ATRP of POSS Monomers Revisited - ACS Publications - American

Article pubs.acs.org/Macromolecules

ATRP of POSS Monomers Revisited: Toward High-Molecular Weight Methacrylate−POSS (Co)Polymers Vladimír Raus,* Eva Č adová, 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 first time, ATRP was successfully employed for homopolymerization of a commercial methacrylate-functionalized polyhedral oligomeric silsesquioxane (POSS) monomer, iBuPOSSMA, to high molecular weights. It was found that iBuPOSSMA has a low ceiling temperature (Tc); 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 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 low dispersity and high Mn, ranging from 23 000 to 460 000, were obtained in a well-controlled ATRP process. Moreover, various block copolymers having high-Mn poly(iBuPOSSMA) blocks were prepared by copolymerization of iBuPOSSMA with methyl methacrylate and styrene.

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INTRODUCTION Polyhedral oligomeric silsesquioxanes (POSS) of the general formula (RSiO1.5)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 Si−O bonds, bear organic substituents at each corner, which gives POSS compatibility or miscibility with many polymers. The unique POSS structure made them attractive as nanofillers in polymer nanocomposites where POSS were employed to enhance 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 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.6−10 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 field have been reviewed recently.13 © 2014 American Chemical Society

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.9,10,14−18 The selfassembly 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 addition− fragmentation 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 (Mn(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 highMW 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 (DPn) Received: July 28, 2014 Revised: October 1, 2014 Published: October 23, 2014 7311

dx.doi.org/10.1021/ma501541g | Macromolecules 2014, 47, 7311−7320

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and then from sodium anthracenide prior to use. The initiator (MBiB) and ligands (BiPy, PMDETA, HMTETA, DBU, Me 6 TREN, Me4Cyclam, TPMA) were used as toluene solutions with concentrations 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 with PMDETA 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 magnetic stirring bar. After thorough deoxygenation by several vacuum-argon cycles, degassed toluene (1.067 mL) was added to dissolve iBuPOSSMA. Subsequently, the PMDETA stock solution in toluene (0.31 mL, 35.7 μmol of PMDETA) was added. After 10 min of stirring at 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 in an oil bath preheated to 60 °C. After 24 h, the polymerization was stopped by adding toluene solution of the radical inhibitor 4-tertoctylcatechol, and the polymerization mixture was cooled down. Afterward, the mixture was diluted with toluene (20 mL) and centrifuged to remove the solids. The supernatant was precipitated in 10-fold excess of methanol (MeOH); the precipitate was filtered on a frit with porosity of 4−16 μm, washed with MeOH, and dried in vacuum at 40 °C for 24 h. If needed, the precipitation was repeated once or twice to remove the residual monomer. Synthesis of Poly(MMA)-block-poly(iBuPOSSMA) Diblock Copolymer. 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), and iBuPOSSMA (1.815 g, 1.923 mmol) were mixed with 13 mL of toluene in a round-bottomed flask, equipped with a stirring bar. After dissolution of the macroinitiator, polymerization was started by the addition of TPMA stock solution in toluene (0.28 mL, 19.23 μmol of TPMA). After 96 h, the polymerization was stopped, and the mixture was processed in the same way as mentioned above. Synthesis of Poly(iBuPOSSMA)-block-poly(MMA) and Poly(iBuPOSSMA)-block-polystyrene Diblock Copolymers. These block copolymers 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 for MMA 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 the isolated (co)polymers was performed at 25 °C with two PLgel MIXED-C columns (300 × 7.5 mm, SDV gel with particle size 5 μm; Polymer Laboratories, USA) and with UV (UVD 250; Watrex, Czech Republic) and RI (RI-101; Shodex, Japan) detectors. Tetrahydrofuran was used as a mobile phase at a flow rate of 1 mL/min. The molecular weight values were calculated using Clarity software (Dataapex, Czech Republic). 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 300.1 MHz. Hexamethyldisiloxane was used as an internal standard. The dn/ dc values of the monomer (4.201 × 10−2 mL/g) and a sample homopolymer of Mn(est) = 171 500 (5.180 × 10−2 mL/g) in THF solutions were determined on a Brookhaven Instruments BI-DNDC differential refractometer.

