3. Because of concern about the monomer droplet stability at the high temperature used, the droplet size distribution was monitored for 6 hours in a nonreacting *inhibited) system. Droplet stability was reasonably good, showing an increase in mean diameter from 151 to 188 nm over 6 h.

Plots of 2ln*1 2 x) versus time were linear once the induction period had ended, and Mn was also linear with respect to conversion [28]. When the TEMPO/KPS ratio was low *1.7) and a high KPS concentration was used, a high initial polydispersity *1.7) and a rapid early increase in Mn were observed, although the ®nal polydispersity was ,1.3. A high radical ¯ux in the aqueous phase with some apparent uncontrolled polymerization, possibly due to insuf®cient nitroxide availability in the aqueous phase, may be the cause. The observed interaction effect was manifested in seemingly anomalous behavior. For example, increasing initiator concentration threefold lead to a relatively small difference in conversion after 6 h for the lower TEMPO/KPS ratio of 1.7, but a much larger difference at TEMPO/KPS 4.0. Furthermore, increasing the TEMPO/KPS ratio had a fairly small effect at the low initiator concentration, but a much more pronounced effect at the higher initiator concentration. Because of this strong interaction between the KPS concentration and the TEMPO/KPS ratio, the effects of changing either variable can depend strongly on the value of the other variable. Initiator ef®ciencies were found to be considerably higher than in conventional KPS-initiated styrene emulsion or miniemulsion polymerizations. Increasing the TEMPO/KPS ratio generally increases the initiator ef®ciency. A TEMPO/KPS ratio of 4.0gave initiator ef®ciencies ,1, signifying virtually all of the KPS-derived radicals initiated chains. In contrast, at TEMPO/KPS 1.7, ef®ciencies are lower *0.63±0.78) but still high compared to typical values of ,10% or less for styrene emulsion polymerization [3]. A decrease in ef®ciency with initiator concentration at a given nitroxide/initiator ratio will directly impact the free nitroxide concentration by effectively increasing the moles of nitroxide available per mole of living chains, thus suppressing the rate as was observed experimentally. Typical n values were ,1022, well below those in classical emulsion or M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1051 miniemulsion polymerization, and similar to values reported by Pan et al. [25]. A more extensive study in which the water solubility of both initiator and nitroxide were systematically varied has shown that initiator ef®ciency, number of chains, concentration of active chains, reaction rate and polydispersity can all be signi®cantly affected by the choice of initiator and the partitioning properties of the nitroxide [32,33]. At an initiator/nitroxide ratio of 1.7:1, for which there will be negligible excess nitroxide, little difference is observed in the conversion±time pro®les using various initiators *BPO, KPS) and nitroxides *TEMPO, OH-TEMPO) since the kinetics are dominated by thermal initiation. However, differences are observed in the molecular weights *i.e. number of chains initiated), particularly when the nitroxide is varied. Evidence suggests that more water-soluble OH-TEMPO-terminated oligomers may take longer to enter particles in the early stages of polymerization. The effect is more pronounced when KPS is used as initiator.

2.2.1.4. Nitroxide partitioning. Ma et al. [26] studied nitroxide partitioning in styrene±water systems. They reported partition coef®cients for three of the most commonly used nitroxides *TEMPO, 4-amino- TEMPO and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy *4-hydroxy-TEMPO)) in styrene±water mixtures at temperatures ranging from 25 to 135 8C *Table 1). The effects of hexadecane, surfactant *SDBS) and polystyrene on the partition coef®cients were studied and found to be negligible. At 120 8C, the partition coef®cient K for TEMPO is 110*[moles/l TEMPO in styrene]/[moles/l TEMPO in water]), indicating that most of the TEMPO resides in the styrene phase. In contrast, 4-amino-TEMPO and 4- hydroxy-TEMPO are considerably more water-soluble than TEMPO. As seen in Fig. 3, KPS-initiated nitroxide-mediated styrene polymerizations using either TEMPO or 4-hydroxy-TEMPO can display considerably different behavior in both rate and molecular weight.

2.2.1.5. Block copolymer synthesis. Synthesis of styrene/n-butyl acrylate block copolymers using nitroxide-mediated polymerization in miniemulsion has recently been reported by Tortosa et al. [27],

Table 1 Measured partition coef®cients for TEMPO, OH-TEMPO and NH-TEMPO between styrene and water at 25, 90, 120 and 135 8C, and the effects of hexadecane, polystyrene and SDBS on nitroxide partitioning. 95% con®dence intervals are shown for each measured value. From Ref. [23]. Reproduced with permission of John Wiley and Sons

21 System Pj *mole% mole% )

25 8C908C 120 8C 135 8C

TEMPO 711.5 ^ 10.5 900.7 ^ 45.4 718.3 ^ 8.5 652.2 ^ 40.6 OH-TEMPO 2.9 ^ 0.2 11.0 ^ 0.5 13.8 ^ 0.4 14.3 ^ 1.4 NH-TEMPO 16.8 ^ 0.7 34.7 ^ 4.5 42.4 ^ 15.1 43.9 ^ 8.9 TEMPO ± ± ± 593.9 ^ 23.6 with added hexadecane TEMPO ± ± ± 640.9 ^ 30.1 with added polystyrene OH-TEMPO ± ± ± 14.1 ^ 2.4 with added SDBS 1052 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067

