Reversible Addition–Fragmentation ChainTransfer/Miniemulsion Polymerization of ButylMethacrylate in the Presence of b-Cyclodextrin

FENG ZHANG, PEIHONG NI, QINGFENG XIONG, ZHANGQING YU

School of Chemistry and Chemical Engineering, The Key Laboratory of Organic Synthesis of Jiangsu Province,Soochow University, Suzhou 215006, China

Received 15 August 2004; accepted 31 January 2005DOI: 10.1002/pola.20759Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: In the presence of �-cyclodextrin (�-CD), reversible addition–fragmenta-tion chain transfer (RAFT) polymerization has been successfully applied to controlthe molecular weight and polydispersity [weight-average molecular weight/number-average molecular weight (Mw/Mn)] in the miniemulsion polymerization of butylmethacrylate, with 2-cyanoprop-2-yl dithiobenzoate as a chain-transfer agent (orRAFT agent) and 2,20-azoisobutyronitrile (AIBN) as an initiator. �-CD acted as both astabilizer and a solubilizer, assisting the transportation of the water-insoluble, low-molecular-weight RAFT agent into the polymerization loca (i.e., droplets or latexparticles) and thereby ensuring that the RAFT agent was homogeneous in the poly-merization loca. The polymers produced in the system of �-CD exhibited narrowerpolydispersity (1.2 < Mw/Mn < 1.3) than those without �-CD. Moreover, the number-average molecular weight in the former case could be controlled by a definite amountof the RAFT agent. Significantly, �-CD was proved to have a favorable effect on thestability of polymer latex, and no coagulum was observed. The effects of the concen-trations of the RAFT agent and AIBN on the conversion, the molecular weight andits distribution, and the particle size of latices were investigated in detail. Further-more, the influences of the variations of the surfactant (sodium dodecyl sulfate) andcostabilizer (hexadecane) on the RAFT/miniemulsion polymerization were alsostudied. VVC 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2931–2940, 2005

Keywords: �-cyclodextrin; butyl methacrylate; kinetics (polym.); latex stability; mini-emulsion polymerization; reversible addition fragmentation chain transfer (RAFT)

INTRODUCTION

There has been substantial interest in reversibleaddition–fragmentation chain transfer (RAFT)polymerization, especially for the developmentof solution and bulk polymerization systems.1–4

Since the industrial manufacturing techniquesfor polymer latex are mature and the productsare environmentally friendly, it is necessary to

conduct RAFT polymerizations in emulsion5–7 orminiemulsion8–10 systems. However, RAFT/emulsion polymerization has not been particu-larly successful because of issues of mass trans-fer of chain-transfer agents (CTAs). Previousstudies have reported high levels of coagulum,thick red layers (phase separation) forming dur-ing the polymerization process, and very slowpolymerization rates.7,11,12 There are similarphenomena in RAFT/miniemulsion polymeriza-tion.

In an ideal case of miniemulsion polymer-ization, there is no need for a RAFT agent to

Correspondence to: P. Ni (E-mail: phni@suda.edu.cn) orZ. Yu (E-mail: yzq62@hotmail.com)

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 2931–2940 (2005)VVC 2005 Wiley Periodicals, Inc.

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transport through the water phase according tothe classical nucleation mechanism of radicalentry into droplets forming particles.13 However,in RAFT/miniemulsion polymerization, not allthe original droplets may be converted to poly-mer particles. The droplets lacking living specieswill act as monomer reservoirs and eventuallysupply monomer to the growing polymer par-ticles. Then, RAFT agents existing in these res-ervoirs have to be transported into the growingparticles across the water phase. There are stilltwo problems to be solved. First, RAFT agentsreaching earlier the locus of polymerization willcontrol the propagation process, whereas thosearriving later will broaden the molar mass dis-tribution. Second, if RAFT agents have beenconverted into dormant species in the aqueousphase, they will become oligomers and form ared layer. Whatever the case, the presence ofa RAFT agent will impact the colloid stabilityof the miniemulsion,11,14–16 bringing on a sepa-rate organic phase and a broad polydispersity.Barner-Kowollik et al.17 summarized the differ-ent aspects of RAFT miniemulsion polymeriza-tion in their highlight article. One idea thatthey proposed is that the polydispersity indices(PDIs) are generally larger than those in corre-sponding bulk or solution processes.