of the isolated homopolymers was deliberately kept relatively low ( 15 were unsuccessful; therefore, relatively short difunctional BA macroinitiator had to be employed to achieve the phaseseparated product. The authors speculated that, due to the steric reasons, the bromine end groups of the growing POSSMA chains may be inaccessible to the catalytic complex after certain DPn is reached. Perhaps this assumption is the reason why other authors did not attempt to use ATRP for synthesis of higher molecular weight POSSMA polymers.6,25−32 Consequently, to our best knowledge, the highest POSSMA homopolymer DPn achieved so far by ATRP of commercial monomers is about 18. In this study, we reinvestigated the applicability of the ATRP method to homopolymerization of a commercially available POSS monomer, methacryl−isobutyl−POSS (iBuPOSSMA). We show that, contrary to former beliefs, it is well possible to homopolymerize a POSSMA monomer via ATRP to high MW when appropriate polymerization conditions are used. In this context, we highlight the influence of the monomer’s low ceiling temperature on the polymerization kinetics. Besides the preparation of a series of iBuPOSSMA homopolymers, synthesis of block copolymers with MMA and styrene having high-MW iBuPOSSMA blocks is also reported.

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EXPERIMENTAL SECTION

Materials. Methyl 2-bromoisobutyrate (MBiB; Aldrich, ≥ 99%), CuBr (Fluka, ≥ 98%), CuBr2 (Fluka, ≥ 99%), 2,2′-bipyridine (BiPy; (Aldrich, ≥ 99%), 1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA; 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-tetraazacyclotetradecane (Me4Cyclam; Aldrich, 98%), tris(2-pyridylmethyl)amine (TPMA; Aldrich, 98%), and 3-{3,5,7,9,11,13,15-hepta(2methylpropyl)-pentacyclo[9.5.1.1.3,91.5,1517,13]-octasiloxan-1-yl}propyl methacrylate (iBuPOSSMA; Hybrid Plastics, molar weight = 943.64 g/ mol) were used as received. Toluene was distilled with lithium aluminum hydride and then from benzophenone ketyl prior to use. Tetrahydrofuran (THF) was distilled with lithium aluminum hydride

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RESULTS AND DISCUSSION Homopolymerization of iBuPOSSMA. Nowadays, RDRP protocols are the methods of choice for controlled synthesis of homo- and copolymeric materials with predetermined composition, molecular weight, and architecture. As previously stated, RAFT and ATRP have been applied to controlled synthesis of various POSS-based materials. It comes as a surprise, however, that the contemporary literature lacks a more detailed evaluation of reaction conditions suitable for ATRP homopolymerization of POSS monomers. In the handful of 7312


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precipitation is often considered troublesome, we would like to note that for the higher-MW poly(iBuPOSSMA) prepared in this study, precipitation in MeOH worked quite well. In most cases, the first precipitation led to monomer content of less than 2%, and additional reprecipitation made the monomer concentration negligible. It needs to be pointed out, however, that MeOH is less than ideal precipitation agent for poly(iBuPOSSMA)/iBuPOSSMA mixtures due to the limited monomer solubility in this solvent. We estimated that at room temperature in MeOH, MeOH/toluene 10:1 (v/v), and MeOH/toluene 20:1 (v/v), the monomer solubility is approximately 0.31, 0.76, and 0.38 g per 100 mL of solvent, respectively. This needs to be taken into account, especially when precipitating low-conversion polymerization mixtures, because the amount of MeOH used for precipitation may not be sufficient to dissolve all the present monomer. Monomer conversions were determined by SEC analysis of the reaction mixture, taking advantage of the fact that the monomer signal is clearly separated from the solvent signal. Conversion was then calculated from the area of the polymer and monomer signals (RI detector) divided by the respective refractive index increments that were determined for this purpose (see the Experimental Section). We consider this method to be substantially more accurate than conversion calculation from NMR spectra (see below). It is noteworthy, that the conversions determined from SEC corresponded very well with values calculated from gravimetry (not reported here). There are three number-average molecular weight (Mn) values given for each entry in Table 1: Mn(theor), Mn(SEC), and

published reports, PMDETA was used as a ligand almost exclusively with the catalyst being either CuCl or CuBr. The only exception was the employment of the CuCl/HMTETA system by Chen et al.25 This means that the majority of authors used the catalytic system proposed by Pyun et al. in the first report 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 possible that steric reasons play an important role in the POSS monomers homopolymerization. In general, the role might be 2-fold. First, due to the POSS bulkiness, the polymerization rate can be expected to be lower compared to less bulky structurally similar monomers (e.g., MMA). Second, potential polymerization rate retardation arising from the inaccessibility of initiation sites at higher-MW polymer chains, suggested by Pyun et al.,24 needs to be taken into consideration. Ligand choice can be essential in both these regards as not only the activity of the catalyst/ligand complex but also the complex size could possibly influence the course of the polymerization. Therefore, we started our investigation by evaluating suitability of various ligands for homopolymerization of the most widely utilized POSS monomer, iBuPOSSMA (Scheme 1). Moreover, Scheme 1. ATRP of iBuPOSSMA

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

ligand

convn (%)b

1 2 3 4 5 6 7

BiPy HMTETA PMDETA DBU Me6TREN TPMA Me4Cyclam

78 75 76 71 81 81 82

Mn(SEC)

Mn(theor)c

Mn(est)d

Đb

10500 9700 11300 9900 13700 9300 31500

22300 21400 21700 20300 23100 23100 n.d.