Fig. 3. *a) Comparison of conversion versus time for TEMPO-mediated *B) and OH-TEMPO-mediated *V) styrene miniemulsion polymerizations. Temperature 135 8C. From Ref. [26]. Reproduced with permission of Wiley. *b) Comparison of number average molecular weight versus conversion for TEMPO-mediated *B) and OH-TEMPO-mediated *V) styrene miniemulsion polymerizations. Temperature 135 8C. From Ref. [26]. Reproduced with permission of Wiley.

Keoshkerian et al. [29] and Farcet et al. [31]. Tortosa et al. [27] prepared nitroxide-terminated polystyrene blocks that were then chain extended with n-butyl acrylate. Blocks with narrow polydispersities and negligible poly*n-butyl acrylate) homopolymer contamination were achieved using TEMPO, but when the more hydrophilic OH-TEMPO was used, the increased partitioning of the nitroxide into the aqueous phase resulted in more homopolymerization and broader polydispersities. The choice of water- soluble versus oil-soluble initiator in preparation of the polystyrene block had little effect on the synthesis of the n-butyl acrylate block. M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1053 Keoshkerian et al. [29] used a modi®ed hexadecane-free TEMPO-mediated miniemulsion polymerization process to achieve .99% conversion of styrene in less than 6 h. These blocks were then chain extended with n-butyl acrylate to yield narrow polydispersity block copolymers with apparently minimal loss of livingness arising from the styrene polymerization, despite the high conversion. Near complete conversion *.99%) of the n-butyl acrylate was achieved. Keoshkerian et al. [30] also reported on the successful homopolymerization of n-butyl acrylate in miniemulsion using a nitroxide synthesized by Benoit et al. [63]. Farcet et al. [31] made poly[n-butyl acrylate-b-*n-butyl acrylate-co-styrene)] polymers using SG1. The n-butyl acrylate block was synthesized ®rst but to less than complete conversion. Styrene was then added, and the residual n-butyl acrylate and styrene polymerized to form the second block. The order of block synthesis was found to be important. When styrene was polymerized ®rst, reinitiation was incomplete, while near-complete reinitiation was reported when the n-butyl acrylate was prepared ®rst.

2.2.1.6. Modeling. A detailed mathematical model for the alkoxyamine-initiated miniemulsion polymerization of styrene, mediated by either TEMPO or OH-TEMPO has recently been developed by Ma et al. [34]. The model, validated by experimental data, accounts for kinetics in both the aqueous and organic phases, including thermal polymerization, as well as phase partitioning of nitroxides and monomer. The populations of both living and dead chains are tracked to yield a comprehensive picture of how molecular weight, polydispersity and livingness evolve under different conditions, including the evolu- tion of the number MWDs of living and dead polymer chains. For styrene, experimental results and model simulations both showed that different partitioning properties of TEMPO and OH-TEMPO do not signi®cantly in¯uence the rate of polymerization, nor the concentration of polymeric radicals and nitroxide within the particles because of the dominant role played by thermal initiation. Aqueous phase nitroxide concentrations did vary signi®cantly in the simulations, however. Nitroxide partitioning was predicted to signi®cantly in¯uence the kinetics in the absence of thermal initiation, an important concern for acrylate polymerizations, and consistent with the experimental results of Tortosa et al. [27]. Charleux [15] also developed a model in her theoretical treatment for systems for which thermal initiation is not important.

2.2.2. Atom transfer radical polymerization

2.2.2.1. Overview. Many of the characteristics described earlier for SFRP also pertain to ATRP but there are critical differences in the phase partitioning behavior of the ATRP activating and deactivating species. With SFRP, a dormant chain is activated by `releasing' nitroxide *unimolecular process) and an active chain is deactivated by a free nitroxide *bimolecular process). However in ATRP, a bimolecular process is involved in both the activation and deactivation steps. Furthermore, different species are involved in the activation/deactivation steps. Because the activating and deactivating species have different oxidation states, they are likely to exhibit widely differing phase partitioning. It is possible, for example, that chain deactivation occurs only in the organic phase. Consequently, the effects of operating in a heterogeneous environment compared to a homogeneous system can be more complex with ATRP.

