α‐Cyanobenzyl dithioester reversible addition–fragmentation chain‐transfer agents for controlled radical polymerizations

A series of new reversible addition–fragmentation chain transfer (RAFT) agents with cyanobenzyl R groups were synthesized. In comparison with other dithioester RAFT agents, these new RAFT agents were odorless or low‐odor, and this made them much easier to handle. The kinetics of methyl methacrylate radical polymerizations mediated by these RAFT agents were investigated. The polymerizations proceeded in a controlled way, the first‐order kinetics evolved in a linear fashion with time, the molecular weights increased linearly with the conversions, and the polydispersities were very narrow (～1.1). A poly[(methyl methacrylate)‐block‐polystyrene] block copolymer was prepared (number‐average molecular weight = 42,600, polydispersity index = 1.21) from a poly(methyl methacrylate) macro‐RAFT agent. These new RAFT agents also showed excellent control over the radical polymerization of styrenics and acrylates. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1535–1543, 2005     

Butyl acrylate RAFT polymerization in miniemulsion

Few successes about butyl acrylate (BA) RAFT miniemulsion homopolymerization were reported, even though styrene, methyl methacrylate, and vinyl acetate had been successfully applied in reversible addition fragmentation transfer (RAFT) miniemulsion polymerization. In this article, four types of RAFT agent with various designed R and Z groups [benzyl dithioisobutyrate (BDIB), 1-phenylethyl phenyldithioacetate (PEPDTA), cumyl dithioisobutyrate (CDIB), benzyl dithiobenzoate] were used to mediate BA miniemulsion polymerization using the conditions (5 wt % hexadance and sodium dodecyl sulfate) effective for styrene and methyl methacrylate systems. All four types of the RAFT agents effectively control over the bulk polymerization. In contrast, only BDIB resulted in a rather narrow molecular weight distribution in the miniemulsion polymerization. A pronounced inhibition and rate retardation were observed in both bulk and miniemulsion polymerizations mediated by CDIB and benzyl dithiobenzoate. When compared with the bulk polymerization, a much longer inhibition period (over eight times) was observed in the CDIB-mediated miniemulsion polymerization. It was concluded that only the RAFT agent with the primary R group and Z group with less stabilizing ability to the intermediate radicals is effective to mediate BA miniemulsion polymerization in terms of achieving a narrow molecular weight distribution and short inhibition period. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2304–2315, 2007     

A new photoiniferter/RAFT agent for ambient temperature rapid and well‐controlled radical polymerization

Phenacyl morpholine‐4‐dithiocarbamate is synthesized and characterized. Its capability to act as both a photoiniferter and reversible addition fragmentation chain transfer agent for the polymerization of styrene is examined. Polymerization carried out in bulk under ultra violet irradiation at above 300 nm at room temperature shows controlled free radical polymerization characteristics up to 50% conversions and produces well‐defined polymers with molecular weights close to those predicted from theory and relatively narrow poyldispersities (M_w/M_n ～ 1.30). End group determination and block copolymerization with methyl acrylate suggest that morpholino dithiocarbamate groups were attained at the end of the polymer. Photolysis and polymerization studies revealed that polymerization proceeds via both reversible termination and RAFT mechanisms. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3387–3395, 2008     

(Thiocarbonyl‐α‐thio)carboxylic acid derivatives as transfer agents in reversible addition–fragmentation chain‐transfer polymerizations

Ethyl S‐(thiobenzoyl)thioacetate, ethyl S‐thiobenzoyl‐2‐thiopropionate, and S‐(thiobenzoyl)thioglycolic acid were used as chain‐transfer agents for the reversible addition–fragmentation chain‐transfer (RAFT) polymerizations of styrene, methyl methacrylate, and butyl acrylate. Of these polymerizations, only those of styrene and butyl acrylate with any of the transfer agents showed molecular weight control corresponding to controlled/living polymerizations. The best molecular weight control was observed for the polymerizations of styrene and butyl acrylate with ethyl (S)‐thiobenzoyl‐2‐thiopropionate. Semiempirical PM3 calculations were performed for the investigation of the relative heats of reaction of the chain‐transfer equilibria between the aforementioned chain‐transfer agents and dimer radicals of the three monomers. The molecular weight control of the polymerizations correlated with the stability trend of the leaving‐group radical of the chain‐transfer agent. This relatively simple computational model offered some value in determining which transfer agents would show the best molecular weight control in RAFT polymerizations. © 2002 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 555–563, 2002; DOI 10.1002/pola.10143     