25800 22800 28600 23600 37500 21500 n.d.

1.44 1.11 1.21 1.13 1.27 1.19 4.82

Standard 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 and DBU. bDetermined by SEC. cCalculated from monomer conversion assuming 100% initiation efficiency. dEstimated from the SEC values. a

we also wanted to critically assess the previous claims about the feasibility of attaining high-MW products. For this reason, the targeted DPn was initially set to about double of the value reported 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, and Me4Cyclam, were trialed under following standard polymerization conditions. Polymerization was initiated by methyl 2bromoisobutyrate (MBiB), and an equimolar amount of copper(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 bidentate ligands (BiPy, DBU), respectively. Polymerizations were carried out in toluene at standard monomer concentration of 0.5 g/mL (monomer/solvent); the [iBuPOSSMA]:[MBiB] ratio was 30:1; the reaction temperature was 60 °C, and the polymerization 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

Mn(est). Mn(theor) is the theoretical molecular weight calculated from the monomer conversion counting with 100% initiation efficiency of the initiator. The Mn(SEC) values were obtained from SEC analysis using a system calibrated with polystyrene standards. As has been previously pointed out by other authors, compared to reality, Mn(SEC) values are considerably undervalued.7,19,20 The difference is most likely caused by the vastly different hydrodynamic volume of poly(iBuPOSSMA) and polystyrene standards of the same molecular weight. Nevertheless, when studying literature data, we noticed that there is a linear correlation between the Mn(SEC) values (polystyrene calibration) and absolute Mn values determined by the SEC MALLS method. Figure S1 (Supporting Information) shows a 7313


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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 2f 3 4 5 6 7 8 9 10 11 12 13g 14f,g

HMTETA HMTETA HMTETA DBU Me6TREN TPMA PMDETA PMDETA PMDETA PMDETA PMDETA PMDETA PMDETA PMDETA

100 100 100 100 100 100 60 100 200 200 500 500 200 700

0.5 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5 1.16 0.5 1.16 0.5 1.0

31 55 58 16 82 79 78 82 69 94 50 93 79 70

12000 20300 21800 9000 27300 24500 17400 23600 34900 50000 39700 88000 43900 114600

29400 52100 54900 15300 77600 74700 48300 77600 130400 177600 236100 439000 149300 462600

31300 62000 67300 20200 87800 77400 51200 74100 115900 171500 133500 311600 148900 409900

1.13 1.12 1.09 1.15 1.35 1.29 1.27 1.24 1.24 1.20 1.34 1.31 1.14 1.19

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

indicates decreased initiation efficiency (≈ 60%) and further supports the assumption of chain inactivation due to radical coupling reactions. The experiment with TPMA as a ligand resulted into a product with quite low dispersity of 1.19, but a small high-MW shoulder was observed in the SEC eluogram (entry 6). The last ligand tested was Me4Cyclam. Unfortunately, this highly active ligand showed to be unsuitable as multimodal product was obtained in this case, which was also reflected 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 (see Table S1 in the Supporting Information). Nevertheless, only small differences were observed, i.e., decreased conversion and dispersity for BiPy, and slightly improved initiation efficiency for PMDETA. Interestingly, in the TPMA experiment, CuBr2 suppressed formation of the unwanted high-MW fraction. The data summarized in Table 1 show that poly(iBuPOSSMA) homopolymers of DPn higher than the previously proclaimed limit of about 15 POSS units24 can be readily prepared via ATRP. The achieved DPn of about 25 was not much higher though. Moreover, the low monomer/initiator ratio and also the relatively long polymerization time were diminishing potential performance differences among the tested ligands. Therefore, in the following experiments, we increased the [iBuPOSSMA]:[MBiB] ratio and again followed the performance of selected ligands. Other polymerization conditions 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 was kept the same as previously, i.e., 0.5 g/mL, and thus the concentrations of other components (initiator, catalyst, and ligand) decreased correspondingly. As follows from the results (entries 1, 4, 5, 6, and 8 in Table 2), the employed ligands fared vastly differently. With HMTETA, a clear increase in MW was observed compared to the experiments with the 30:1 [iBuPOSSMA]:[MBiB] ratio. Nevertheless, the attained Mn(theor) of 29 400 corresponded to 31% conversion only (entry 1), implying rather low rate of polymerization under given conditions. Prolonging the reaction time to 48 h led to considerably higher conversion of 55% (entry 2). Even slower polymerization was observed with DBU (entry 4). Here,

