atrp of poss monomers revisited: toward high-molecular weight methacrylate–poss (co)polymers

10
ATRP of POSS Monomers Revisited: Toward High-Molecular Weight MethacrylatePOSS (Co)Polymers Vladimír Raus,* Eva C ̌ adova ́ , Larisa Starovoytova, and Miroslav Janata Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic * S Supporting Information ABSTRACT: For the rst time, ATRP was successfully employed for homopolymerization of a commercial meth- acrylate-functionalized polyhedral oligomeric silsesquioxane (POSS) monomer, iBuPOSSMA, to high molecular weights. It was found that iBuPOSSMA has a low ceiling temperature (T c ); therefore, low temperatures and/or high initial monomer concentrations need to be used in order to avoid low degrees of polymerization that had been observed previously. The values of T c , as well as of the polymerization enthalpy ΔH p and entropy ΔS p were determined to be 130 °C (at [M] 0 = 1 M), 41 kJ mol 1 , and 101 J mol 1 K 1 , respectively. Under optimized conditions, poly(iBuPOSSMA) homopolymers having low dispersity and high M n , ranging from 23 000 to 460 000, were obtained in a well-controlled ATRP process. Moreover, various block copolymers having high-M n poly(iBuPOSSMA) blocks were prepared by copolymerization of iBuPOSSMA with methyl methacrylate and styrene. INTRODUCTION Polyhedral oligomeric silsesquioxanes (POSS) of the general formula (RSiO 1.5 ) 8 rank among the most studied members of the silsesquioxane family. Their nanometer sized, cube-like molecules, made of silicon atoms linked together through stable SiO bonds, bear organic substituents at each corner, which gives POSS compatibility or miscibility with many polymers. The unique POSS structure made them attractive as nanollers in polymer nanocomposites where POSS were employed to enhance mechanical and thermal properties. 15 Importantly, some of the corner substituents can be polymerizable groups, which oers immense opportunities for synthesis of organicinorganic hybrid polymeric materials, especially considering the fact that several such POSS-based monomers are today commercially available. Contemporary methods of controlled polymerization such as living anionic polymerization or reversible-deactivation radical polymerization (RDRP) are potentially powerful tools for synthesis of POSS (co)polymers with diverse composition and topology. In this regard, particular attention has recently been paid to preparation of POSS-based block copolymers and studies of their self-assembly behavior. 610 Compared to statistical/random copolymerization, 11,12 synthesis of block copolymers is more demanding as homopolymerization of the bulky POSS monomer needs to be achieved. Steric hindrance arising from the POSS bulkiness can interfere with the polymerization process and complicate attaining high molecular weights (MW). 2 Still, various POSS monomers were homopolymerized or block copolymerized in a controlled fashion via several methods. The achievements in this eld have been reviewed recently. 13 Living anionic polymerization was successfully implemented in synthesis of POSS polymers by Hirai and co-workers who prepared homopolymers of methacrylate-functionalized POSS (POSSMA) and also synthesized its block copolymers with methyl methacrylate (MMA) and styrene. 9,10,1418 The self- assembly characteristics of the block copolymers were then studied with lithography applications in mind. In this context, it was pointed out that both low dispersity and high degree of polymerization of POSS chain are essential for the formation of the desired hierarchical nanostructures. 9,14 Compared to anionic polymerization, RDRP methods such as atom transfer radical polymerization (ATRP) or reversible additionfragmentation chain transfer (RAFT) are generally less experimentally demanding and somewhat more versatile. Surprisingly enough, literature on RDRP use for POSS monomer homopolymerization has been rather limited so far, and attempts to optimize the polymerization conditions to achieve high-MW products have been even scarcer. For instance, using RAFT, Mya et al. synthesized quite high-MW polymer (M n(SEC) = 32 300) by employing a high monomer/ RAFT agent ratio. 19 However, the polymerization was plagued by broadening of the MW distributions at higher conversions, resulting in relatively high dispersity (Đ 1.6). In another important report, Deng and co-workers also aimed for high- MW products by employing monomer/RAFT agent ratios of 30:1 and 60:1. 7 Unfortunately, a loss of control was observed at higher conversions, and so the degree of polymerization (DP n ) Received: July 28, 2014 Revised: October 1, 2014 Published: October 23, 2014 Article pubs.acs.org/Macromolecules © 2014 American Chemical Society 7311 dx.doi.org/10.1021/ma501541g | Macromolecules 2014, 47, 73117320

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Page 1: ATRP of POSS Monomers Revisited: Toward High-Molecular Weight Methacrylate–POSS (Co)Polymers

ATRP of POSS Monomers Revisited: Toward High-Molecular WeightMethacrylate−POSS (Co)PolymersVladimír Raus,* Eva Cadova, Larisa Starovoytova, and Miroslav Janata

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, CzechRepublic

*S Supporting Information

ABSTRACT: For the first time, ATRP was successfullyemployed for homopolymerization of a commercial meth-acrylate-functionalized polyhedral oligomeric silsesquioxane(POSS) monomer, iBuPOSSMA, to high molecular weights. Itwas found that iBuPOSSMA has a low ceiling temperature(Tc); therefore, low temperatures and/or high initial monomerconcentrations need to be used in order to avoid low degreesof polymerization that had been observed previously. Thevalues of Tc, as well as of the polymerization enthalpy ΔHp and entropy ΔSp were determined to be 130 °C (at [M]0 = 1 M),−41 kJ mol−1, and −101 J mol−1 K−1, respectively. Under optimized conditions, poly(iBuPOSSMA) homopolymers having lowdispersity and high Mn, ranging from 23 000 to 460 000, were obtained in a well-controlled ATRP process. Moreover, variousblock copolymers having high-Mn poly(iBuPOSSMA) blocks were prepared by copolymerization of iBuPOSSMA with methylmethacrylate and styrene.

■ INTRODUCTION

Polyhedral oligomeric silsesquioxanes (POSS) of the generalformula (RSiO1.5)8 rank among the most studied members ofthe silsesquioxane family. Their nanometer sized, cube-likemolecules, made of silicon atoms linked together through stableSi−O bonds, bear organic substituents at each corner, whichgives POSS compatibility or miscibility with many polymers.The unique POSS structure made them attractive as nanofillersin polymer nanocomposites where POSS were employed toenhance mechanical and thermal properties.1−5 Importantly,some of the corner substituents can be polymerizable groups,which offers immense opportunities for synthesis of organic−inorganic hybrid polymeric materials, especially considering thefact that several such POSS-based monomers are todaycommercially available.Contemporary methods of controlled polymerization such as

living anionic polymerization or reversible-deactivation radicalpolymerization (RDRP) are potentially powerful tools forsynthesis of POSS (co)polymers with diverse composition andtopology. In this regard, particular attention has recently beenpaid to preparation of POSS-based block copolymers andstudies of their self-assembly behavior.6−10 Compared tostatistical/random copolymerization,11,12 synthesis of blockcopolymers is more demanding as homopolymerization ofthe bulky POSS monomer needs to be achieved. Sterichindrance arising from the POSS bulkiness can interfere withthe polymerization process and complicate attaining highmolecular weights (MW).2 Still, various POSS monomers werehomopolymerized or block copolymerized in a controlledfashion via several methods. The achievements in this field havebeen reviewed recently.13