Emulsion and miniemulsion polymerizations with an oil‐soluble initiator in the presence and absence of an aqueous‐phase radical scavenger

Butyl acrylate conventional emulsion (macroemulsion) and miniemulsion polymerizations were carried out with an oil‐soluble initiator (azobisisobutyronitrile) in the presence or absence of an aqueous‐phase radical scavenger. For macroemulsion polymerization, in the presence of an aqueous‐phase radical scavenger, no particle nucleation occurred, whereas in the absence of an aqueous‐phase radical scavenger, particle nucleation proceeded as expected. For miniemulsion polymerization, the rate of polymerization was much higher in the absence of an aqueous‐phase radical scavenger than in its presence. Furthermore, in the absence of an aqueous‐phase radical scavenger, the miniemulsion polymerization rate increased with reduced droplet size, whereas in the presence of an aqueous‐phase radical scavenger, the trend was reversed. It is concluded that (1) for macroemulsion polymerization, the contribution from free radicals originating in the aqueous‐phase is predominant in the micellar nucleation of particles; (2) free radicals originating in the particle phase contribute to the rate of polymerization and the contribution increases with an increase in the particle size; and (3) for polymer particles with diameters of up to approximately 100 nm, polymerization is initiated from free radicals originating in the aqueous phase. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3200–3211, 2002     

Immobilization of RAFT agents on silica nanoparticles utilizing an alternative functional group and subsequent surface‐initiated RAFT polymerization

Silica–polystyrene core‐shell particles were successfully prepared by surface‐mediated reversible addition fragmentation chain transfer (RAFT) polymerization of styrene monomer from the surfaces of the silica‐supported RAFT agents. Initially, macro‐RAFT agents were synthesized by RAFT polymerization of γ‐methacryloxypropyltrimethoxysilane (MPS) in the presence of chain transfer agents (CTAs). Immobilization of CTAs onto the silica surfaces was then performed by reacting silica with macro‐RAFT agents via a silane coupling. Grafting of polymer onto silica forms core‐shell nanostructures and shows a sharp contrast between silica core and polymer shell in the phase composition. The thickness of grafted‐polymer shell and the diameter of core‐shell particles increase with the increasing ratio of monomer to silica. A control experiment was carried out by conventional free radical emulsion copolymerization of MPS‐grafted silica and styrene under comparable conditions. The resulting data provide further insight into the chemical composition of grafted‐polymers that are grown from the silica surface through RAFT process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 467–484, 2009     

Low temperature TEMPO‐mediated styrene polymerization in miniemulsion

The 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO)‐mediated stable free radical polymerization of styrene in miniemulsion at 100 °C is demonstrated. Although this temperature is 20–35 °C lower than typical temperatures used for TEMPO‐mediated polymerizations, reasonable reaction rates were achieved by the addition of ascorbic acid or a free radical initiator. More importantly, the living character of the chains was preserved; the degree of polymer “livingness” was comparable to polymerizations conducted at 135 °C. Polydispersities were broader than that observed in well‐controlled systems, ranging from ～1.4–1.6, and consistent with expectations for systems having a low activation rate. The results are significant for two reasons. They will facilitate TEMPO‐mediated minemulsion polymerizations in nonpressurized (or minimally pressurized) reactors, and they reveal the potential to expand the traditional temperature range of TEMPO and possibly other nitroxides in bulk, solution, and miniemulsion. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 232–242, 2006     

An Emulsifier‐Free RAFT‐Mediated Process for the Efficient Synthesis of Cerium Oxide/Polymer Hybrid Latexes

Hybrid latexes based on cerium oxide nanoparticles are synthesized via an emulsifier‐free process of emulsion polymerization employing amphiphatic macro‐RAFT agents. Poly(butyl acrylate‐co‐acrylic acid) random oligomers of various compositions and chain lengths are first obtained by RAFT copolymerization in the presence of a trithiocarbonate as controlling agent. In a second step, the seeded emulsion copolymerization of styrene and methyl acrylate is carried out in the presence of nanoceria with macro‐RAFT agents adsorbed at their surface, resulting in a high incorporation efficiency of cerium oxide nanoparticles in the final hybrid latexes, as evidenced by cryo‐transmission electron microscopy.     