To obtain a homogeneous RAFT agent distrib-uted in each nuclear particle, several strategieshave been employed to get rid of the red layer.Prescott et al.12 reported that using acetone as asolubilizer to assist the transport of the RAFTagent into seed particles allowed the formationof latices in a controlled fashion. De Brouweret al.15 and Tsavalas et al.16 found that the redlayer was present in ionically stabilized minie-mulsions and absent in miniemulsions stabilizedwith a nonionic surfactants. Uzulina et al.18

used a macro-CTA as a hydrophobe to stabilizethe miniemulsion. They found that the polymer-ization was devoid of flocs, but a competitiveprocess occurred between the RAFT polymeriza-tion in miniemulsion droplets and the genera-tion of new particles in which uncontrolled poly-merization occurred.

Cyclodextrins (CDs) are well known as cyclicspecies that can solubilize hydrophobic com-pounds in aqueous media.19 The solubilizationof organic compounds is affected by the com-plexation of the water-insoluble species withinthe hydrophobic cavity of the CD. This inclu-sion phenomenon leads to dramatic changes ofthe solution properties of the guest molecule.

Rimmer and a coworker20,21 used CD to solubi-lize highly hydrophobic monomers (e.g., octa-decyl methacrylate and dodecyl methacrylate) inemulsion polymerization. They discovered thatthese hydrophobic monomers could be homopoly-merized in an aqueous emulsion with CD totransport oil-soluble components into the aque-ous phase. Storsberg et al.22 first described theATRP of CD-complexed methyl methacrylatefrom a homogeneous aqueous solution.

Until now, using CDs in the RAFT/miniemul-sion polymerization of butyl methacrylate(BMA) has not been described. In this article,we first show that the miniemulsion polymeriza-tion process allows the synthesis of a stablelatex via a RAFT mechanism with �-cyclodex-trin (�-CD) as an efficient stabilizer and solubil-izer in the miniemulsion system. The influenceof reaction parameters, such as �-CD, theamounts of the RAFT agent, the surfactant, andthe costabilizer, on the molecular weight andPDI is examined in detail.

EXPERIMENTAL

Materials

BMA was obtained from Shanghai ChemicalReagent Co., Ltd. (Shanghai, China), and puri-fied by distillation under reduced pressure. 2,20-Azoisobutyronitrile (AIBN; Shanghai ChemicalReagent; 98%) was recrystallized from ethanoland kept in a refrigerator under 4 8C. 2-Cyano-prop-2-yl dithiobenzoate (CPDB) was preparedaccording to the method reported by Mitsukamiet al.23 and Tang et al.24 �-CD was purchasedfrom Shanghai Chemical Reagent and recrystal-lized from demineralized water. Hexadecane(HD; Merck), sodium dodecyl sulfate (SDS; Suz-hou Zhenya Chemical Works, Suzhou, China),cyclohexane, and hydroquinone were used asreceived. Deionized water was used throughoutthe experiments.

Measurement of the Solubilization of b-CDfor a RAFT Agent

Typically, 0.125 g of a RAFT agent (CPDB) wasdissolved in 250 mL of cyclohexane (CH). Thesolution (10 mL) was diluted to 250 mL withcyclohexane again, and a 0.02 g/L CPDB–CHdilute solution was obtained. The 0.02 g/LCPDB–CH solution (20 mL) was added to 20 mL

2932 ZHANG ET AL.

of a �-CD aqueous solution (with different con-centrations of �-CD ranging from 0 to 0.04 g/L)with vigorous stirring at 70 8C for 30 min.CPDB was well dispersed and gave a light pinkdisperse system. The mixed solution was pouredinto a separatory funnel and kept for 30 min. Theoil phase was separated for further measure-ment of residual CPDB in cyclohexane. UV–visspectra were recorded with a PerkinElmerLambda 17 UV–vis spectrophotometer. The max-imum absorbance was found at 297 nm. The sol-ubilization of �-CD was evaluated by the absorb-ance obtained at various �-CD concentrations.