Living anionic polymerization was successfully implementedin synthesis of POSS polymers by Hirai and co-workers whoprepared homopolymers of methacrylate-functionalized POSS(POSSMA) and also synthesized its block copolymers withmethyl methacrylate (MMA) and styrene.9,10,14−18 The self-assembly characteristics of the block copolymers were thenstudied with lithography applications in mind. In this context, itwas pointed out that both low dispersity and high degree ofpolymerization of POSS chain are essential for the formation ofthe desired hierarchical nanostructures.9,14 Compared toanionic polymerization, RDRP methods such as atom transferradical polymerization (ATRP) or reversible addition−fragmentation chain transfer (RAFT) are generally lessexperimentally demanding and somewhat more versatile.Surprisingly enough, literature on RDRP use for POSSmonomer homopolymerization has been rather limited so far,and attempts to optimize the polymerization conditions toachieve high-MW products have been even scarcer. Forinstance, using RAFT, Mya et al. synthesized quite high-MWpolymer (Mn(SEC) = 32 300) by employing a high monomer/RAFT agent ratio.19 However, the polymerization was plaguedby broadening of the MW distributions at higher conversions,resulting in relatively high dispersity (Đ ≈ 1.6). In anotherimportant report, Deng and co-workers also aimed for high-MW products by employing monomer/RAFT agent ratios of30:1 and 60:1.7 Unfortunately, a loss of control was observed athigher conversions, and so the degree of polymerization (DPn)

Received: July 28, 2014Revised: October 1, 2014Published: October 23, 2014

Article

pubs.acs.org/Macromolecules

© 2014 American Chemical Society 7311 dx.doi.org/10.1021/ma501541g | Macromolecules 2014, 47, 7311−7320

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of the isolated homopolymers was deliberately kept relativelylow (<30). Deng’s group later synthesized numerous low-DPnpoly(POSSMA) polymers which they successfully exploited asmacroinitiators in synthesis of diverse block copolymers.8,20,21

Further, RAFT was also utilized for block copolymerization ofPOSSMA initiated by poly(poly(ethylene glycol) methyl ethermethacrylate)12 and poly(ε-caprolactone)22 macroinitiators; aDPn value of up to about 60 was reached.ATRP was the first RDRP method employed in POSSMA

homopolymerization; however, it has been so far unsuccessfulin achieving high-MW homopolymers. This is startling,especially when considering the general robustness andversatility of the ATRP method. The first report, publishedalready in 2000 by Pyun and Matyjaszewski, described ATRP ofa POSSMA monomer, catalyzed by a CuCl/PMDETAcomplex.23 The prepared homopolymer had a low DPn of 14and a narrow MW distribution (Đ = 1.14). Triblock and star-shaped block copolymers bearing short polyPOSS blocks (DPn< 14) were also synthesized using butyl acrylate (BA) andmethyl acrylate as comonomers. The same group subsequentlyprepared poly(POSSMA)-b-poly(BA)-b-poly(POSSMA) ABAtriblock copolymers and studied their microstructure andproperties by various methods.24 Importantly, the desiredphase-separated microstructure was obtained only with a blockcopolymer having high-enough POSSMA/BA monomeric unitratio. Attempts to synthesize POSSMA blocks of DPn > 15were unsuccessful; therefore, relatively short difunctional BAmacroinitiator had to be employed to achieve the phase-separated product. The authors speculated that, due to thesteric reasons, the bromine end groups of the growingPOSSMA chains may be inaccessible to the catalytic complexafter certain DPn is reached. Perhaps this assumption is thereason why other authors did not attempt to use ATRP forsynthesis of higher molecular weight POSSMA polymers.6,25−32

Consequently, to our best knowledge, the highest POSSMAhomopolymer DPn achieved so far by ATRP of commercialmonomers is about 18.In this study, we reinvestigated the applicability of the ATRP

method to homopolymerization of a commercially availablePOSS monomer, methacryl−isobutyl−POSS (iBuPOSSMA).We show that, contrary to former beliefs, it is well possible tohomopolymerize a POSSMA monomer via ATRP to high MWwhen appropriate polymerization conditions are used. In thiscontext, we highlight the influence of the monomer’s lowceiling temperature on the polymerization kinetics. Besides thepreparation of a series of iBuPOSSMA homopolymers,synthesis of block copolymers with MMA and styrene havinghigh-MW iBuPOSSMA blocks is also reported.

■ EXPERIMENTAL SECTIONMaterials. Methyl 2-bromoisobutyrate (MBiB; Aldrich, ≥ 99%),

CuBr (Fluka, ≥ 98%), CuBr2 (Fluka, ≥ 99%), 2,2′-bipyridine (BiPy;(Aldrich, ≥ 99%), 1,1,4,7,7-pentamethyldiethylenetriamine (PMDE-TA; Aldrich, 99%), 1,1,4,7,10,10-hexamethyltriethylenetetramine(HMTETA; Aldrich, 97%), 1,8-diazabicyclo[5.4.0]undec-7-ene(DBU; Fluka, >99%), tris[2-(dimethylamino)ethyl]amine(Me6TREN; Aldrich, 97%), 1,4,8,11-tetramethyl-1,4,8,11-tetraazacy-clotetradecane (Me4Cyclam; Aldrich, 98%), tris(2-pyridylmethyl)-amine (TPMA; Aldrich, 98%), and 3-{3,5,7,9,11,13,15-hepta(2-methylpropyl)-pentacyclo[9.5.1.1.3,91.5,1517,13]-octasiloxan-1-yl}propylmethacrylate (iBuPOSSMA; Hybrid Plastics, molar weight = 943.64 g/mol) were used as received. Toluene was distilled with lithiumaluminum hydride and then from benzophenone ketyl prior to use.Tetrahydrofuran (THF) was distilled with lithium aluminum hydride

and then from sodium anthracenide prior to use. The initiator (MBiB)and ligands (BiPy, PMDETA, HMTETA, DBU, Me6TREN,Me4Cyclam, TPMA) were used as toluene solutions with concen-trations 10 mg/mL (MBiB) and 20 mg/mL (ligand/solution),respectively. Syntheses were carried out under argon atmosphere.

ATRP of iBuPOSSMA. A Typical Polymerization Procedure withPMDETA as a Ligand ([iBuPOSSMA]:[MBiB]:[CuBr]:[PMDETA] =30:1:1:1). CuBr (5.13 mg, 35.7 μmol) and iBuPOSSMA (1.012 g,1.07 mmol) were placed in a reaction flask, equipped with a magneticstirring bar. After thorough deoxygenation by several vacuum-argoncycles, degassed toluene (1.067 mL) was added to dissolveiBuPOSSMA. Subsequently, the PMDETA stock solution in toluene(0.31 mL, 35.7 μmol of PMDETA) was added. After 10 min of stirringat room temperature, a stock solution of MBiB in toluene (0.647 mL,35.7 μmol of MBiB) was added. The reaction flask was then placed inan oil bath preheated to 60 °C. After 24 h, the polymerization wasstopped by adding toluene solution of the radical inhibitor 4-tert-octylcatechol, and the polymerization mixture was cooled down.Afterward, the mixture was diluted with toluene (20 mL) andcentrifuged to remove the solids. The supernatant was precipitated in10-fold excess of methanol (MeOH); the precipitate was filtered on afrit with porosity of 4−16 μm, washed with MeOH, and dried invacuum at 40 °C for 24 h. If needed, the precipitation was repeatedonce or twice to remove the residual monomer.