2.2.2.2. ATRP in emulsion polymerization. ATRP has been conducted with varying degrees of success in suspension [36,37], emulsion [38±45] and miniemulsion [46]. Gaynor et al. [39] studied n-butyl 1054 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 methacrylate *n-BMA) polymerization using ethyl 2-bromoisobutyrate *EBiB) as initiator, CuBr as catalyst, and dNbpy *4,40-di*5-nonyl)-2,20-bipyridine) as the ligand. Using SDS as surfactant, they observed high molecular weights but also high polydispersities in emulsion polymerization. The broad molecular weight distributions were attributed to the reaction of copper *II) bromide or chloride with sulfate anions to form copper *II) sulfate and NaBr, with the consequence that growing radicals were not deactivated. Well-de®ned polymer was produced in the presence of the nonionic surfactants Brij 97 and Brij 98, although colloidal stability was not satisfactory with the Brij 97, with coagulation occurring polymerization. For n-butyl methacrylate at 90 8C, very little difference was observed in the conversion pro®les of bulk and emulsion polymerizations, consistent with absence of compartmentalization effects and the absence of effects arising from either phase partitioning or mass transfer. Styrene, butyl acrylate and methyl methacrylate were also successfully polymerized in emulsion. The rates in emulsion for styrene and butyl acrylate were signi®cantly slower than in bulk, although the reason for the difference was not clear. Varying the water-solubility of the ligands showed that copper complexes preferentially soluble in the aqueous phase gave uncontrolled polymerization as a result of the deactivator *copper *II) halide) residing mostly in the water and not at the polymerization locus *particles). For the well-controlled polymerizations, the molecular weights were generally close to theoretical values and polydispersities were typically in the 1.2±1.3 range. In conventional ATRP, the initiator *e.g. EBib*ethyl 2-bromoisobutyrate)) is monomer-soluble. Because particle nucleation requires transfer of initiating radicals through the aqueous phase into micelles, mechanistic interpretation of kinetic data from emulsion ATRP is not straightforward. However by using reverse ATRP *Scheme 5), a water-soluble initiator can be used, thereby more closely approximating a traditional emulsion polymerization with respect to particle nucleation. In ATRP, the initiator is added to a catalyst in its lower oxidation state *i.e. activator). The initiator is therefore the `dormant' species. In reverse ATRP, conventional free radical initiators are used to thermally generate radicals *i.e. activated species) and the metal complex is added its higher oxidation state so that it functions as a deactivator. The initiators should decompose rapidly so that all chains are initiated within a narrow time frame. Qiu et al. explored the use of reverse ATRP in the emulsion polymerization of n- BMA [41] using water-soluble initiators *KPS, V-50, V-044) and a catalyst of Cu*II) dibromide with 4,40-dialkyl-2,20-bipyridine ligands. Brij 98 was used as surfactant. The ligands were chosen to ensure

Scheme 5. Reverse atom transfer radical polymerization. M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1055 preferential partitioning into the organic phase of the deactivating species, considered a requirement for effective control of the polymerization. When KPS was used in reverse emulsion ATRP of n-BMA, poor results were generally obtained. No polymerization was observed unless a buffer was used because of accelerated initiator decomposition. Polymerization was achieved at 70 8C but at 90 8C the control was poor, likely due to increased irreversible termination arising from a high radical ¯ux. Latexes were generally stable, although buffering was required *low pH reduces initiation ef®ciency and may affect catalyst activity). The diameter was generally .2 mm, more closely resembling suspension than emulsion polymerization. The use of water-soluble azo compounds as initiators gave well-controlled polymerizations *polydispersities were ,1.2±1.4) without requiring a buffer, although initiator ef®ciencies were observed to be relatively low *,30±40%). Actual molecular weights were typically ,2±3 times theoretical values, re¯ecting the low initiation ef®ciency *Fig. 4). The lower than expected initiation ef®ciency was attributed to the scaven- ging of aqueous phase radicals by uncomplexed CuBr2. With the azo compounds, the particle diameters were ,300 nm, more resembling true emulsion polymerization. Fig. 5 shows conversion plots for different [CuBr2]/[V-50] ratios. A more detailed mechanistic study on reverse ATRP in emulsion was subsequently published by Qiu et al. [42] using the same system described earlier. The phase partitioning behavior of the copper complexes in both oxidation states and of the Brij 98 surfactant in n-BMA/water mixtures was considered. *Partitioning of the copper complexes in the presence of surfactant was not considered.) Phase partitioning behavior of the catalyst is more complex than in the case of nitroxides. CuBr2/2dNbpy partitioning was found to be dependent on both temperature and concentration. The ligand dNbpy probably partitions negligibly into the aqueous phase on its own. However, it was suggested that the 1 coordination between Cu*II) and dNbpy is partially reversible, so that the complex [Cu*dNbpy)2Br] partially dissociates, releasing Cu21 *highly water-soluble) and free dNbpy. The free dNbpy and the remainder of the complex locate in the organic phase. The Cu*I) complex is more soluble in the organic phase compared to the Cu*II) complex. At 90 8C, approximately 20±30% of the initial Cu*I) was

Fig. 4. Dependence of molecular weight on conversion for emulsion polymerization by reverse ATRP. Temperature 90 8C. Initiator is V-50. From Ref. [41]. Reproduced with permission of American Chemical Society. 1056 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067

Fig. 5. Kinetics of emulsion reverse ATRP under different [CuBr2]0/[I]0 ratios. Temperature 90 8C. Initiator is V-50. From Ref. [41]. Reproduced with permission of American Chemical Society.