Thermal self‐initiation of styrene in the presence of TEMPO radicals: Bulk and miniemulsion

In TEMPO (2,2,6,6,‐tetramethyl‐1‐piperidinyloxy) controlled styrene radical polymerizations, the thermal self‐initiation reaction of styrene monomer is one of the main sources for the deviations from ideal living polymerization. However, it is also important because it continuously generates radicals to compensate for the loss of radicals caused by the termination reactions and thereby maintains a reasonable reaction rate. In this report, different initial TEMPO concentrations were used in styrene miniemulsion polymerizations without any added initiator. The consumption rate of TEMPO or radical generation rate was calculated from the length of the induction period and the increasing total number of polymer chains. It was found that there is little difference between the miniemulsions and the corresponding bulk systems in terms of the length of the induction period, which increases linearly with initial TEMPO concentration. After the induction period, the consumption rate of TEMPO or radical generation rate was reduced to a lower level, and a faster initial polymerization rate was found in the bulk system compared to the corresponding miniemulsion system. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4921–4932, 2004     

Kinetic model of reversible addition‐fragmentation chain transfer polymerization in microemulsions

A simplified kinetic model for RAFT microemulsion polymerization has been developed to facilitate the investigation of the effects of slow fragmentation of the intermediate macro‐RAFT radical, termination reactions, and diffusion rate of the chain transfer agent to the locus of polymerization on the control of the polymerization and the rate of monomer conversion. This simplified model captures the experimentally observed decrease in the rate of polymerization, and the shift of the rate maximum to conversions less than the 39% conversion predicted by the Morgan model for uncontrolled microemulsion polymerizations. The model shows that the short, but finite, lifetime of the intermediate macro‐RAFT radical (1.3 × 10^?4–1.3 × 10^?2 s) causes the observed rate retardation in RAFT microemulsion polymerizations of butyl acrylate with the chain transfer agent methyl‐2‐(O‐ethylxanthyl)propionate. The calculated magnitude of the fragmentation rate constant (k_f = 4.0 × 10¹–4.0 × 10³ s^?1) is greater than the literature values for bulk RAFT polymerizations that only consider slow fragmentation of the macro‐RAFT radical and not termination (k_f = 10^?2 s^?1). This is consistent with the finding that slow fragmentation promotes biradical termination in RAFT microemulsion polymerizations. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 604–613, 2010     

乳液体系中的RAFT可控/活性自由基聚合研究进展

可逆加成-断裂链转移聚合(RAFT)是新近发展起来的可控/活性自由基聚合方法。由于该方法具有适用单体范围广、反应条件温和、可采用多种聚合实施方法等优点,已成为一种有效的分子设计手段。本文总结了近几年文献报道的在乳液和细乳液体系中实施RAFT聚合反应的研究进展,对非均相体系的稳定性、聚合反应过程中的动力学特点、以及聚合产物的分子量及其分布等方面的研究进行了综述。     

Recent advances in controlled/living radical polymerization in emulsion and dispersion

Effective ways to conduct controlled/living radical polymerization (CRP) in emulsion systems are necessary for commercial latex production without significant modification of current industrial facilities. Conducting CRP in emulsion media is more complicated and more challenging than its application in homogeneous bulk. These challenges come from the intrinsic kinetics of emulsion polymerization. They include mass transport, slow chain growth mechanism, and exit of short radicals from polymeric particles. This review describes the recent developments of CRP in heterogeneous dispersion, including miniemulsion, microemulsion, dispersion, and especially emulsion. Various approaches for conducting emulsion CRP are detailed, including controlled seeded emulsion polymerization, nanoprecipitation, use of short oligomers as macroinitiators for in situ block copolymerization, and RAFT‐mediated self‐assembly. In addition many remaining challenges of the current methods barring wide spread industrial application of emulsion CRP are also suggested. © Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6983–7001, 2008     

RAFT‐mediated emulsion polymerization of styrene using brush copolymer as surfactant macro‐RAFT agent: Effect of the brush copolymer sequence and chemical composition