Synthesis of Poly(MMA)-block-poly(iBuPOSSMA) Diblock Copoly-mer. In a typical experiment, poly(MMA) (1 g, Mn = 52 000, nBr =19.23 μmol), prepared by ATRP according to a published procedure,33

CuBr (2.76 mg, 19.23 μmol), CuBr2 (0.86 mg, 3.85 μmol), andiBuPOSSMA (1.815 g, 1.923 mmol) were mixed with 13 mL oftoluene in a round-bottomed flask, equipped with a stirring bar. Afterdissolution of the macroinitiator, polymerization was started by theaddition of TPMA stock solution in toluene (0.28 mL, 19.23 μmol ofTPMA). After 96 h, the polymerization was stopped, and the mixturewas processed in the same way as mentioned above.

Synthesis of Poly(iBuPOSSMA)-block-poly(MMA) and Poly-(iBuPOSSMA)-block-polystyrene Diblock Copolymers. These blockcopolymers were synthesized in a similar way as described above.Poly(iBuPOSSMA) of Mn(est) = 60 500 was used as a macroinitiator.[Monomer]:[macroinitiator]:[CuBr]:[ligand] ratio was 560:1:1:1 forMMA and 1980:1:1:1 for styrene (carried out in block). MMA/toluene (v/v) was 1:1. MMA was polymerized at 60 °C for 22 h,styrene at 100 °C for 23 h. Products were precipitated in MeOH,filtered, and dried in a vacuum at 40 °C for 24 h.

Characterization. Size exclusion chromatography (SEC) of theisolated (co)polymers was performed at 25 °C with two PLgelMIXED-C columns (300 × 7.5 mm, SDV gel with particle size 5 μm;Polymer Laboratories, USA) and with UV (UVD 250; Watrex, CzechRepublic) and RI (RI-101; Shodex, Japan) detectors. Tetrahydrofuranwas used as a mobile phase at a flow rate of 1 mL/min. The molecularweight values were calculated using Clarity software (Dataapex, CzechRepublic). Calibration with polystyrene standards (PSS, Germany)was used. 1H NMR spectra were measured in deuterochloroform(CDCl3) at 57 °C using a Bruker DPX 300 spectrometer at 300.1MHz. Hexamethyldisiloxane was used as an internal standard. The dn/dc values of the monomer (4.201 × 10−2 mL/g) and a samplehomopolymer of Mn(est) = 171 500 (5.180 × 10−2 mL/g) in THFsolutions were determined on a Brookhaven Instruments BI-DNDCdifferential refractometer.

■ RESULTS AND DISCUSSIONHomopolymerization of iBuPOSSMA. Nowadays, RDRP

protocols are the methods of choice for controlled synthesis ofhomo- and copolymeric materials with predeterminedcomposition, molecular weight, and architecture. As previouslystated, RAFT and ATRP have been applied to controlledsynthesis of various POSS-based materials. It comes as asurprise, however, that the contemporary literature lacks a moredetailed evaluation of reaction conditions suitable for ATRPhomopolymerization of POSS monomers. In the handful of

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published reports, PMDETA was used as a ligand almostexclusively with the catalyst being either CuCl or CuBr. Theonly exception was the employment of the CuCl/HMTETAsystem by Chen et al.25 This means that the majority of authorsused the catalytic system proposed by Pyun et al. in the firstreport on ATRP homopolymerization of POSS monomers,despite the alleged limitations associated with this catalyst/ligand combination.23,24

As was pointed out in the pioneering works, it is well possiblethat steric reasons play an important role in the POSSmonomers homopolymerization. In general, the role might be2-fold. First, due to the POSS bulkiness, the polymerization ratecan be expected to be lower compared to less bulky structurallysimilar monomers (e.g., MMA). Second, potential polymer-ization rate retardation arising from the inaccessibility ofinitiation sites at higher-MW polymer chains, suggested byPyun et al.,24 needs to be taken into consideration. Ligandchoice can be essential in both these regards as not only theactivity of the catalyst/ligand complex but also the complex sizecould possibly influence the course of the polymerization.Therefore, we started our investigation by evaluating suitabilityof various ligands for homopolymerization of the most widelyutilized POSS monomer, iBuPOSSMA (Scheme 1). Moreover,

we also wanted to critically assess the previous claims about thefeasibility of attaining high-MW products. For this reason, thetargeted DPn was initially set to about double of the valuereported as the maximal one, i.e., to 30.In the first set of experiments, seven common ATRP ligands,

i.e., BiPy, HMTETA, PMDETA, DBU, Me6TREN, TPMA, andMe4Cyclam, were trialed under following standard polymer-ization conditions. Polymerization was initiated by methyl 2-bromoisobutyrate (MBiB), and an equimolar amount ofcopper(I) bromide was used as a catalyst. The [CuBr]:[ligand]ratio was kept at 1:1 and 1:2 for multidentate (HMTETA,PMDETA, Me6TREN, TPMA, and Me4Cyclam) and bidentateligands (BiPy, DBU), respectively. Polymerizations were carriedout in toluene at standard monomer concentration of 0.5 g/mL(monomer/solvent); the [iBuPOSSMA]:[MBiB] ratio was30:1; the reaction temperature was 60 °C, and the polymer-ization was allowed to proceed for 24 h.The polymers were isolated by precipitation of their toluene

solutions in MeOH. Since the POSS polymer isolation by

precipitation is often considered troublesome, we would like tonote that for the higher-MW poly(iBuPOSSMA) prepared inthis study, precipitation in MeOH worked quite well. In mostcases, the first precipitation led to monomer content of lessthan 2%, and additional reprecipitation made the monomerconcentration negligible. It needs to be pointed out, however,that MeOH is less than ideal precipitation agent forpoly(iBuPOSSMA)/iBuPOSSMA mixtures due to the limitedmonomer solubility in this solvent. We estimated that at roomtemperature in MeOH, MeOH/toluene 10:1 (v/v), andMeOH/toluene 20:1 (v/v), the monomer solubility isapproximately 0.31, 0.76, and 0.38 g per 100 mL of solvent,respectively. This needs to be taken into account, especiallywhen precipitating low-conversion polymerization mixtures,because the amount of MeOH used for precipitation may notbe sufficient to dissolve all the present monomer.Monomer conversions were determined by SEC analysis of