RAFT/Miniemulsion Polymerization

All reactions were carried out according to thefollowing procedure. A typical miniemulsion wasprepared by the mixing of an oil phase (contain-ing monomer BMA, costabilizer HD, RAFTagent CPDB, and initiator AIBN) with a waterphase (containing surfactant SDS and solubil-izer �-CD, as mentioned previously) under vigo-rous magnetic agitation for 15 min for homoge-nization. Then, the crude emulsion was soni-cated by a probe (CPS-2, Shanghai BransonUltrasonic Co., China) at an amplitude of 60%of output in the pulse mode for a period of5 min. The miniemulsion was immediatelytransferred into a four-necked, 250-mL, round-bottom flask equipped with a mechanical stirrer,a nitrogen inlet, a thermometer, and a refluxcondenser. The flask was then immersed into anoil bath that had been preheated to the reactiontemperature (70 8C), and RAFT/miniemulsionpolymerization was carried out under a nitrogenatmosphere for approximately 9 h.

Kinetic Analysis

The conversion of the monomer to the poly-mer was followed through dry solids (gravimet-ric analysis). Samples were taken regularlythroughout the polymerization, immediatelyquenched with hydroquinone, and dried in avacuum oven.

Gel Permeation Chromatography (GPC) Analysis

The number-average molecular weight (Mn) andmolecular weight distribution [weight-averagemolecular weight/number-average molecularweight (Mw/Mn), denoted by PDI] were recordedon a Waters 1515 GPC instrument with tetra-

hydrofuran as an eluent at a flow rate of1.0 mL/min. The molecular weights were deter-mined from standard polystyrene calibration.

Particle Size and Size Distribution of Latices

The particle size and size distribution of BMApolymer latices were obtained with an HPPShpp5001 high-performance particle size instru-ment at 25 8C. Original latex samples werediluted with deionized water before the meas-urements to adjust the light strength suitable tothe measurement conditions. The cumulantmethod was chosen for measuring the z-averagehydrodynamic diameter and size polydispersity.

Transmission Electron Microscopy (TEM)

TEM analysis was performed with a Hitachi H-600. The synthesized latex was diluted withdeionized water to about 1:100 and then stainedwith 2 drops of a 1 wt % tungstophosphoric acidaqueous solution. One drop of the stained latexwas placed on the coated side of a 200-mesh cop-per grid in a petri dish. After 18–24 h of dryingat the ambient temperature, the samples wereready to be observed.

RESULTS AND DISCUSSION

A basic recipe for the RAFT/miniemulsion poly-merization of BMA is listed in Table 1.

To investigate the influence of variable condi-tions on the kinetics of RAFT/miniemulsion pol-ymerization and on the molecular weights andmolecular weight distributions, we devised aseries of polymerization conditions for under-standing the controlled free-radical polymeriza-tion in a heterogeneous system. As necessary,

Table 1. Basic Recipe of MiniemulsionPolymerization

IngredientsAmount

(g/100 g of Miniemulsion)

Monomer (BMA) 10–20Surfactant (SDS) 0.2–0.6Initiator (AIBN) 0.1–0.3Costabilizer (HD) 0.2–0.6RAFT agent (CPDB) 0.2–0.6Solubilizer (�-CD) 0.05–0.6Deionized water 80–90

RAFT/MINIEMULSION POLYMERIZATION 2933

two cases of RAFT polymerization of BMA inminiemulsions with and without �-CD werecompared in terms of the morphology of thelatex and the molecular weight distribution(Mw/Mn).

Effect of b-CD

It is well known that �-CDs are cyclic oligoamy-loses that exhibit a torus-shaped structure witha hydrophobic cavity and a hydrophilic outerside. In this work, �-CD was used as both a sol-ubilizer for an oil-soluble RAFT agent and a sta-bilizer for polymeric latex because it couldenclose smaller hydrophobic molecules to trans-port them across the water phase to the poly-merizing loca.