2 reported to be in the aqueous phase *probably as *CuBr2) ), which is suf®cient to allow adequate transport rates through the aqueous phase. For this system, initially the monomer droplets would contain CuBr2/dNbpy, excess dNbpy and some surfactant. The aqueous phase would contain dissolved monomer, micelles, initiator, surfactant, CuBr2 and a small amount of CuBr2/dNbpy. Several variables in the emulsion polymerization were also examined in Ref. [34], including temperature and concentrations of catalyst *CuBr2/dNbpy in a 1:2 ratio), surfactant *Brij 98) and initiator *V-50). Typical emulsion reverse ATRPs exhibited a 30±45 induction period, accompanied by a color change indicating the conversion of CuBr2/dNbpy to CuBr/dNbpy as radicals from the aqueous phase entered the organic phase *probably mostly micelles) to be deactivated into halide-terminated oligomers. Initi- ally, there would not be enough Cu*II) in the organic phase to trap the entering aqueous oligoradicals, and therefore Cu*II) in the aqueous phase would diffuse into the organic phase, complexing with the free dNbpy. The length of the induction period thus corresponds to the ratio of CuBr2/dNbpy to initiator. The polymerization rate increases to a maximum between 10and 30%conversion and then slows. Typically, the observed relationship between Mn and conversion was linear, with polydispersities ,1.5. As mentioned earlier, initiator ef®ciencies *,30±40%) were low compared to bulk reverse ATRP *,70%). Aqueous termination between radicals was considered the most probable cause of radical loss, although the possibility of radical-CuBr2 reaction was discussed. The particle size did not affect the rate but was itself a function of the amount of complex used, likely due to ionic strength effects. Polymerization commences when the radicals have been converted to alkyl halides and the Cu*II) complex converted to Cu*I). Early in the polymerization, oligomers may be able to exit and re-enter particles, depending on their water-solubility. The initial ratio of the CuBr2/dNbpy complex to initiator, besides determining the length of the induction period, has a major in¯uence on the course of reaction. The maximum rate depends on this ratio, with a lower ratio yielding a higher maximum rate. The actual amount of deactivator remaining after deactivation also depends on the concentration dependent partitioning of Cu*II). The slope of the Mn versus conversion pro®le *i.e. the number of chains) is independent M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1057 of the activator, although for these experiments there was always enough deactivator for the primary radicals arising from initiator decomposition. Temperature affects the initiator decomposition rate, the equilibrium constant between active and dormant chains and the propagation rate constant [42]. Lower temperatures lower the primary radical ¯ux and would therefore be expected to increase initiator ef®ciency but this also results in an extended period for initiation of all chains and therefore molecular weight distribution broadening. At higher temperatures, fewer chains will result because of higher termination early in the polymerization, therefore giving higher molecular weights and also lower polydispersities *reduced initiation period). An increase in initiator concentration should increase the number of chains, and therefore the expected rate. However in emulsion, the situation is not that straightforward. Higher initiator concentration also results in lower initiator ef®ciency *lower than expected number of chains) and an increase in the deactivating Cu*II) concentration, both of which act to reduce rate [42]. The net effect can be a seemingly low dependence on initiator concentration, as observed in SFRP miniemulsion [28]. The average number of radicals per particle was observed to be ,0.06 [42], similar to those reported by Cunningham et al. [28] for SFRP, verifying that the dominant chain-stopping event is indeed reversible termination. More irreversible termination was reported to occur in reverse ATRP in emulsion than in bulk because of partitioning of the Cu*II) species into the aqueous phase.

2.2.2.3. ATRP in miniemulsion polymerization. Application of reverse ATRP in miniemulsion was reported by Matyjaszewski et al. [46] using a similar system to the previously described emulsion polymerization studies *n-BMA, Brij 98, CuBr2/dNbpy). Hexadecane was used as costabilizer. Experi- ments run at 90 8C with AIBN gave near complete conversions with polydispersities ,1.5 but the relatively slow initiator decomposition led to the number of chains increasing until ,70% conversion.

Mn was greater than the theoretically expected value. The miniemulsion experiments were compared with bulk runs, but also with a bulk phase that was separated from the aqueous phase immediately following dispersion and then polymerized. The miniemulsion was slower than the bulk runs, caused by an excess of deactivator arising from low initiator ef®ciency. The isolated bulk phase polymerized faster than the bulk polymerization but was poorly controlled. This can be attributed to the initial loss of Cu*II) into the aqueous phase when the dispersion was created, leading to a de®ciency of deactivator in the droplets. Using the initiator V-50, which decomposes faster than AIBN, gave a lower polymerization rate due to more aqueous phase termination *lower initiator ef®ciency). Analogous experiments on direct ATRP were also conducted [46]. The polymerizations are much faster than reverse ATRP, with no observed induction period since activator is dominant early in the reaction. CuBr was used as catalyst, since the Cu*I) complex is highly air sensitive. Curvature was observed in the plot of ln*M0/M) versus time. The curvature was greater than for bulk, indicating a less- controlled reaction which is due to Cu*II) partitioning into the aqueous phase. Mn increased sharply at low conversion followed by a period of linear growth with conversion.