the reaction mixture, taking advantage of the fact that themonomer signal is clearly separated from the solvent signal.Conversion was then calculated from the area of the polymerand monomer signals (RI detector) divided by the respectiverefractive index increments that were determined for thispurpose (see the Experimental Section). We consider thismethod to be substantially more accurate than conversioncalculation from NMR spectra (see below). It is noteworthy,that the conversions determined from SEC corresponded verywell with values calculated from gravimetry (not reportedhere).There are three number-average molecular weight (Mn)

values given for each entry in Table 1: Mn(theor), Mn(SEC), and

Mn(est). Mn(theor) is the theoretical molecular weight calculatedfrom the monomer conversion counting with 100% initiationefficiency of the initiator. The Mn(SEC) values were obtainedfrom SEC analysis using a system calibrated with polystyrenestandards. As has been previously pointed out by other authors,compared to reality, Mn(SEC) values are considerably under-valued.7,19,20 The difference is most likely caused by the vastlydifferent hydrodynamic volume of poly(iBuPOSSMA) andpolystyrene standards of the same molecular weight. Never-theless, when studying literature data, we noticed that there is alinear correlation between the Mn(SEC) values (polystyrenecalibration) and absolute Mn values determined by the SECMALLS method. Figure S1 (Supporting Information) shows a

Scheme 1. ATRP of iBuPOSSMA

Table 1. ATRP of iBuPOSSMA at [iBuPOSSMA]:[MBiB] =30:1a

Mn

entry ligandconvn(%)b Mn(SEC) Mn(theor)

c Mn(est)d Đb

1 BiPy 78 10500 22300 25800 1.442 HMTETA 75 9700 21400 22800 1.113 PMDETA 76 11300 21700 28600 1.214 DBU 71 9900 20300 23600 1.135 Me6TREN 81 13700 23100 37500 1.276 TPMA 81 9300 23100 21500 1.197 Me4Cyclam 82 31500 n.d. n.d. 4.82

aStandard polymerization conditions: 24 h, 60 °C, Cmon = 0.5 g/mL,[iBuPOSSMA]:[MBiB]: [CuBr]:[ligand] = 30:1:1:1 for HMTETA,PMDETA, Me6TREN, TPMA, Me4Cyclam and 30:1:1:2 for BiPy andDBU. bDetermined by SEC. cCalculated from monomer conversionassuming 100% initiation efficiency. dEstimated from the SEC values.

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plot of this dependence; data used for the plot constructionwere published by Hirai et al., who prepared poly-(iBuPOSSMA) with very low dispersity (Đ ≈ 1.04) by anionicpolymerization.9 The linear regression equation Mn(MAALS) =3.6906 × Mn(SEC) − 12982 can then be used for surprisinglyaccurate estimates of absolute molecular weights of poly-(iBuPOSSMA) homopolymers. This is illustrated by the Mn(est)values in Table 1, calculated from the above-mentionedequation, and their very good agreement with Mn(theor) values.Quite obviously, this approach gives the best results only forsamples having relatively low dispersity; therefore, the Mn(est)was not calculated for polymers with wide distribution of MW(e.g., entry 7 in Table 1). It is noteworthy that Mn of thehomopolymers can also be estimated from their 1H NMRspectra by the end group method using the signals of theinitiator fragment. Nevertheless, we deem this approach to belimited mainly to low-molecular weight poly(iBuPOSSMA)polymers where we obtained a good agreement betweenMn(NMR) values and Mn values determined by other means.However, signal overlap and low relative intensity of the signalsused for the Mn(NMR) calculation considerably decrease themethod accuracy for the high-MW products relevant to thispaper. For this reason, we did not systematically employ NMRfor molecular weight determination.From the data collected in Table 1, it is clear that under the

standard reaction conditions conversions around 75% wereachieved no matter which ligand was used. BiPy use resulted ina product with broader distribution of MW (Đ = 1.44), andSEC analysis revealed signs of MW distribution bimodality(entry 1). Polymerizations with HMTETA, PMDETA, andDBU yielded products with low dispersity in the range of 1.11−1.21 and unimodal SEC traces (entries 2−4). When thepolymerization time was prolonged to 48 h for HMTETA,practically identical conversion, MW, and dispersity wereachieved (data not shown), implying that certain DPn limithad been already reached in 24 h. With Me6TREN, relativelylow dispersity product was isolated, but its SEC trace showedstrong tailing pointing to the presence of a considerable low-MW fraction (entry 5). This fraction probably originates fromradical recombination in the early stages of the polymerization.The discrepancy between the Mn (est) and Mn (theor) values

indicates decreased initiation efficiency (≈ 60%) and furthersupports the assumption of chain inactivation due to radicalcoupling reactions. The experiment with TPMA as a ligandresulted into a product with quite low dispersity of 1.19, but asmall high-MW shoulder was observed in the SEC eluogram(entry 6). The last ligand tested was Me4Cyclam. Unfortu-nately, this highly active ligand showed to be unsuitable asmultimodal product was obtained in this case, which was alsoreflected in a very high dispersity value (entry 7).We also tested how the addition of the CuBr2 deactivator

would influence the outcome of these polymerizations (seeTable S1 in the Supporting Information). Nevertheless, onlysmall differences were observed, i.e., decreased conversion anddispersity for BiPy, and slightly improved initiation efficiencyfor PMDETA. Interestingly, in the TPMA experiment, CuBr2suppressed formation of the unwanted high-MW fraction.The data summarized in Table 1 show that poly-

(iBuPOSSMA) homopolymers of DPn higher than thepreviously proclaimed limit of about 15 POSS units24 can bereadily prepared via ATRP. The achieved DPn of about 25 wasnot much higher though. Moreover, the low monomer/initiatorratio and also the relatively long polymerization time werediminishing potential performance differences among the testedligands. Therefore, in the following experiments, we increasedthe [iBuPOSSMA]:[MBiB] ratio and again followed theperformance of selected ligands. Other polymerizationconditions remained the same. Results are shown in Table 2.In the first set of experiments, the [iBuPOSSMA]:[MBiB]

ratio was set to 100:1. The initial monomer concentration waskept the same as previously, i.e., 0.5 g/mL, and thus theconcentrations of other components (initiator, catalyst, andligand) decreased correspondingly. As follows from the results(entries 1, 4, 5, 6, and 8 in Table 2), the employed ligands faredvastly differently. With HMTETA, a clear increase in MW wasobserved compared to the experiments with the 30:1[iBuPOSSMA]:[MBiB] ratio. Nevertheless, the attainedMn(theor) of 29 400 corresponded to 31% conversion only(entry 1), implying rather low rate of polymerization undergiven conditions. Prolonging the reaction time to 48 h led toconsiderably higher conversion of 55% (entry 2). Even slowerpolymerization was observed with DBU (entry 4). Here,