plot of this dependence; data used for the plot construction were published by Hirai et al., who prepared poly(iBuPOSSMA) with very low dispersity (Đ ≈ 1.04) by anionic polymerization.9 The linear regression equation Mn(MAALS) = 3.6906 × Mn(SEC) − 12982 can then be used for surprisingly accurate estimates of absolute molecular weights of poly(iBuPOSSMA) homopolymers. This is illustrated by the Mn(est) values in Table 1, calculated from the above-mentioned equation, and their very good agreement with Mn(theor) values. Quite obviously, this approach gives the best results only for samples 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 the homopolymers can also be estimated from their 1H NMR spectra by the end group method using the signals of the initiator fragment. Nevertheless, we deem this approach to be limited mainly to low-molecular weight poly(iBuPOSSMA) polymers where we obtained a good agreement between Mn(NMR) values and Mn values determined by other means. However, signal overlap and low relative intensity of the signals used for the Mn(NMR) calculation considerably decrease the method accuracy for the high-MW products relevant to this paper. For this reason, we did not systematically employ NMR for molecular weight determination. From the data collected in Table 1, it is clear that under the standard reaction conditions conversions around 75% were achieved no matter which ligand was used. BiPy use resulted in a product with broader distribution of MW (Đ = 1.44), and SEC analysis revealed signs of MW distribution bimodality (entry 1). Polymerizations with HMTETA, PMDETA, and DBU yielded products with low dispersity in the range of 1.11− 1.21 and unimodal SEC traces (entries 2−4). When the polymerization time was prolonged to 48 h for HMTETA, practically identical conversion, MW, and dispersity were achieved (data not shown), implying that certain DPn limit had been already reached in 24 h. With Me6TREN, relatively low dispersity product was isolated, but its SEC trace showed strong tailing pointing to the presence of a considerable lowMW fraction (entry 5). This fraction probably originates from radical recombination in the early stages of the polymerization. The discrepancy between the Mn (est) and Mn (theor) values 7314