A special experiment was performed to demon-strate the solubilization of �-CD for the RAFTagent. The water-insoluble RAFT agent was firstdissolved in cyclohexane and then placed in inti-mate contact with a �-CD aqueous solution. Thehydrophobic cavity encapsulated organic com-pounds to form an inclusion complex (CPDB–CD)and allowed the RAFT agent to be dissolved inwater. After the removal of the water phase, theoil phase was determined with a UV–vis spectro-photometer, and the absorbance of residual CPDBin cyclohexane was obtained. The results areshown in Table 2. The absorbance of the �-CDsystems was much lower than that of the systemwithout �-CD. The solubilization of �-CD for veryhydrophobic monomers was also confirmed byRimmer and a coworker20,21 and Yuan et al.25

Three typical experiments were designed tostudy the action of �-CD in RAFT/miniemulsionpolymerization. The amounts of the initial ingre-dients were kept constant (10 g of BMA, 0.20 gof SDS, 0.10 g of AIBN, 0.20 g of HD, 0.20 g ofCPDB, and 90 g of deionized water). The addedamounts of �-CD in these experiments were

changed (0.05, 0.20, and 0.60 g for runs 1, 2,and 3, respectively).

The conversion–time plots for the BMA mini-emulsion polymerizations with different amountsof �-CD are shown in Figure 1(a). With anincrease in the polymerization time, the conver-sions of the BMA monomer increased.

In the absence of �-CD, the hydrophobicRAFT agents (CPDB) transferred slowly fromone unreacted oil droplet to another one contain-ing a free radical or oligomer radical; this led touneven concentrations of the CTA in the RAFT/miniemulsion polymerization. Therefore, themolecular weight distributions of the productsbecame broad.

On the contrary, the system containing �-CDwas improved in its polydispersity. Figure 1(b)depicts the effects of different amounts of �-CDon the molecular weight distribution (Mw/Mn) ofthe evolution for the miniemulsion polymeriza-tion of BMA. A large decline of Mw/Mn is evidentfrom a comparison of the results of run 2 (0.2 gof �-CD) and those of run 1 (0.05 g of �-CD),indicating that moderate �-CD can act as anefficient solubilizer in the RAFT/miniemulsionsystem.

The decrease in PDI can be mainly attributedto the following factor: the addition of �-CD tothe system expedited the transfer of the oil-solu-ble RAFT agent (CPDB) and stabilized low-molecular-weight radicals. This resulted in auniform distribution of both the CTA andoligomer radicals in the polymerization loca(droplets or latices). At the same time, no redlayer or flocculation was observed. When exces-sive �-CD was used in the miniemulsion system(run 3, 0.6 g of �-CD), however, the PDI values(Mw/Mn ¼ 1.24–1.34) were slightly higher thanthose in run 2 (Mw/Mn ¼ 1.21–1.29). This factcan be explained by the postulate that there isan optimal proportion (1:1 w/w) between theRAFT agent (CPDB) and its solubilizer (�-CD).Much more �-CD is likely to enclose partial sur-factants and weaken the effect of emulsification,yielding poorly controlled polymerization andbroad molecular weight distributions.

The Mn values for three experiments with dif-ferent amounts of �-CD are shown in Figure1(c). The first observation is that the plots of Mn

versus the conversion are linear, and this indi-cates that the polymerization proceeded in acontrolled fashion. The next observation is thatincreasing the amount of �-CD had little effecton Mn in all cases.

Table 2. Absorbances of Residual CPDB inCyclohexane after the Solubilization Experiments

RunConcentrationsof �-CD (g/L)

Absorbanceof CPDB

1 0 1.7962 0.005 1.0273 0.010 1.1284 0.020 0.9595 0.025 0.8146 0.040 0.813

2934 ZHANG ET AL.

To further study the effect of �-CD on thestability and morphology of polymer latices, wecompared TEM images of poly(butyl methacry-

late) (PBMA) prepared with and without �-CD,as shown in Figure 2(a,b), respectively. TheTEM photographs show that PBMA particlesproduced in the presence of �-CD had asmoother surface and more uniform particle sizethan those prepared in the absence of any �-CD.

Figure 2. TEM images of PBMA latex particles (a)in the presence and (b) in the absence of �-CD.

Figure 1. (a) Conversion–time profiles, (b) PDI–con-version profiles, and (c) Mn–conversion profiles forBMA miniemulsion polymerizations with differentamounts of �-CD and constant amounts of AIBN,CPDB, SDS, and HD.