Table 2. ATRP of iBuPOSSMA with Higher [iBuPOSSMA]:[MBiB] Ratiosa

Mn

entry ligand [M]:[MBiB] Cmon (g/mL)b convn (%)c Mn(SEC) Mn(theor)d Mn(est)

e Đc

1 HMTETA 100 0.5 31 12000 29400 31300 1.132f HMTETA 100 0.5 55 20300 52100 62000 1.123 HMTETA 100 1.0 58 21800 54900 67300 1.094 DBU 100 0.5 16 9000 15300 20200 1.155 Me6TREN 100 0.5 82 27300 77600 87800 1.356 TPMA 100 0.5 79 24500 74700 77400 1.297 PMDETA 60 0.5 78 17400 48300 51200 1.278 PMDETA 100 0.5 82 23600 77600 74100 1.249 PMDETA 200 0.5 69 34900 130400 115900 1.2410 PMDETA 200 1.16 94 50000 177600 171500 1.2011 PMDETA 500 0.5 50 39700 236100 133500 1.3412 PMDETA 500 1.16 93 88000 439000 311600 1.3113g PMDETA 200 0.5 79 43900 149300 148900 1.1414f,g PMDETA 700 1.0 70 114600 462600 409900 1.19

aStandard polymerization conditions: 24 h, 60 °C, [MBiB]:[CuBr]:[ligand] = 1:1:1 for HMTETA, PMDETA, Me6TREN, and TPMA and 1:1:2 forDBU. bConcentration of monomer (monomer weight per volume of toluene). cDetermined from SEC. dCalculated from monomer conversionassuming 100% initiation efficiency of the initiator. eEstimated from the SEC values. fPolymerization time was 48 h. gCarried out at 40 °C.

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basically the same Mn(theor) was attained after 24 h as in theexperiment with the 30:1 monomer/initiator ratio above. Incontrast, Me6TREN, TPMA, and PMDETA proved to besubstantially more efficient ligands, and conversions around80% were generally obtained under the standard conditions,which corresponded to DPn ∼ 80. This finding is important asit suggests that the former concerns about the feasibility ofattaining high-MW POSS homopolymers via ATRP wereunjustified. The use of these three active ligands resulted indispersity values slightly higher than those obtained withHMTETA and DBU. Especially in the cases of Me6TREN andTPMA, the higher dispersity was reflected in certain low-MWtailing of the polymer SEC curves. Moreover, in the TPMAexperiment, a small high-MW shoulder was observed in a SECeluogram, similar to that in the 30:1 monomer/initiator ratioexperiment discussed above.Among the tested ligands, PMDETA seemed to be a good

compromise between polymerization rate and controlledbehavior of the polymerization, i.e., low dispersity, unimodaldistributions of MW, and good initiation efficiency. Therefore,this ligand was selected for experiments with other[iBuPOSSMA]:[MBiB] ratios. Experiments having this ratioset to 60:1, 200:1, and 500:1 (entries 7, 9, and 11, respectively)were performed at 60 °C while still maintaining the monomerconcentration at 0.5 g/mL as before. With the 60:1 ratio,similar monomer conversion (78%) as for the 100:1 ratio wasreached. However, when increasing the ratio to 200:1 and500:1, lower conversions of 69 and 50%, respectively, wereattained in 24 h. Such polymerization rate slowdown is to beexpected considering the decreased concentrations of theinitiator and the catalytic system in these experiments,originating from the above-mentioned dilution. Note that inthe latter case (entry 11), rather high discrepancy betweenMn(est) and Mn(theor) values can be seen. This is probably relatedto the presence of a substantial low-MW fraction detectable asnoticeable tailing in the SEC eluogram of the polymer (Figure1). Considering the number-average Mn sensitivity to low-MWspecies, Mn(SEC), and consequently also Mn(est), can beunderestimated. When the molecular weight-average of themain polymeric fraction (Mp) is used for the calculation insteadof Mn, more reasonable Mn(est) = 217 000 is obtained.The experiments presented so far were conducted at the

constant monomer concentration of 0.5 g/mL. It is striking that

the limiting conversion around 80% was achieved in all cases,provided an active enough ligand was used and sufficientpolymerization time was allowed. The situation changed,however, when the monomer concentration was increased(entries 3, 10, and 12 in Table 2). In all cases, the conversionsrose substantially. In the HMTETA experiment (entry 3),almost the same conversion (58%) was effectively reached inhalf the time when compared to the 48 h experiment employingthe standard monomer concentration (entry 2). Further,repetition of the entry 9 experiment with a higher monomerconcentration (1.16 g/mL) led not only to a noticeable increasein conversion but also to almost no tailing of the polymer SECeluogram (Figure 1), which was also reflected in lowerdispersity value (entry 10). This could be attributed to thehigher concentration of the initiator and of the catalytic systemas the monomer concentration increase was simply achieved bydecreasing the amount of solvent in the reaction mixture.Similarly, repeating the entry 11 experiment with highermonomer concentration (1.16 g/mL) resulted in highconversion and somewhat decreased dispersity (Đ = 1.31),albeit the SEC eluogram (Figure 1) showed relativelypronounced tailing (entry 12). As could be expected,considering the high dispersity value, the Mn(est) and Mn(theor)values differed substantially in this experiment; nevertheless,calculation of Mn(est) from Mp gave much closer value of Mn(est)= 499 700.Observation of a limiting conversion dependent on the initial

monomer concentration is typical for polymerization ofmonomers having a low ceiling temperature (Tc). Steric factorsare considered to be responsible for the decreased Tc.

34 Sincethe present study deals with a considerably bulky monomer, wedecided to investigate also the influence of temperature oniBuPOSSMA polymerization. For this purpose, kinetic experi-ments were carried out at 60 and 40 °C, and the results areshown in Figure 2. From the conversion curves (bottom left), itis obvious that the conversion plateau was reached at a highervalue (90%) when the polymerization was carried out at 40 °C,compared to only 75% attained at 60 °C. This observation isconsistent with the assumption of a relatively low Tc of thestudied monomer and, consequently, an important role ofdepropagation in later polymerization stages. The limitingconversion values then correspond to the equilibriummonomer concentrations for the given temperatures. Atequilibrium, polymerization and depolymerization proceed atthe same rate, and the molecular weight is not furtherincreased. The semilogarithmic plots (Figure 2, top left)show a considerable curvature as the polymerizations proceedsto higher conversions. Normally, this could be a sign oftermination processes.35 However, in the present case, wepresume that the curvature can be largely ascribed to theexistence of the above-mentioned polymerization-depolymeri-zation equilibrium. In the first stage of the polymerization, thesemilogarithmic plots are obviously linear, but as theconversion increases, the monomer concentration approachesthe equilibrium concentration, and the rate constant ofdepropagation increases. Thus, the apparent rate constant ofpropagation kapp = kp

eff [P*] is not reduced by decreasing theconcentration of active chains [P*], but instead by decreasingthe effective rate constant of propagation kp

eff that is dependenton rate constants of propagation (kp) and depropagation (kd):kp

eff = kp − kd/[M].36,37 In addition to this phenomenon,limited mobility of polymeric chains and the bulky monomer inan environment of continuously increasing viscosity could

Figure 1. SEC eluograms of poly(iBuPOSSMA) homopolymers(labels correspond to data in Table 2).