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basically the same Mn(theor) was attained after 24 h as in the experiment with the 30:1 monomer/initiator ratio above. In contrast, Me6TREN, TPMA, and PMDETA proved to be substantially more efficient ligands, and conversions around 80% were generally obtained under the standard conditions, which corresponded to DPn ∼ 80. This finding is important as it suggests that the former concerns about the feasibility of attaining high-MW POSS homopolymers via ATRP were unjustified. The use of these three active ligands resulted in dispersity values slightly higher than those obtained with HMTETA and DBU. Especially in the cases of Me6TREN and TPMA, the higher dispersity was reflected in certain low-MW tailing of the polymer SEC curves. Moreover, in the TPMA experiment, a small high-MW shoulder was observed in a SEC eluogram, similar to that in the 30:1 monomer/initiator ratio experiment discussed above. Among the tested ligands, PMDETA seemed to be a good compromise between polymerization rate and controlled behavior of the polymerization, i.e., low dispersity, unimodal distributions of MW, and good initiation efficiency. Therefore, this ligand was selected for experiments with other [iBuPOSSMA]:[MBiB] ratios. Experiments having this ratio set to 60:1, 200:1, and 500:1 (entries 7, 9, and 11, respectively) were performed at 60 °C while still maintaining the monomer concentration at 0.5 g/mL as before. With the 60:1 ratio, similar monomer conversion (78%) as for the 100:1 ratio was reached. However, when increasing the ratio to 200:1 and 500:1, lower conversions of 69 and 50%, respectively, were attained in 24 h. Such polymerization rate slowdown is to be expected considering the decreased concentrations of the initiator and the catalytic system in these experiments, originating from the above-mentioned dilution. Note that in the latter case (entry 11), rather high discrepancy between Mn(est) and Mn(theor) values can be seen. This is probably related to the presence of a substantial low-MW fraction detectable as noticeable tailing in the SEC eluogram of the polymer (Figure 1). Considering the number-average Mn sensitivity to low-MW species, Mn(SEC), and consequently also Mn(est), can be underestimated. When the molecular weight-average of the main polymeric fraction (Mp) is used for the calculation instead of 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 sufficient polymerization 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 conversions rose substantially. In the HMTETA experiment (entry 3), almost the same conversion (58%) was effectively reached in half the time when compared to the 48 h experiment employing the standard monomer concentration (entry 2). Further, repetition of the entry 9 experiment with a higher monomer concentration (1.16 g/mL) led not only to a noticeable increase in conversion but also to almost no tailing of the polymer SEC eluogram (Figure 1), which was also reflected in lower dispersity value (entry 10). This could be attributed to the higher concentration of the initiator and of the catalytic system as the monomer concentration increase was simply achieved by decreasing the amount of solvent in the reaction mixture. Similarly, repeating the entry 11 experiment with higher monomer concentration (1.16 g/mL) resulted in high conversion and somewhat decreased dispersity (Đ = 1.31), albeit the SEC eluogram (Figure 1) showed relatively pronounced 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 of monomers having a low ceiling temperature (Tc). Steric factors are considered to be responsible for the decreased Tc.34 Since the present study deals with a considerably bulky monomer, we decided to investigate also the influence of temperature on iBuPOSSMA polymerization. For this purpose, kinetic experiments were carried out at 60 and 40 °C, and the results are shown in Figure 2. From the conversion curves (bottom left), it is obvious that the conversion plateau was reached at a higher value (90%) when the polymerization was carried out at 40 °C, compared to only 75% attained at 60 °C. This observation is consistent with the assumption of a relatively low Tc of the studied monomer and, consequently, an important role of depropagation in later polymerization stages. The limiting conversion values then correspond to the equilibrium monomer concentrations for the given temperatures. At equilibrium, polymerization and depolymerization proceed at the same rate, and the molecular weight is not further increased. The semilogarithmic plots (Figure 2, top left) show a considerable curvature as the polymerizations proceeds to higher conversions. Normally, this could be a sign of termination processes.35 However, in the present case, we presume that the curvature can be largely ascribed to the existence of the above-mentioned polymerization-depolymerization equilibrium. In the first stage of the polymerization, the semilogarithmic plots are obviously linear, but as the conversion increases, the monomer concentration approaches the equilibrium concentration, and the rate constant of depropagation increases. Thus, the apparent rate constant of propagation kapp = kpeff [P*] is not reduced by decreasing the concentration of active chains [P*], but instead by decreasing the effective rate constant of propagation kpeff that is dependent on rate constants of propagation (kp) and depropagation (kd): kpeff = kp − kd/[M].36,37 In addition to this phenomenon, limited mobility of polymeric chains and the bulky monomer in an environment of continuously increasing viscosity could

Figure 1. SEC eluograms of poly(iBuPOSSMA) homopolymers (labels correspond to data in Table 2). 7315


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

possibly also contribute to a gradual decrease in the polymerization rate. Importantly, despite the rate retardation, the polymerization still proceeded in a well-controlled manner, which is illustrated by the linear development of MW with monomer conversion (Figure 2, top right). It is also noteworthy that the Mn(est) values were close to the theoretical conversion-based Mn values, represented in the graph by the solid line, and so the initiation efficiency in these polymerizations was high. Dispersity values did not decrease gradually with polymerization time, contrary to what is a typical feature of many ATRP processes.35 In the experiment carried out at 40 °C, dispersity remained about 1.15 for most of the polymerization, while at 60 °C, it steadily increased to ca. 1.19 (Figure 2, bottom right). When the latter experiment was prolonged to 144 h, the dispersity further gradually increased to 1.22 (data not shown here). Gradual broadening of MW distribution is a common feature of living systems reaching the propagationdepropagation equilibrium.38−40 It is also possible that side reactions (e.g., termination and transfer), which can still occur after the net propagation ceases, might contribute to the MW distribution broadening. To further support the hypothesis of the presence of a Tcrelated equilibrium, an additional kinetic experiment was performed. In this experiment, after carrying out the polymerization at 60 °C for 24 h, the temperature was increased to 90 °C for another 24 h. The conversion curve in Figure 3 shows that the typical conversion plateau had been reached at 60 °C, but, as expected, depolymerization occurred after the temperature increase to 90 °C. This was accompanied by a drop in Mn (est) of the polymer from about 50 000 at 24 h to about 40 000 at 48 h. Moreover, the temperature-induced depolymerization resulted in bimodality of the MW distribution (minor lowMW fraction), which was reflected in considerably increased