The nonionic amphiphilic brush polymers such as poly[poly(ethylene oxide) methyl ether vinylphenyl‐co‐styrene] trithiocarbonate [P(mPEGV‐co‐St)‐TTC] and poly[poly(ethylene oxide) methyl ether vinylphenyl‐b‐styrene‐b‐poly(ethylene oxide) methyl ether vinylphenyl] trithiocarbonate [P(mPEGV‐b‐St‐b‐mPEGV)‐TTC] with different monomer sequence and chemical composition are synthesized and their application as macro‐RAFT agent in the emulsion RAFT polymerization of styrene is explored. It is found that the monomer sequence in the brush polymers exerts great influence on the emulsion RAFT polymerization kinetics, and the fast polymerization with short induction period in the presence of P(mPEGV‐co‐St)‐TTC is demonstrated. Besides, the chemical composition in the brush polymer macro‐RAFT agent effect on the emulsion RAFT polymerization is investigated, and the macro‐RAFT agent with high percent of the hydrophobic PS segment leads to fast and well controlled polymerization. The growth of triblock copolymer colloids in the emulsion polymerization is checked, and it reveals that the colloidal morphology is ascribed to the hydrophobic PS block extension, and the P(mPEGV‐co‐St) block almost have no influence just on the size of the colloids. This may be the first example to study the monomer sequence and the chemical composition in the macro‐RAFT agent on emulsion RAFT polymerization, and will be useful to reveal the block copolymer particle growth. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Kinetic model of reversible addition fragmentation chain transfer polymerization of styrene in seeded emulsion

A detailed model describing the kinetics of living polymerization mediated by reversible addition‐fragmentation chain transfer (RAFT) in seeded emulsion polymerization is developed. The model consists of a set of population balance equations of the different radical species in the aqueous phase and in the particle phase (accounting for radical segregation) as well as for the dormant species in the particle phase. The entire population of radicals was divided into several distinguished species, based on their length and their chain end group. The model results are helpful in understanding inhibition and retardation phenomena that are typical for RAFT emulsion polymerizations. While inhibition is due to the radical loss in form of the RAFT leaving group, retardation is mostly caused by a small amount of short dormant chains in the particle phase, leading to a slight increase of radical loss via RAFT exchange with radicals entering a particle. The model results are compared to a series of experiments, using cumyl dithiobenzoate as a RAFT agent in polymerizations of styrene. The agreement between experimental and model results is good and, notably, the only parameters considered adjustable were the RAFT exchange rate coefficients. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6114–6135, 2006     

Reversible addition–fragmentation chain transfer polymerization of 2‐naphthyl acrylate with 2‐cyanoprop‐2‐yl 1‐dithionaphthalate as a chain‐transfer agent

The reversible addition–fragmentation chain transfer (RAFT) polymerizations of 2‐naphthyl acrylate (2NA) initiated by 2,2′‐azobisisobutyronitrile were investigated with 2‐cyanoprop‐2‐yl 1‐dithionaphthalate (CPDN) as a RAFT agent at various temperatures in a benzene solution. The results of the polymerizations showed that 2NA could be polymerized in a controlled way by RAFT polymerization with CPDN as a RAFT agent; the polymerization rate was first‐order with respect to the monomer concentration, and the molecular weight increased linearly with the monomer conversion. The polydispersities of the polymer were relatively low up to high conversions in all cases. The chain‐extension reactions of poly(2‐naphthyl acrylate) (P2NA) with methyl methacrylate and styrene successfully yielded poly(2‐naphthyl acrylate)‐b‐poly(methyl methacrylate) and poly(2‐naphthyl acrylate)‐b‐polystyrene block polymers, respectively, with narrow polydispersities. The P2NA obtained by RAFT polymerization had a strong ultraviolet absorption at 270 nm, and the molecular weights had no apparent effect on the ultraviolet absorption intensities; however, the fluorescence intensity of P2NA increased as the molecular weight increased and was higher than that of 2NA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2632–2642, 2005     

Role of sodium dodecylbenzenesulfonate in 2,2,6,6‐tetramethy‐1‐piperidinyloxy‐mediated styrene miniemulsion polymerization