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possibly also contribute to a gradual decrease in thepolymerization rate. Importantly, despite the rate retardation,the polymerization still proceeded in a well-controlled manner,which is illustrated by the linear development of MW withmonomer conversion (Figure 2, top right). It is alsonoteworthy that the Mn(est) values were close to the theoreticalconversion-based Mn values, represented in the graph by thesolid line, and so the initiation efficiency in these polymer-izations was high. Dispersity values did not decrease graduallywith polymerization time, contrary to what is a typical featureof many ATRP processes.35 In the experiment carried out at 40°C, dispersity remained about 1.15 for most of the polymer-ization, while at 60 °C, it steadily increased to ca. 1.19 (Figure2, bottom right). When the latter experiment was prolonged to144 h, the dispersity further gradually increased to 1.22 (datanot shown here). Gradual broadening of MW distribution is acommon feature of living systems reaching the propagation-depropagation equilibrium.38−40 It is also possible that sidereactions (e.g., termination and transfer), which can still occurafter the net propagation ceases, might contribute to the MWdistribution broadening.To further support the hypothesis of the presence of a Tc-

related equilibrium, an additional kinetic experiment wasperformed. In this experiment, after carrying out the polymer-ization at 60 °C for 24 h, the temperature was increased to 90°C for another 24 h. The conversion curve in Figure 3 showsthat the typical conversion plateau had been reached at 60 °C,but, as expected, depolymerization occurred after the temper-ature increase to 90 °C. This was accompanied by a drop inMn (est) of the polymer from about 50 000 at 24 h to about 40000 at 48 h. Moreover, the temperature-induced depolymeriza-tion resulted in bimodality of the MW distribution (minor low-MW fraction), which was reflected in considerably increased

dispersity as is evident from the plotted Đ values. It waspredicted that formation of shorter chains can result from thepositive temperature jump in equilibrated systems of similartype.39

Although a low Tc is commonly associated with α-substitutedstyrenes and certain α-substituted acrylic esters,34,36 theinfluence that bulky side groups have on Tc of methacrylateswas also noticed in literature, e.g. for ortho-substituted phenylmethacrylates.41,42 Furthermore, marked dependence ofachievable monomer conversion on the initial monomerconcentration and on temperature in free radical polymer-ization of triphenylmethyl methacrylate (TrMA) was describedby Okamoto group.43 Importantly, Ishitake et al. have recentlyreported exactly the same effects as discussed here in their

Figure 2. Kinetics of iBuPOSSMA polymerization at 40 and 60 °C ([MBiB]:[CuBr]:[PMDETA]:[iBuPOSSMA] = 1:1:1:60, Cmon = 0.5 g/mL).

Figure 3. Effects of the temperature increase from 60 to 90 °C on thekinetics of iBuPOSSMA polymerization ([MBiB]:[CuBr]:[PMDE-TA]:[iBuPOSSMA] = 1:1:1:60, Cmon = 0.5 g/mL).

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studies on RAFT polymerization of TrMA and othermethacrylates with bulky side groups.44−46 In these works,highly isotactic stereogradient polymers were obtained as aresult of the thermodynamic control in later polymerizationstages. By analogy, it is reasonable to expect that the(iBuPOSSMA) homopolymers could have increased stereo-regularity as well. Unfortunately, in contrast to poly(TrMA)homopolymers, the transformation of poly(iBuPOSSMA) intoeasily analyzable poly(MMA) is not straightforward, whichlargely precludes definite stereoregularity assessment.It is clear that a low Tc has great practical implications for the

polymerization of iBuPOSSMA and other POSS monomers.Therefore, we decided to determine the Tc and also to estimatethe polymerization enthalpy (ΔHp) and entropy (ΔSp) for ourcurrent system. To achieve this, iBuPOSSMA was polymerizedunder standard conditions at five different temperatures (40−80 °C), and the equilibrium monomer concentration [M]eq wasdetermined from the monomer conversion obtained from SEC.Figure 4 shows the plot of ln [M]eq against 1/T. From the slope

and intercept of the linear regression line, values of ΔHp = −41kJ mol−1 and ΔSp = −101 J mol−1 K−1 were calculated. Tc forunimolar monomer concentration was then determined to be130 °C. This is not far from the value reported for TrMApolymerized via RAFT in toluene (Tc = 104 °C).44 Thedetermined value of ΔHp is considerably higher than that forMMA (about -55 kJ mol−1).47 This can be attributed to sterichindrance caused by the bulky substituted POSS side group.The calculated ΔSp value lies on the higher end of the interval−120 to −100 J mol−1 K−1, typical for most monomers,perhaps because the loss of translational entropy is lessprominent for the bulky iBuPOSSMA monomer.48 It can bereasonably expected that POSSMA monomers bearing othersubstituents on the POSS cage such as phenyl, cyclopentyl, andcyclohexyl, to name a few common variants, will also showdecreased Tc, which needs to be taken into account whenattempting their polymerization.The knowledge of Tc allows tailoring appropriate conditions

for iBuPOSSMA polymerization. It is clear that to attain highmonomer conversion in a reasonable time, use of a high initialmonomer concentration and lower polymerization temperatureis advisable. It also appears that lower temperature (40 °C)contributes positively to low dispersity of the synthesizedpolymers. Examples of such optimized experiments are

represented by the last two entries in Table 2. In entry 13,lower temperature led to increased conversion and noticeablylower dispersity, compared to the entry 9 experiment carriedout at 60 °C. Since in neither of those two experiments thepolymerization-depolymerization equilibrium was reached, thehigher conversion attained at lower temperature can be rathersurprising. It needs to be borne in mind, however, that in thelatter experiment, termination in early polymerization stageswas probably much more prominent, as illustrated by SECcurve tailing (Figure 1). This would eventually lead to furtherdecrease of (already quite low) active chains concentration andaccumulation of Cu2+ species (persistent radical effect).49 Boththese effects would efficiently retard the rate of polymerizationand, in turn, also the conversion achieved in 24 h. In entry 14, ahigh [iBuPOSSMA]:[MBiB] ratio of 700:1 was employed at 40°C. The amount of solvent was decreased (Cmon = 1.0 g/mL) inorder to increase the polymerization rate. In 48 h, theconversion reached 70%, which corresponded to Mn(theor) of462 600 and DPn = 490. This is several times higher than thehighest DPn values reported so far in the literature forcontrolled polymerization of POSS monomers (iBuPOSSMA),i.e., about 100 for anionic polymerization9 and about 160 (ourestimate from the reported conversion data) for RAFTpolymerization.19 The attained dispersity was low (1.19), andthe SEC eluogram showed minimal tailing (Figure 1).In the light of the results presented in this study, it can be