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

dispersity as is evident from the plotted Đ values. It was predicted that formation of shorter chains can result from the positive temperature jump in equilibrated systems of similar type.39 Although a low Tc is commonly associated with α-substituted styrenes and certain α-substituted acrylic esters,34,36 the influence that bulky side groups have on Tc of methacrylates was also noticed in literature, e.g. for ortho-substituted phenyl methacrylates.41,42 Furthermore, marked dependence of achievable monomer conversion on the initial monomer concentration and on temperature in free radical polymerization of triphenylmethyl methacrylate (TrMA) was described by Okamoto group.43 Importantly, Ishitake et al. have recently reported exactly the same effects as discussed here in their 7316


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represented by the last two entries in Table 2. In entry 13, lower temperature led to increased conversion and noticeably lower dispersity, compared to the entry 9 experiment carried out at 60 °C. Since in neither of those two experiments the polymerization-depolymerization equilibrium was reached, the higher conversion attained at lower temperature can be rather surprising. It needs to be borne in mind, however, that in the latter experiment, termination in early polymerization stages was probably much more prominent, as illustrated by SEC curve tailing (Figure 1). This would eventually lead to further decrease of (already quite low) active chains concentration and accumulation of Cu2+ species (persistent radical effect).49 Both these effects would efficiently retard the rate of polymerization and, in turn, also the conversion achieved in 24 h. In entry 14, a high [iBuPOSSMA]:[MBiB] ratio of 700:1 was employed at 40 °C. The amount of solvent was decreased (Cmon = 1.0 g/mL) in order to increase the polymerization rate. In 48 h, the conversion reached 70%, which corresponded to Mn(theor) of 462 600 and DPn = 490. This is several times higher than the highest DPn values reported so far in the literature for controlled polymerization of POSS monomers (iBuPOSSMA), i.e., about 100 for anionic polymerization9 and about 160 (our estimate from the reported conversion data) for RAFT polymerization.19 The attained dispersity was low (1.19), and the 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 have accounted for the limiting DPn values observed in the early studies on POSSMA homopolymerization via ATRP. In other words, high temperature and/or too diluted polymerization mixture would necessarily lead to limited conversions and low DPn. Unfortunately, this assumption is hard to verify, as the pioneering studies do not specify the conditions under which synthesis of higher-MW poly(POSSMA) was attempted.24 It is clear that the low-Tc limitations potentially apply also to RAFT polymerization of POSS monomers. In this regard, it is interesting to analyze the first two reports on POSSMA homopolymerization via RAFT as they contain valuable kinetic data illustrating the polymerization course. It appears that Mya et al. largely evaded most problems arising from the polymerization−depolymerization equilibrium by employing a high initial iBuPOSSMA concentration (1.06 M) and ending the polymerization before the conversion plateau could be reached.19 Nevertheless, gradually increasing dispersity values reaching about 1.6 were observed, i.e. a trend similar to our results. In the second report, Deng et al. utilized lower initial monomer concentrations in the range of 0.416 to 0.50 M in RAFT polymerization of iBuPOSSMA and cyclohexyl-substituted POSSMA (cyPOSSMA) at 65 °C. Analyzing the published data, it appears that when long enough polymerization time was allowed, similar effects as reported here were observed, i.e., deviation of the semilogarithmic plot from linearity at certain conversion and formation of the typical conversion plateau. This was most pronounced in polymerization of cyPOSSMA with [M]0 = 0.416 M and 32 h polymerization time. For both monomers, significant broadening of the MW distribution was observed for conversions higher than about 50% when the 60:1 [monomer]:[RAFT] ratio was applied. It can be speculated that increased role of depropagation in later stages of the polymerization can, at least partially, account for these effects. At the time of the submission of this work for publication, another study was published where modified iBuPOSSMA

studies on RAFT polymerization of TrMA and other methacrylates with bulky side groups.44−46 In these works, highly isotactic stereogradient polymers were obtained as a result of the thermodynamic control in later polymerization stages. By analogy, it is reasonable to expect that the (iBuPOSSMA) homopolymers could have increased stereoregularity as well. Unfortunately, in contrast to poly(TrMA) homopolymers, the transformation of poly(iBuPOSSMA) into easily analyzable poly(MMA) is not straightforward, which largely 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 estimate the polymerization enthalpy (ΔHp) and entropy (ΔSp) for our current system. To achieve this, iBuPOSSMA was polymerized under standard conditions at five different temperatures (40− 80 °C), and the equilibrium monomer concentration [M]eq was determined from the monomer conversion obtained from SEC. Figure 4 shows the plot of ln [M]eq against 1/T. From the slope