RAFT/MINIEMULSION POLYMERIZATION 2935

Moreover, the storage stability of polymerlatex increased with an increase in the amountof �-CD. Coagulation occurred in the systemwith less �-CD, whereas good colloid stabilityresulted from more �-CD for the RAFT/mini-emulsion polymerization. As can be clearly seenfrom these phenomena, the addition of �-CDprovides a beneficial effect to the latex stability.

Table 3 shows the influence of the amount of�-CD on the particle size and particle size distri-bution of PBMA latex particles. The z-averageparticle size (Dz) of the PBMA latex increased asthe amount of �-CD increased, whereas the par-ticle size distribution (size PDI) became nar-rower. This fact can be explained with theassumption that partial surfactants wereenclosed to form complexes by �-CD. As a result,the number of oil droplets was reduced duringthe particle formation phase, and less but biggerparticles formed in the polymerization process.26

Effect of the RAFT Agent (CPDB)

Conversion–time, Mn–conversion, and PDI–con-version profiles for BMA miniemulsion polymer-izations with different amounts of the RAFTagent (CPDB) are shown in Figure 3. A distinctdecrease in the polymerization rate, Mn, andPDI of PBMA can be observed with an increas-ing amount of CPDB. This is in agreement withpreviously published results.5,10,11

More CPDB led to an increase in the times ofchain transfer and chain equilibrium in minie-mulsion polymerizations, and this resulted inlower polymerization rates and not high Mn val-ues, as well as narrow molecular weight distri-butions.

Effect of the Initiator Amount

In an attempt to find an optimal proportionbetween the initiator and monomer, a series ofexperiments were investigated with differentamounts of the initiator, constant amounts of

BMA (20 g), SDS (0.4 g), HD (0.4 g), and H2O(80 g), and a constant ratio of AIBN to CPDB to�-CD (1:2:2 w/w). The results are shown in Fig-ure 4. The first observation is that there was lit-

Table 3. Effects of Amounts of �-CD on the ParticleSize and Size Polydispersity of PBMA Latex Particles

Amountof �-CD (g) Dz (nm)

SizePolydispersity

0 230 0.2380.2 240 0.1460.6 369 0.116

Figure 3. (a) Conversion–time profiles, (b) Mn–con-version profiles, and (c) PDI–conversion profiles forBMA miniemulsion polymerizations with differentamounts of CPDB and constant amounts of AIBN,SDS, and HD.

2936 ZHANG ET AL.

tle change in the polymerization rate with anincreasing amount of the initiator because theratio of AIBN to CPDB was always kept con-stant. The second observation is that Mn

decreased with an increasing amount of the ini-tiator, but the plots of Mn versus the conversionare linear; this indicates that the polymerizationproceeded in a controlled fashion. The thirdobservation is that the PDI of PBMA obtainedwith 0.2 g of AIBN and 0.4 g of CPDB was nar-rower than those of the other two experiments,although the ratio of AIBN to CPDB was main-tained at 1:2 (w/w). Therefore, the optimal pro-portion between the initiator and monomer was0.2:100 (w/w) when a constant ratio of AIBN toCPDB of 1:2 (w/w) was kept.

Effect of the Surfactant

The influence of SDS on the first-order rate, Mn,and PDI of PBMA is revealed in Figure 5. Thebasic ingredients in our experiments includedBMA (20 g), HD (0.4 g), H2O (80 g), AIBN (0.2 g),CPDB (0.4 g), and �-CD (0.4 g). The amount ofSDS was varied from 0.2 to 0.6 g. Figure 5(a)shows that the first-order rate plot is more lin-ear as the amount of SDS increases. At thesame time, the polymerization rate, final conver-sion, and Mn increased linearly as the amountof SDS increased, as shown in Figure 5(b). Anacceptable explanation for these results could bethat the increase in the amount of SDS madethe polymerization system produce many moreparticles that were smaller in size, and thisincreased the opportunity of radical capture.This resulted in the increase of both the poly-merization rate and the final conversion. Whenthe amount of SDS was increased from 0.2 to0.6 g, Mw/Mn of PBMA became narrower, rang-ing from 1.59 to 1.25 and indicating a stationarypolymerization process in the system [seeFig. 5(c)]. The reason is that a good effect ofemulsification facilitates homogeneous minie-mulsion. However, an excess of the surfactantwill result in significant amounts of micellarnucleation. The polymerization in these secon-dary particles would then take place via a con-ventional free-radical mechanism, which wouldlead to a fraction of high-molecular-weight poly-mer in the system. To avoid bimodal nucleation,the amount of SDS should be within a certainrange.13,27