2.3. Aqueous dispersed systems with reversible transfer

2.3.1. Overview In an emulsion polymerization with reversible transfer, many of the previously described principles apply. The transfer agent distributes between the monomer droplets, aqueous phase and micelles, and must be transported from the droplets through the aqueous phase to the particles. Oligomeric radicals 1058 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 growing in the aqueous phase can react with the transfer agent to yield a dormant chain that remains water-soluble or enter a micelle/particle if they reach suf®cient chain length. An entering oligoradical can propagate, react with the transfer agent or terminate with another radical. The partition coef®cient and aqueous phase diffusivity will determine if the transfer agent can diffuse to the particles quickly enough to maintain equilibrium concentrations. Chain transfer is known to be diffusion limited in conventional emulsion polymerization when the transfer agent is very reactive and has low water solubility [47]. When unreacted transfer agent enters a particle, it remains capable of exiting that particle unless it reacts with a growing macroradical of suf®cient length to render it insoluble in the aqueous phase. A transfer agent may still exit if it is ®xed to a short oligomeric radical. Once a minimum chain length for the growing chains is reached, the transfer agent will remain in the particles, unlike nitroxides, for example, which are always capable of exit when dissociated from a radical. The fate of living but dormant oligomeric radicals in the aqueous phase is similar to that of reversible termination systems *i.e. potential fates include propagation of the dormant chain with possible particle entry, or aqueous phase termination). Miniemulsion polymerizations using reversible transfer are not expected to have particle nucleation nor require transport of the transfer agent through the aqueous phase. Phase partitioning, entry and exit must still be considered.

2.3.2. Reversible addition±fragmentation transfer

2.3.2.1. RAFT in emulsion polymerization. The use of RAFT in emulsion was ®rst reported by Le et al. [48] who noted that the transfer agent selected should partition primarily into the aqueous phase yet have suf®cient water-solubility to diffuse through the aqueous phase from monomer droplets to particles. More recently, there have been a number of publications dealing with the use of RAFT *and MADIX) in emulsion [49±56] and miniemulsion [57±60]. RAFT is applicable to a wide variety of monomers, and reaction conditions *e.g. temperature) are the same as those used in conventional radical polymerizations. Since it is a reversible transfer process, the addition of a RAFT agent to a polymerization ideally should not affect the polymerization rate. It will be seen in the following discussion, however, that this is not always true in heterogeneous systems where the presence of aqueous phase reactions can affect the overall kinetics. The effects of heterogeneity have proven to be an issue in successfully conducting RAFT polymerizations, although for somewhat different reasons than in SFRP or ATRP. Although in principle reversible transfer systems exhibit compartmentalization, they are not necessarily the same as seen in conventional emulsion polymerization. Monteiro et al. [49] noted that RAFT systems may not exhibit the expected zero-one kinetics in styrene polymerization. For typical conditions used in styrene emulsion polymerization, the entry of an aqueous oligoradical with one growing radical into a particle results in instantaneous termination, and thus the particles have either zero or one active radical at any time. However in the presence of a RAFT agent, an entering oligoradical can ®rst transfer its activity to a long radical. The probability of termination between two long radicals is much lower than between a short and long radical [3], and therefore termination may not be instantaneous. Exit of the leaving group of the RAFT agent was studied by Monteiro et al. [49] using seeded styrene emulsion polymerizations. Use of a polymethylmethacrylate seed allowed the seed molecular weight distribution to be distinguished from that of the polymer produced in the living radical polymerization via use of a UV±VIS detector on their gel permeation chromatograph. Exit is expected to be directly related to the particle size distribution *probability of exit is proportional to [radius]22). In these seeded M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1059 styrene emulsion polymerizations with dithiobenzoates, nonideal behavior was observed in both rate retardation and high polydispersities. Two high activity RAFT agents, with leaving groups of different water solubility *C*CH3)2Ph and C*CH3)2CO2Et), were used. Probabilities of exit for the two leaving groups were calculated as 0.29 for the cumyl radical *C*CH3)2Ph) and 0.56 for the 2-*ethoxycarbonyl)- prop-2-yl radical *C*CH3)2CO2Et). Observed rate retardation early in the polymerization was attributed to exit of the leaving group that lowered the radical concentration in the particles. Increased water- solubility of the RAFT leaving group led to increased exit and greater rate retardation. This effect will only persist until all of the RAFT agent has been fully consumed. For highly reactive RAFT agents, as used in this study *Ctr , 6000), full consumption will normally occur within a few percent conversion unless severe diffusional limitations exist. However, the high polydispersities were believed to be caused by the continuous transport of dithiobenzoate through the aqueous phase during the polymerization. Recall these transfer agents are both highly active and water-insoluble, thereby creating the potential for diffusionally limited transfer. A study by Monteiro and de Barbeyrac [55] further showed how the selection of the RAFT agent can in¯uence the course of polymerization through exit and re-entry of the fragments resulting from transfer.