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

and intercept of the linear regression line, values of ΔHp = −41 kJ mol−1 and ΔSp = −101 J mol−1 K−1 were calculated. Tc for unimolar monomer concentration was then determined to be 130 °C. This is not far from the value reported for TrMA polymerized via RAFT in toluene (Tc = 104 °C).44 The determined value of ΔHp is considerably higher than that for MMA (about -55 kJ mol−1).47 This can be attributed to steric hindrance 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 less prominent for the bulky iBuPOSSMA monomer.48 It can be reasonably expected that POSSMA monomers bearing other substituents on the POSS cage such as phenyl, cyclopentyl, and cyclohexyl, to name a few common variants, will also show decreased Tc, which needs to be taken into account when attempting their polymerization. The knowledge of Tc allows tailoring appropriate conditions for iBuPOSSMA polymerization. It is clear that to attain high monomer conversion in a reasonable time, use of a high initial monomer concentration and lower polymerization temperature is advisable. It also appears that lower temperature (40 °C) contributes positively to low dispersity of the synthesized polymers. Examples of such optimized experiments are 7317


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monomer was polymerized to high MW by ATRP.50 The monomer modification, introduced to relax strain caused by the bulky POSS cage, consisted in incorporating an O-SiMe2 linker to prolong the spacer between the POSS cage and the methacrylate function. Importantly, the authors observed a correlation between the initial monomer concentration and the achieved conversion, with only limited conversions achieved at lower [M]0. Although no definite explanation for this behavior was given, limited thermodynamic polymerizability or termination/transfer reactions were suggested as possible causes. Considering our results presented here, it is likely that the propagation-depropagation equilibrium is still the main cause of limited conversions in this modified system. Similar approach has also been used recently by Zhang et al., who carried out a three-step synthesis to prepare an iBuPOSSMA-type monomer bearing a long 11-atom spacer.51,52 Interestingly, RAFT polymerization of this monomer still showed clear signs of thermodynamic control resulting in a limited conversion.52 On the whole, these results imply that even though the spacer incorporation can be helpful to some extent, careful optimization of the reaction conditions is still necessary, and the use of a commercial monomer such as iBuPOSSMA might be sufficient. Block Copolymerization of iBuPOSSMA. Besides preparation of high-MW poly(iBuPOSSMA) homopolymers, we also, for the first time, employed ATRP for synthesis of diblock copolymers with high-MW poly(POSSMA) blocks. Such copolymers have already attracted considerable attention,9,10,14−19 and we believe that ATRP could become a preferred method of their preparation. For this purpose, we selected MMA and styrene as model comonomers. Generally, two approaches to synthesis of the desired block copolymers exist. In the first one, iBuPOSSMA polymerization is triggered from a suitable polymeric macroinitiator while in the second approach, poly(iBuPOSSMA) block is synthesized beforehand and polymerization of the comonomer is initiated by it. We investigated 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 already been tested by other authors, but was dismissed as severely limited due to low solubility of poly(MMA) in POSS monomer solutions and because of very low attained conversions.7,21 Indeed, poly(MMA) does not go readily into iBuPOSSMA solution in toluene (or THF). Nevertheless, a clear lowconcentration solution can be achieved. For instance, initial monomer 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 to appropriately decrease polymerization temperature when working with dilute mixtures if too low limiting conversions are to be avoided. Therefore, the polymerization was carried out at 40 °C with PMDETA as a ligand; the [iBuPOSSMA]: [initiator] ratio was set to 200:1. Unfortunately, due to the substantial dilution, the reaction proceeded very slowly, reaching only 15% conversion (estimated from gravimetry) after 113 h. This translates to the poly(iBuPOSSMA) block DPn of about 29 and Mn (theor) = 27 500. If we calculate and add the equivalent Mn(SEC) value (ca. 11 000) to the Mn of the macroinitiator, we arrive to the theoretical value of Mn(SEC) = 63 000, which agrees very well to the Mn(SEC) = 63800 determined for the block copolymer. The SEC eluogram showed slight tailing, but the dispersity (Đ = 1.20) was fairly low (Figure 5).