Effect of the Costabilizer (HD)

As stated earlier, the destabilization of nano-droplets by Ostwald ripening can be efficientlyslowed by the addition of a hydrophobic agent

Figure 4. (a) Conversion–time profiles, (b) Mn–con-version profiles, and (c) PDI–conversion profiles forBMA miniemulsion polymerizations with differentamounts of the initiator, constant amounts of SDSand HD, and constant proportions of AIBN andCPDB.

RAFT/MINIEMULSION POLYMERIZATION 2937

(costabilizer) to the dispersed phase, whichcounteracts the droplet or Laplace pressure ofthe droplet.28 In this study, HD was used as acostabilizer, and a steady-state miniemulsifica-

tion was obtained after an efficient homogeniza-tion process.

The influence of HD on the polymerizationrate, Mn, and PDI is presented in Figure 6. With

Figure 5. (a) First-order rate plots, (b) Mn–conver-sion profiles, and (c) PDI–conversion profiles for BMAminiemulsion polymerizations with different amountsof SDS and constant amounts of AIBN, CPDB, HD,and �-CD.

Figure 6. (a) Conversion–time profiles, (b) Mn–con-version profiles, and (c) PDI–conversion profiles forBMA miniemulsion polymerizations with differentamounts of HD and constant amounts of AIBN,CPDB, SDS, and �-CD.

2938 ZHANG ET AL.

the aforementioned recipe (20 g of BMA, 0.4 g ofSDS, 80 g of H2O, 0.2 g of AIBN, 0.4 g of CPDB,and 0.4 g of �-CD), increasing the amount of HD(from 0.2 to 0.6 g) resulted in a decrease in thepolymerization rate because the hydrophobiccostabilizer had an obstructive effect on theentry of the radicals into the droplets. It isobvious that increasing the amount of HD hadlittle effect on the molecular weight but stronglyinfluenced PDI of PBMA. As expected, the addi-tion of HD to the system generated more stabledroplets and an even distribution of the RAFTagent in polymerizing loca, and this led to con-trolled behavior, as exhibited by a linearincrease in Mn.

CONCLUSIONS

RAFT techniques were successfully applied tothe miniemulsion polymerization of BMA, whichretained a controlled character, as demonstratedby some results, such as the control of the finalmolecular weight based on the amount of theRAFT agent used, the evolution of the molecularweight distribution as a function of conversion,and the low polydispersities of the obtained poly-mers.

The most important innovation of this studyis that �-CD can be used as a stabilizer and asolubilizer to assist the transport of water-insoluble RAFT agents into polymerizing loca;this results in a stationary polymerization proc-ess. The high levels of coagulum and thick redlayer, as reported by previous studies, have beeneliminated completely.

The addition of �-CD to the system had littleeffect on the molecular weight and polymeriza-tion rate but strongly influenced Mw/Mn, theparticle size, and the particle size distribution ofPBMA latex particles. The particle size distribu-tion of the PBMA latex became narrower andthe number-average particle size increased asthe amount of �-CD was increased. The molecu-lar weight distribution decreased as the amountof �-CD increased in a certain range (0.05–0.6 g).However, when the amount of �-CD increasedbeyond the range, there was a small increase inthe PDI of the polymer. The optimal ratio ofCPDB to �-CD was 1:1 (w/w).

The optimal ratio of the initiator to the mono-mer was 0.2:100 (w/w) when a constant ratio ofAIBN to CPDB of 1:2 (w/w) was used. When theamount of SDS was increased from 0.2 to 0.6 g,

Mw/Mn of PBMA became narrower, ranging from1.59 to 1.25. All the PBMA latices were very sta-ble under the experimental conditions.

The authors gratefully acknowledge the financial sup-port of the National Natural Science Foundation ofChina (20276044) and the Natural Science Founda-tion of the Educational Department of Jiangsu Prov-ince, China (03KJD150188).

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