In studying 2,2,6,6‐tetramethy‐1‐piperidinyloxy (TEMPO)‐mediated styrene miniemulsions, we have observed that the surfactant sodium dodecylbenzenesulfonate (SDBS) not only provides colloidal stability but also influences the rate of polymerization. Increasing the SDBS concentration results in higher polymerization rates, although the molecular weight distribution and particle size distribution are not significantly impacted. We have also examined another common sulfonate surfactant, DOWFAX 8390. In contrast to SDBS, DOWFAX 8390 does not affect the polymerization rate. Furthermore, DOWFAX‐stabilized polymerizations are slower than SDBS‐stabilized polymerizations. TEMPO‐mediated bulk styrene polymerizations are also accelerated significantly in the presence of SDBS. Although the mechanism for the rate acceleration is unknown, the experimental evidence suggests that SDBS is participating in the generation of radicals capable of propagating, thereby reducing the TEMPO concentration within the particles. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5974–5986, 2006     

AB and ABA type butyl acrylate and styrene block copolymers via RAFT‐mediated miniemulsion polymerization

Two trithiocarbonate reversible addition fragmentation chain transfer (RAFT) agents are compared in miniemulsion polymerization of styrene and butyl acrylate and the formation of seeded emulsion block copolymers. The order of block synthesis and the number of block segments per polymer are discussed. The use of nonionic surfactants is examined and the type of surfactant in relation to the monomer used is found to have a significant affect on latex formation. Conditions are shown by which AB and ABA type block copolymers can be successfully prepared via a seeded RAFT‐mediated emulsion polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 588–604, 2007     

Influence of camphorsulfonic acid in nitroxide‐mediated styrene miniemulsion polymerization

The rate‐accelerating effects of camphorsulfonic acid (CSA) on nitroxide‐mediated styrene miniemulsion polymerization were studied. Polymerizations were initiated with benzoyl peroxide (BPO) as an initiator and mediated with either 2,2,6,6‐tetramethylpiperidinyloxy (TEMPO) or 4‐hydroxy‐2,2,6,6‐tetramethylpiperidinyloxy (OH‐TEMPO). Although CSA has been used to accelerate the rate in bulk nitroxide‐mediated polymerizations, it has not been well studied in emulsion/miniemulsion. With dispersed systems, the effectiveness of CSA is likely to be affected by partitioning between the aqueous and organic phases. In styrene miniemulsion experiments performed over a range of conditions, the effect of adding CSA varied from negligible to significantly increasing the final conversion and molecular weight, depending on the nitroxide:BPO ratio. At a ratio of nitroxide:BPO = 1.7, the effect of CSA addition is small, whereas the final conversion and molecular weight are dramatically enhanced by CSA addition when the nitroxide:BPO ratio is 3.6. CSA is most effective in enhancing the rate and molecular weight when the initial free‐nitroxide concentration is higher. The magnitude of the rate and molecular weight enhancement was similar for TEMPO and OH‐TEMPO despite their differences in water solubility. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2828–2841, 2002     

The influence of RAFT on the rates and molecular weight distributions of styrene in seeded emulsion polymerizations

Seeded emulsion polymerizations of styrene in the presence of two reversible addition–fragmentation chain‐transfer (RAFT) agents were studied. We designed the seed to be small to observe the effects of exit and, we made the seed of poly(methyl methacrylate) so that the molecular weight distributions of poly(styrene) by gel permeation chromatography could be obtained independently by UV detection. The rates were significantly retarded by the presence of a RAFT agent, with the retardation being greater with an EMA RAFT agent [2‐(ethoxycarbonyl)propyl‐2‐yl dithiobenzoate] than with a cumyl RAFT agent (2‐phenylprop‐2‐yl dithiobenzoate). We propose that exit from the particles after fragmentation was the main cause of retardation. In addition, the number‐average molecular weight and polydispersities (broad) did not resemble the characteristic living behavior found in bulk or solution. This was a result of the continuous transportation of RAFT agent into the particles during interval II and the transportation of a small amount during interval III. A conspicuous red layer was also observed at the beginning of the polymerization. The red layer consisted of low molecular weight dormant species swollen with monomer. Once the switch from interval II to interval III occurred, the low molecular weight species coalesced to form a red coagulant. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3864–3874, 2000