The agent 1-*O-ethylxanthyl)ethylbenzene, having a transfer constant to styrene of Ctr , 0.8, was used. Exit and re-entry of the RAFT fragment during the nucleation process when micelles are still present affects the ®nal particle number, and thus particle size and reaction rate. Upon analyzing the molecular weight data *the number of chains was much lower than theoretically expected), the authors concluded that the actual concentration of RAFT agent in the particles was lower than the overall bulk value, and suggested the RAFT agent may be partitioning at the particle interface. Taking advantage of this proposed feature, they synthesized block copolymer particles with core±shell morphology, in which the cores were polystyrene, and the shells were poly*n-butyl acrylate-co-acetoacetoxyethyl methacrylate). Block copolymers of poly*4-acetoxystyrene-b-styrene) in emulsion were also reported by Kanagasabapathy et al. [56]. Charmot et al. [53] reported that a MADIX polymerization yielded no rate retardation in ab initio emulsion polymerizations of styrene and butyl acrylate. The polydispersities were generally higher than observed for high activity RAFT agents *.2.1 for styrene but as low as 1.4 for butyl acrylate). Good agreement, however, was observed between the theoretical and experimental molecular weights. It was found advantageous to operate in semi-batch mode, since this ensured ,100% dormant chains were formed early in the process. Using MADIX to make butyl acrylate±styrene block copolymers, Monteiro et al. [50] also found a semi-batch starved-fed approach was advantageous. Purer blocks were obtained, which was believed to be due to lower radical entry ef®ciency and therefore less irreversible termination and lower polystyrene homopolymer formation.

2.3.2.2. RAFT in miniemulsion polymerization. Miniemulsion polymerization used with nonionic surfactants was able to yield well-behaved living radical polymerizations of styrene and methacrylates [59]. de Brouwer et al. [59] and Tsavalas et al. [60], using dithiobenzoates, found both anionic and cationic surfactants failed to give stable latexes in miniemulsion but stability was achieved using nonionics *Igepal 890and Brij 98) with either hexadecane or Kraton L1203as hydrophobe. With ionic stabilizers, droplet nucleation was inef®cient and the polydispersity increased with conversion *Fig. 6), although it is known that polydispersity in RAFT systems should decrease with conversion. The RAFT agent caused signi®cant rate retardation, including near cessation of the polymerization after ,70% conversion. Fig. 6 also reveals that the experimentally observed molecular weight agreed well with ideal theoretical values 1060 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067

Fig. 6. Number average molecular weight for RAFT miniemulsion polymerizations of styrene carried out at 75 8C. All experiments initiated by KPS. Mn,th1 is theoretical Mn accounting for RAFT-derived chains only. Mn,th2 is theoretical Mn accounting for RAFT-derived chains and initiator-derived chains. From Ref. [60]. Reproduced with permission of American Chemical Society. until ,30% conversion. However, when a correction was made for the presence of initiator-derived radicals that terminate irreversibly, experimental results agreed with theoretical values throughout the polymerization. Rate retardation was considered as potentially being due to two effects [60]. First is the exit of transferred radicals to the aqueous phase and subsequent aqueous phase termination. Second is the termination of an entering radical or leaving group radical with the intermediate radical *Scheme 3) formed by the dithiobenzoate. The ®rst explanation was considered only to be potentially important at low conversions. The second explanation was considered the most likely explanation for retardation observed during the remainder of the polymerization. When nonionic stabilizers were used in place of ionic stabilizers, there was either no surface organic layer observed, or it disappeared during polymerization [59]. The polymerization was also better controlled, giving polydispersities of usually ,1.20. Butte et al. [57] combined experimental studies with mathematical modeling in their study of RAFT miniemulsion polymerization. Polymerizations of styrene, methyl methacrylate and n-butyl acrylate were done using different RAFT agents. Block copolymers *poly*methyl methacrylate-b-styrene), poly*styrene-b-n-butyl acrylate)) were also made. The importance of the partitioning properties of the RAFT leaving group in in¯uencing rate was noted, since a greater tendency to diffuse into the aqueous phase reduces the radical concentration within the particles and therefore the rate. Issues related to diffusional transport of RAFT agents in emulsion polymerizations are cited as the incentive for using miniemulsion, even if those issues can be partially resolved through judicious choice of RAFT agent. The inability of costabilizers to completely inhibit monomer diffusion between more and less reactive particles was also cited as a potential problem, since it can result in inhomogeneity in the RAFT agent concentration between particles if the RAFT agents are not suf®ciently mobile.