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

In another experiment, we attempted to facilitate the polymerization by using a more active ligand, TPMA (KATRP of TPMA is about 2 orders of magnitude higher than that of PMDETA).53 Because the initial monomer concentration was only 0.12 M, the experiment was performed at room temperature to suppress depolymerization; the [iBuPOSSMA]:[initiator] ratio was 100:1. In 96 h, the conversion reached 59%, corresponding to poly(iBuPOSSMA) DPn of 59 and Mn(theor) = 55 300. Applying the same procedure as above, we can estimate the theoretical Mn(SEC) of the copolymer to be 70 500, which is in fair agreement with the measured Mn(SEC) = 65 700. SEC analysis of the copolymer also revealed slightly higher dispersity value (Đ = 1.29) stemming from certain tailing of the SEC signal, signaling the presence of a low-MW fraction (Figure 5). These results show that, despite some limitations, the poly(MMA) macroinitiator approach is viable and can be used for synthesis of block copolymers with highMW poly(iBuPOSSMA) blocks, provided low-enough temperature and an active-enough ligand are used. In this context, it is worth noticing that ATRP has a certain advantage over RAFT at low temperatures, as the latter method counts on thermal decomposition of a radical initiator in the traditional setup. To evaluate the second approach outlined above, a poly(iBuPOSSMA) macroinitiator was first synthesized by ATRP at 40 °C with PMDETA as a ligand and the [iBuPOSSMA]:[MBiB] ratio of 100:1. The polymerization was deliberately ended at low conversion (52%) to help preserve the chain end functionality. The macroinitiator was purified by repeated precipitation in MeOH and analyzed by SEC, which revealed low dispersity of 1.14 and Mn(SEC) value of 19 900, i.e., Mn(est) = 60 500. In the first experiment, the poly(iBuPOSSMA) initiated ATRP of styrene, carried out without solvent at 100 °C for 23 h with PMDETA as a ligand. From the weight of the isolated copolymer, the Mn(theor) of the polystyrene block was estimated to be 69 900, assuming 100% initiation efficiency. SEC analysis of the copolymer revealed Mn(SEC) = 97 200 and Đ = 1.14. The Mn(SEC) value is slightly higher than the one we calculated from molecular weights of the two blocks (about 90 000). This discrepancy can be ascribed to slightly decreased initiation efficiency of the macroinitiator. Indeed, a very small signal corresponding to the macroinitiator can be found in the SEC chromatogram of the isolated copolymer (Figure 6). In another experiment, 7318


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straightforward synthesis of new hybrid (co)polymers bearing POSS moiety.

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ASSOCIATED CONTENT

S Supporting Information *

A plot of Mn(SEC) vs Mn(SEC MALLS) for poly(iBuPOSSMA) homopolymers and a table with experimental data regarding ATRP of iBuPOSSMA using the CuBr2 deactivator. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (V.R.). Notes

The authors declare no competing financial interest.

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

ACKNOWLEDGMENTS This work has been supported by the Grant Agency of the Czech Republic (Grant P106/12/0844). The authors thank Mr. Štěpán Adamec for technical assistance, Mrs. Dana Kaňková for measurement of NMR spectra, and Dr. Peter Č ernoch for performing the refractometric analysis.

MMA was used for synthesis of the second block. [MMA]: [initiator] ratio of 560:1 was utilized, and HMTETA was employed as a ligand in this case. Conversion of 77% was reached, but the SEC eluogram of the copolymer showed a noticeable shoulder representing unreacted macroinitiator. This fraction (about 13 wt %) was successfully removed by extraction of the product with cyclohexane. The purified block copolymer had Mn(SEC) = 73 200, which is in a good accordance with the calculated value Mn = 75 500, obtained by addition of the macroinitiator Mn(est) and Mn(theor) of the poly(MMA) block (derived from conversion taking the decreased initiation efficiency into consideration). Figure 6 shows the SEC trace of the purified block copolymer. Comparing the results obtained with styrene and MMA, it appears that the poly(iBuPOSSMA) macroinitiator shows higher reinitiation efficiency with styrene. Such trend has been previously observed in synthesis of POSSMA diblock copolymers by RAFT.7

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REFERENCES

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CONCLUSIONS In this study, the commercially available POSS monomer, iBuPOSSMA, was homopolymerized via CuBr catalyzed ATRP. It was shown that various ligands can be successfully utilized for this purpose; PMDETA was identified as a good compromise between the polymerization rate and controllability of the process. Poly(iBuPOSSMA) homopolymers of very high MW and low dispersity were prepared in a controlled way under optimized conditions. Low degrees of polymerization achieved in some of the former studies can be plausibly ascribed to the effects originating from the low Tc of the monomer. The values of Tc as well as of the polymerization enthalpy and entropy were estimated, which should greatly facilitate designing future polymerization experiments involving iBuPOSSMA and similar POSS monomers. Moreover, this work also provides some practical information concerning isolation of the products and determination of monomer conversion and MW of polymers, which could be beneficial for future research. Finally, various block copolymers having high-MW poly(iBuPOSSMA) blocks were successfully synthesized using two different strategies. On the whole, these results should help establish ATRP as the method of choice for controlled polymerization of POSSMA monomers and can open new opportunities for 7319


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