speculated that nonoptimal experimental conditions could haveaccounted for the limiting DPn values observed in the earlystudies on POSSMA homopolymerization via ATRP. In otherwords, high temperature and/or too diluted polymerizationmixture would necessarily lead to limited conversions and lowDPn. Unfortunately, this assumption is hard to verify, as thepioneering studies do not specify the conditions under whichsynthesis of higher-MW poly(POSSMA) was attempted.24 It isclear that the low-Tc limitations potentially apply also to RAFTpolymerization of POSS monomers. In this regard, it isinteresting to analyze the first two reports on POSSMAhomopolymerization via RAFT as they contain valuable kineticdata illustrating the polymerization course. It appears that Myaet al. largely evaded most problems arising from thepolymerization−depolymerization equilibrium by employing ahigh initial iBuPOSSMA concentration (1.06 M) and endingthe polymerization before the conversion plateau could bereached.19 Nevertheless, gradually increasing dispersity valuesreaching about 1.6 were observed, i.e. a trend similar to ourresults. In the second report, Deng et al. utilized lower initialmonomer concentrations in the range of 0.416 to 0.50 M inRAFT polymerization of iBuPOSSMA and cyclohexyl-sub-stituted POSSMA (cyPOSSMA) at 65 °C. Analyzing thepublished data, it appears that when long enough polymer-ization time was allowed, similar effects as reported here wereobserved, i.e., deviation of the semilogarithmic plot fromlinearity at certain conversion and formation of the typicalconversion plateau. This was most pronounced in polymer-ization of cyPOSSMA with [M]0 = 0.416 M and 32 hpolymerization time. For both monomers, significant broad-ening of the MW distribution was observed for conversionshigher than about 50% when the 60:1 [monomer]:[RAFT]ratio was applied. It can be speculated that increased role ofdepropagation in later stages of the polymerization can, at leastpartially, account for these effects.At the time of the submission of this work for publication,

another study was published where modified iBuPOSSMA

Figure 4. Plot of ln [M]eq against 1/T for determination of the ceilingtemperature (ln [M]eq = ΔHp/RT − ΔSp/R).

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monomer was polymerized to high MW by ATRP.50 Themonomer modification, introduced to relax strain caused by thebulky POSS cage, consisted in incorporating an O-SiMe2 linkerto prolong the spacer between the POSS cage and themethacrylate function. Importantly, the authors observed acorrelation between the initial monomer concentration and theachieved conversion, with only limited conversions achieved atlower [M]0. Although no definite explanation for this behaviorwas given, limited thermodynamic polymerizability or termi-nation/transfer reactions were suggested as possible causes.Considering our results presented here, it is likely that thepropagation-depropagation equilibrium is still the main cause oflimited conversions in this modified system. Similar approachhas also been used recently by Zhang et al., who carried out athree-step synthesis to prepare an iBuPOSSMA-type monomerbearing a long 11-atom spacer.51,52 Interestingly, RAFTpolymerization of this monomer still showed clear signs ofthermodynamic control resulting in a limited conversion.52 Onthe whole, these results imply that even though the spacerincorporation can be helpful to some extent, carefuloptimization of the reaction conditions is still necessary, andthe use of a commercial monomer such as iBuPOSSMA mightbe sufficient.Block Copolymerization of iBuPOSSMA. Besides prep-

aration of high-MW poly(iBuPOSSMA) homopolymers, wealso, for the first time, employed ATRP for synthesis of diblockcopolymers with high-MW poly(POSSMA) blocks. Suchcopolymers have already attracted considerable atten-tion,9,10,14−19 and we believe that ATRP could become apreferred method of their preparation. For this purpose, weselected MMA and styrene as model comonomers. Generally,two approaches to synthesis of the desired block copolymersexist. In the first one, iBuPOSSMA polymerization is triggeredfrom a suitable polymeric macroinitiator while in the secondapproach, poly(iBuPOSSMA) block is synthesized beforehandand polymerization of the comonomer is initiated by it. Weinvestigated both these approaches.The first approach was employed in polymerization of

iBuPOSSMA initiated by poly(MMA), synthesized by ATRP(Mn = 52 000, Đ = 1.14). Apparently, this method has alreadybeen tested by other authors, but was dismissed as severelylimited due to low solubility of poly(MMA) in POSS monomersolutions and because of very low attained conversions.7,21

Indeed, poly(MMA) does not go readily into iBuPOSSMAsolution in toluene (or THF). Nevertheless, a clear low-concentration solution can be achieved. For instance, initialmonomer concentration of about 0.16 M for iBuPOSSMA/poly(MMA) (w/w) = 3.6 was employed in the first experiment.As follows from the previous discussion, it is necessary toappropriately decrease polymerization temperature whenworking with dilute mixtures if too low limiting conversionsare to be avoided. Therefore, the polymerization was carriedout at 40 °C with PMDETA as a ligand; the [iBuPOSSMA]:[initiator] ratio was set to 200:1. Unfortunately, due to thesubstantial dilution, the reaction proceeded very slowly,reaching only 15% conversion (estimated from gravimetry)after 113 h. This translates to the poly(iBuPOSSMA) blockDPn of about 29 and Mn (theor) = 27 500. If we calculate and addthe equivalent Mn(SEC) value (ca. 11 000) to the Mn of themacroinitiator, we arrive to the theoretical value ofMn(SEC) = 63000, which agrees very well to the Mn(SEC) = 63800 determinedfor the block copolymer. The SEC eluogram showed slighttailing, but the dispersity (Đ = 1.20) was fairly low (Figure 5).

In another experiment, we attempted to facilitate thepolymerization by using a more active ligand, TPMA (KATRPof TPMA is about 2 orders of magnitude higher than that ofPMDETA).53 Because the initial monomer concentration wasonly 0.12 M, the experiment was performed at roomtemperature to suppress depolymerization; the [iBuPOSS-MA]:[initiator] ratio was 100:1. In 96 h, the conversionreached 59%, corresponding to poly(iBuPOSSMA) DPn of 59and Mn(theor) = 55 300. Applying the same procedure as above,we can estimate the theoretical Mn(SEC) of the copolymer to be70 500, which is in fair agreement with the measured Mn(SEC) =65 700. SEC analysis of the copolymer also revealed slightlyhigher dispersity value (Đ = 1.29) stemming from certaintailing of the SEC signal, signaling the presence of a low-MWfraction (Figure 5). These results show that, despite somelimitations, the poly(MMA) macroinitiator approach is viableand can be used for synthesis of block copolymers with high-MW poly(iBuPOSSMA) blocks, provided low-enough temper-ature and an active-enough ligand are used. In this context, it isworth noticing that ATRP has a certain advantage over RAFTat low temperatures, as the latter method counts on thermaldecomposition of a radical initiator in the traditional setup.To evaluate the second approach outlined above, a

poly(iBuPOSSMA) macroinitiator was first synthesized byATRP at 40 °C with PMDETA as a ligand and the[iBuPOSSMA]:[MBiB] ratio of 100:1. The polymerizationwas deliberately ended at low conversion (52%) to helppreserve the chain end functionality. The macroinitiator waspurified by repeated precipitation in MeOH and analyzed bySEC, which revealed low dispersity of 1.14 and Mn(SEC) value of19 900, i.e., Mn(est) = 60 500. In the first experiment, thepoly(iBuPOSSMA) initiated ATRP of styrene, carried outwithout solvent at 100 °C for 23 h with PMDETA as a ligand.From the weight of the isolated copolymer, the Mn(theor) of thepolystyrene block was estimated to be 69 900, assuming 100%initiation efficiency. SEC analysis of the copolymer revealedMn(SEC) = 97 200 and Đ = 1.14. The Mn(SEC) value is slightlyhigher than the one we calculated from molecular weights ofthe two blocks (about 90 000). This discrepancy can beascribed to slightly decreased initiation efficiency of themacroinitiator. Indeed, a very small signal corresponding tothe macroinitiator can be found in the SEC chromatogram ofthe isolated copolymer (Figure 6). In another experiment,

Figure 5. SEC eluograms of the poly(MMA) macroinitiator (solidline), the MMA520-b-iBuPOSSMA29 block copolymer (dashed line),and the MMA520-b-iBuPOSSMA59 block copolymer (dotted line).