2.3.2.3. Colloidal stability in heterogeneous RAFT polymerizations. The issue of colloidal stability in RAFT emulsion and miniemulsion polymerizations has been reported in different studies [49,51,55,58±

60]. Uzulina et al. [51], using S-thiobenzoyl-thioglycolic acid *Ctr , 6) in styrene emulsion polymerization, observed a polymerization with living characteristics until about 40% conversion when massive M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1061 ¯occulation occurred, attributed to precipitation of the chain transfer agent. Another common problem is formation of a monomer-rich layer containing dormant oligomers with broad molecular weight distribution *polydispersity , 3±5) on the latex surface, and typically deeply colored by the presence of the RAFT agent [49,59,60]. This has been reported in seeded styrene emulsion polymerization [49] and miniemulsion polymerization of styrenes and methacrylates with high activity dithiobenzoates

*Ctr , 6000) [59,60]. Tsavalas et al. [60] extensively studied the stability issue in ionically stabilized miniemulsions. The formation of the organic layer containing dormant oligomers, monomer and dithiobenzoate, occurs only after initiator addition. It occurs independently of monomer type *styrene, butyl methacrylate, ethyl hexyl methacrylate) and polymerization rate. Low droplet nucleation ef®ciency was also observed, which was attributed to low radical ¯ux *a consequence of low initiator concentration and a requirement for livingness in RAFT systems). However, the low ¯ux and ensuing low droplet nucleation ef®ciency result in high polydispersities. Increasing the radical ¯ux by using a redox initiating system gave improved control of the polymerization. A distinguishing feature of RAFT polymerizations with high activity transfer agents is that at low conversions, essentially all the polymer exists as oligomers, while with either conventional emulsion/ miniemulsion polymerization or even RAFT with lower activity transfer agents, high polymer is produced at low conversions. The presence of a large oligomer population appears to play a role in destabilizing these systems, however there also appears to be an interaction with the surfactant used *SDS), and the authors suggested displacement of the surfactant from the droplet/particle surface was responsible for the observed instability [60]. Luo et al. [58] presented a theory of particle swelling based on solution thermodynamics that postulates the existence of a superswollen state in the droplets/particles caused by the presence of high oligomer concentrations. The superswelling phenomenon, which is considered to be the likely cause of observed instabilities with some RAFT systems, is thought to be sensitive to the formulation used. Particles with high oligomer concentrations may undergo superswelling, inducing transport of monomer from droplets, thereby broadening or even destabilizing the miniemulsion. The authors believe the phenomenon could be eliminated by increasing costabilizer concentration and/or use of a polymeric surfactant. A similar phenomenon has been reported in ATRP emulsion polymerization [40]. While this behavior has not been reported for nitroxide-mediated systems, most of this work has been conducted at elevated temperatures *.100 8C) in pressurized steel reactors. In such vessels, the phenomena may occur but not be observed.

2.3.3. Degenerative transfer using iodine exchange Degenerative transfer has been used in emulsion [13] and miniemulsion [13,14,61] polymerization based on exchange of the iodine atom of per¯uorohexyl iodide *C6F13I). This transfer agent, while commercially available, has a relatively low transfer constant *Ctr 1.4 at 70 8C) and is therefore not ideal for producing narrow molecular weight distributions as it is consumed slowly, leading to more dead chains early in the polymerization and therefore higher polydispersity. Butte et al. [14] polymerized styrene in miniemulsion using SDS with hexadecane as costabilizer. When they employed a water-soluble initiator, no control of the polymerization was observed, probably due to a predominance of homogeneous nucleation over droplet nucleation. However, when the monomer-soluble AIBN was used, a well-controlled polymerization was achieved and at a rate much faster than in bulk. Typically, .90% conversion was reached in 2 h, giving ®nal polydispersities of 1.5± 1.7. The average number of radicals per particle was ,0.5, in accord with the expected value for 1062 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 compartmentalized emulsion/miniemulsion polymerization of styrene. Plots of Mn versus conversion displayed the expected sharp initial increase in molecular weight *due to slow transfer agent consumption) followed by a more linear relationship between Mn and conversion. When used in styrene emulsion polymerization with SDS as surfactant and at 70 8C, Lansalot et al. [13] found that transfer of the per¯uorohexyl iodide from the monomer droplets through the aqueous phase to the particles was severely retarded by its low water solubility, thus preventing it from controlling the polymerization. The C6F13I was not fully consumed even when the monomer conversion was complete, and the ef®ciency of the C6F13I was calculated to be only ,50%. The molecular weights were higher than observed for bulk polymerizations run with the same C6F13I concentrations, re¯ecting the inef®cient use of the transfer agent *Fig. 7). However, when the same system was then employed in styrene miniemulsion, near 100% ef®ciency was achieved, and stable latexes were formed using the water-soluble initiator 4,40-azobis*4-cyanopen- tanoic acid) *ACPA) [13]. SDS was used as surfactant, and 1% of polystyrene *Mw , 330 000) was also added to enhance droplet nucleation, although it has subsequently been shown that this effect is an artifact of enhanced monomer droplet stability [62]. The transfer agent itself also appears to function as a hydrophobe. The observed ®nal values of molecular weight agreed well with theoretically expected values, although molecular weights at lower conversions were higher than theoretical values *Fig. 8). This difference at lower conversions is caused by the slow consumption of transfer agent due to its low transfer constant. Polydispersities decreased from ,2 at low conversions to 1.5±1.6 at higher conversions, and Mn values reached 45 000±50 000. The livingness of the system was veri®ed by adding a second shot of monomer *continuous addition) and monitoring the growth in Mn with conversion. During this stage of the reaction, a linear increase in Mn with conversion was observed, although as noted earlier this only establishes that some, and not necessarily all of the chains are living. Polydispersity also increased during this stage, which was attributed to a decrease in the rate constant for activation/ deactivation with increasing chain length.