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MMA was used for synthesis of the second block. [MMA]:[initiator] ratio of 560:1 was utilized, and HMTETA wasemployed as a ligand in this case. Conversion of 77% wasreached, but the SEC eluogram of the copolymer showed anoticeable shoulder representing unreacted macroinitiator. Thisfraction (about 13 wt %) was successfully removed byextraction of the product with cyclohexane. The purifiedblock copolymer had Mn(SEC) = 73 200, which is in a goodaccordance with the calculated value Mn = 75 500, obtained byaddition of the macroinitiator Mn(est) and Mn(theor) of thepoly(MMA) block (derived from conversion taking thedecreased initiation efficiency into consideration). Figure 6shows the SEC trace of the purified block copolymer.Comparing the results obtained with styrene and MMA, itappears that the poly(iBuPOSSMA) macroinitiator showshigher reinitiation efficiency with styrene. Such trend hasbeen previously observed in synthesis of POSSMA diblockcopolymers by RAFT.7

■ CONCLUSIONSIn this study, the commercially available POSS monomer,iBuPOSSMA, was homopolymerized via CuBr catalyzed ATRP.It was shown that various ligands can be successfully utilized forthis purpose; PMDETA was identified as a good compromisebetween the polymerization rate and controllability of theprocess. Poly(iBuPOSSMA) homopolymers of very high MWand low dispersity were prepared in a controlled way underoptimized conditions. Low degrees of polymerization achievedin some of the former studies can be plausibly ascribed to theeffects originating from the low Tc of the monomer. The valuesof Tc as well as of the polymerization enthalpy and entropywere estimated, which should greatly facilitate designing futurepolymerization experiments involving iBuPOSSMA and similarPOSS monomers. Moreover, this work also provides somepractical information concerning isolation of the products anddetermination of monomer conversion and MW of polymers,which could be beneficial for future research. Finally, variousblock copolymers having high-MW poly(iBuPOSSMA) blockswere successfully synthesized using two different strategies.On the whole, these results should help establish ATRP as

the method of choice for controlled polymerization ofPOSSMA monomers and can open new opportunities for

straightforward synthesis of new hybrid (co)polymers bearingPOSS moiety.

■ ASSOCIATED CONTENT*S Supporting InformationA plot of Mn(SEC) vs Mn(SEC MALLS) for poly(iBuPOSSMA)homopolymers and a table with experimental data regardingATRP of iBuPOSSMA using the CuBr2 deactivator. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] (V.R.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work has been supported by the Grant Agency of theCzech Republic (Grant P106/12/0844). The authors thankMr. Ste pan Adamec for technical assistance, Mrs. DanaKankova for measurement of NMR spectra, and Dr. PeterCernoch for performing the refractometric analysis.

■ REFERENCES(1) Li, G. Z.; Wang, L. C.; Ni, H. L.; Pittman, C. U. J. Inorg.Organomet. Polym. 2001, 11, 123−154.(2) Wu, J.; Mather, P. T. Polymer Rev. 2009, 49, 25−63.(3) Cordes, D. B.; Lickiss, P. D.; Rataboul, F. Chem. Rev. 2010, 110,2081−2173.(4) Kuo, S.-W.; Chang, F.-C. Prog. Polym. Sci. 2011, 36, 1649−1696.(5) Zhang, W.; Muller, A. H. E. Prog. Polym. Sci. 2013, 38, 1121−1162.(6) Hussain, H.; Tan, B. H.; Seah, G. L.; Liu, Y.; He, C. B.; Davis, T.P. Langmuir 2010, 26, 11763−73.(7) Deng, Y.; Bernard, J.; Alcouffe, P.; Galy, J.; Dai, L.; Gerard, J.-F. J.Polym. Sci., Part A: Polym. Chem. 2011, 49, 4343−4352.(8) Deng, Y.; Yang, C.; Yuan, C.; Xu, Y.; Bernard, J.; Dai, L.; Gerard,J.-F. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4558−4564.(9) Hirai, T.; Leolukman, M.; Jin, S.; Goseki, R.; Ishida, Y.;Kakimoto, M.; Hayakawa, T.; Ree, M.; Gopalan, P. Macromolecules2009, 42, 8835−8843.(10) Tada, Y.; Yoshida, H.; Ishida, Y.; Hirai, T.; Bosworth, J. K.;Dobisz, E.; Ruiz, R.; Takenaka, M.; Hayakawa, T.; Hasegawa, H.Macromolecules 2012, 45, 292−304.(11) Kim, D.-G.; Sohn, H.-S.; Kim, S.-K.; Lee, A.; Lee, J.-C. J. Polym.Sci., Part A: Polym. Chem. 2012, 50, 3618−3627.(12) Kim, S.-K.; Kim, D.-G.; Lee, A.; Sohn, H.-S.; Wie, J. J.; Nguyen,N. A.; Mackay, M. E.; Lee, J.-C. Macromolecules 2012, 45, 9347−9356.(13) Hussain, H.; Shah, S. M. Polym. Int. 2014, 63, 835−847.(14) Hirai, T.; Leolukman, M.; Hayakawa, T.; Kakimoto, M.;Gopalan, P. Macromolecules 2008, 41, 4558−4560.(15) Hirai, T.; Leolukman, M.; Liu, C. C.; Han, E.; Kim, Y. J.; Ishida,Y.; Hayakawa, T.; Kakimoto, M.; Nealey, P. F.; Gopalan, P. Adv. Mater.2009, 21, 4334−4338.(16) Ishida, Y.; Tada, Y.; Hirai, T.; Goseki, R.; Kakimoto, M.;Yoshida, H.; Hayakawa, T. J. Photopolym Sci. Technol. 2010, 23, 155−159.(17) Ishida, Y.; Hirai, T.; Goseki, R.; Tokita, M.; Kakimoto, M. A.;Hayakawa, T. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 2653−2664.(18) Jin, S.; Hirai, T.; Ahn, B.; Rho, Y.; Kim, K.-W.; Kakimoto, M.;Gopalan, P.; Hayakawa, T.; Ree, M. J. Phys. Chem. B 2010, 114, 8033−8042.(19) Mya, K. Y.; Lin, E. M. J.; Gudipati, C. S.; Shen, L.; He, C. J. Phys.Chem. B 2010, 114, 9119−9127.

Figure 6. SEC eluograms of the poly(iBuPOSSMA) macroinitiator(solid line), the iBuPOSSMA64-b-MMA555 block copolymer (dashedline), and the iBuPOSSMA64-b-styrene671 block copolymer (dottedline).

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