Fig. 7. Number average molar mass versus conversion for batch emulsion polymerization of styrene using ACPA initiator at

70 8C. E1: [C6F13I]0 0.1 mol/l *styrene); E2: [C6F13I]0 0.2 mol/l *styrene). From Ref. [13]. Reproduced with permission of American Chemical Society. M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1063

Fig. 8. Number average molar mass versus conversion for batch miniemulsion polymerization of styrene using ACPA *ME1,

ME2, ME5) or AIBN *ME3, ME4) initiator at 70 8C. ME1: [C6F13I]0 0.1 mol/l *styrene); ME2: [C6F13I]0 0.2 mol/l *styrene). ME3: [C6F13I]0 0.1 mol/l *styrene); ME4: [C6F13I]0 0.2 mol/l *styrene). ME5: [C6F13I]0 0.05 mol/l *styrene). From Ref. [13]. Reproduced with permission of American Chemical Society.

Butte et al. and Lansalot et al. reported different successes in the use of water-soluble initiators and interference from homogeneous nucleation. These differences may be attributable to the much higher SDS concentration used by Butte et al. in their experiments that would be conducive to increased homogeneous nucleation.

3. Block copolymer synthesis

The preparation of block copolymers in emulsion or miniemulsion using living radical polymerization, an application of considerable interest, has been reported by several authors [27,29,31,45,50,52,54±57,59,61]. In keeping with the intended scope of this review, this topic will not be discussed separately. Material relevant to the heterogeneous nature of the reactions has been included in the foregoing discussion. One noteworthy feature of making block copolymers in emulsion/ miniemulsion systems is that nucleation of new particles *that do not contain the ®rst block) during the polymerization of the second block can be particularly detrimental to the purity of the block copolymer. The ®rst block will not be able to diffuse through the aqueous phase to new particles. Therefore, secondary nucleation in these systems should be carefully avoided.

4. Concluding remarks

The heterogeneous nature of emulsion or miniemulsion polymerization can impact the kinetics and molecular weight development of living radical polymerizations in several ways. Reversible termination systems *SFRP, ATRP) and reversible transfer systems *RAFT, degenerative transfer, MADIX) behave fundamentally differently. Reversible termination systems have lower radical concentrations and do not exhibit compartmentalization effects. The dominant chain-stopping event is reversible deactivation of a 1064 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 growing chain. The kinetics therefore broadly resemble bulk kinetics, with the important exceptions that chain initiation, particle initiation, entry and exit of radicals, and phase partitioning complicate the overall reaction mechanism. The average number of radicals is typically p 0:5; and the reaction rates are similar to those in bulk. With reversible transfer, the radical concentration is not affected and therefore compartmentalization can in principle be realized. Reaction rates are faster than in bulk systems. Miniemulsion polymerization offers improved control of the polymerization for most living radical systems, either reversible transfer or reversible termination. By eliminating the particle nucleation step *ideally), issues related to diffusion of the controlling agent through the aqueous phase are largely solved. However, issues and questions remain around the entry of radicals into miniemulsion droplets or particles. The phase partitioning of the controlling agent, and how it affects the growth rate of aqueous oligoradicals, is an important area for research. The high oligomer concentration at low conversions in living radical systems appears to be have a role in problems with colloidal stability, although this has yet to be de®nitively established. All of the living radical systems are capable of yielding reasonably well-controlled heterogeneous polymerizations under suitable conditions, although each also has distinctive features that can be either advantageous or disadvantageous. Most systems can be run at the same temperatures usually used in emulsion polymerization *,100 8C), which is an important industrial concern. Most nitroxides, with the exception of SG1, require polymerization temperatures of ,115±135 8C, and therefore must be run under pressure *,300 kPa). Interaction of the controlling agent with ionic surfactants has been an issue for some RAFT and ATRP agents, but not for nitroxides. RAFT and ATRP have been shown to be suitable for a wide range of monomers, whereas nitroxides have traditionally been successful for mostly styrenics. However, recent progress in nitroxide design suggests they can also be suitable for a wide range of monomers [63,64]. ATRP systems raise concern about removal of the metal complex, while RAFT agents can give odor problems. High reaction rates and near complete conversions can be readily achieved with RAFT and degenerative transfer, while this is currently a much greater challenge for SFRP and ATRP. Priority areas for future research in heterogeneous living radical polymerization include greater understanding of chain initiation, entry of aqueous radicals or deactivated oligomers into droplets/ particles, phase partitioning and interphase transport of the controlling agent. Issues related to colloidal stability and the fate of aqueous radicals are also important. Continued development in this area provides an excellent opportunity for the widespread and inexpensive commercialization of living radical polymerization.

Acknowledgements

The assistance provided by Marcus Lin in the preparation of this manuscript is gratefully acknowledged.

References

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