Living Radical Polymerization - American

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Chapter 14

Atom Transfer Radical Polymerization of Methyl Methacrylate Utilizing an Automated Synthesizer Huiqi Zhang, Martin W. M. Fijten, Richard Hoogenboom, and Ulrich S. Schubert* Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology and Dutch Polymer Institute, P . O . Box 513, 5600 M B Eindhoven, The Netherlands

The homogeneous atom transfer radical polymerization of methyl methacrylate mediated by CuBr/N-(n-hexyl)-2-pyridylmethanimine was successfully carried out using an automated synthesizer. The effects of initiators, solvents and reactant ratios on the polymerization were investigated. Three different kinds of initiators, namely ethyl 2-bromoisobutyrate, (1-bromoethyl)benzene, and p-toluenesulfonyl chloride, were utilized to initiate the polymerization and ethyl 2-bromoisobutyrate was proven to be the best initiator for the studied system in terms of molecular weight control and narrow polydispersities of the obtained polymers. The solvents used (i.e., toluene, p-xylene, and n-butylbenzene) were found to have strong effects on the polymerization. The reactions in toluene and p-xylene were well-controlled and almost the same polymerization rates were observed. However, the reaction rate dramatically increased in the case of n-butylbenzene, resulting in radical termination during the polymerization. The initiator and Cu(I) concentrations had a positive effect on the polymerization rate. In the meantime, all the reactions were controlled and polymers with predetermined molecular weights and low polydispersity indices (< 1.3) were obtained.

© 2003 American Chemical Society

193

194


Introduction Controlled "living" radical polymerization (CRP) systems have attracted great attention from both the academic research and industry during the past years because of their versatility (1,2). Among the most successful CRPs are nitroxide-mediated polymerization (NMP) (3), atom transfer radical polymerization (ATRP) (4,5,6), and reversible addition-fragmentation chain transfer (RAFT) polymerization (7). All these techniques allow the preparation of well-defined polymers with predetermined molecular weights, low polydispersities, functional groups, and various architectures under relatively mild reaction conditions. ATRP has been the most extensively studied CRP system (8,9). The success depends largely on a reversible dynamic equilibrium between the dormant species (P -X) and the active species (radicals, P ) (Scheme 1). n

n

P -X + Cu(I)-Y/L n

(Χ, Y = Br, CI)

m

Scheme 1

The equilibrium determines the radical concentration and subsequently the rates of polymerization and termination. A successful ATRP system should have a very low equilibrium constant (K^ = kjk ~10" ) (10), which keeps the active species at a very low concentration (approximately 10" to 10" M) (//) and thus greatly minimizes the radical termination reactions. Many parameters in ATRP such as the structure and concentration of the utilized monomers, catalysts (metals and iigands), initiators, solvents, reactant ratios, and the reaction temperature can significantly influence the equilibrium, which makes the optimization of the reaction conditions very time-consuming, in particular when a new reaction system is investigated. Therefore, practical techniques for a rapid parallel and maybe even automated synthesis, which would allow an efficient high-throughput screening to obtain optimal reaction conditions and to understand the polymerization parameters, are highly suitable for this research direction. Recently, an automated parallel synthetic approach has been applied in ATRP, but only the screening of the molecular weights of the copolymers prepared via the ATRP of styrene and butyl acrylate was presented (12,13). In addition, no detailed information concerning the utilized process has been 7

d

8

7

195


provided. In this paper, the investigation of the homogeneous ATRP of methyl methacrylate (MMA) mediated by CuBr/iV*-(«-hexyl)-2-pyridylmethanimine (NHPMI) utilizing an automated synthesizer is reported. The experimental setup, the parallel synthetic procedure, the reproducibility of the parallel approach, and the comparison with the conventional laboratory experiments are described. Furthermore, the effects of initiators, solvents and reactant ratios on the polymerization are also presented.

Experimental Section Materials M M A (Aldrich, 99%) was washed twice with an aqueous solution of sodium hydroxide (5%) and twice with distilled water, dried with anhydrous magnesium sulfate overnight, and then distilled over calcium hydride under vacuum. The distillate was stored at -18 °C before use. CuBr (Aldrich, 98%) was stirred with acetic acid for 12 h, washed with ethanol and diethyl ether, and then dried under vacuum at 75 °C for 3 days. The purified CuBr was stored in an argon atmosphere. NHPMI was synthesized by condensation of pyridine-2carboxaldehyde (Acros, 99%) and w-hexylamine (Acros, 99%) as described elsewhere (14). Toluene (Biosolve Ltd., AR) was distilled over calcium hydride. /?-Xylene (Aldrich, 99+%, anhydrous), w-butylbenzene (Acros, 99+%), ethyl 2bromoisobutyrate (EBIB, Aldrich, 98%), (l-bromoethyi)benzene (BEB, Aldrich, 97%), p-toluenesulfonyl chloride (TSC, Acros, 99+%), deactivated neutral aluminium oxide (Merck, for column chromatography), and all the other chemicals were used as received.

Instruments and Measurements The reactions were carried out in a computer-controlled Chemspeed ASW 2000 automated synthesizer (Figure 1). Five reactor blocks could be used in parallel and each block had 4 to 16 reaction vessels depending on their volumes (100 to 13 mL). Each reaction vessel was jacketed with an oil bath and was equipped with a cold-finger reflux condenser. The temperature of the oil bath was controlled by a Huber Unistat 390 W Cryostat and could vary from -90 to 150 °C and the temperature of the reflux liquid was controlled by a Huber ministat compatible control and could change from -10 to 50 °C. The reaction vessels were connected with a membrane pump, which could be utilized for inertization or evaporation processes. Mixing was performed by a vortex process

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(0 to 1400 rpm). A glove box was available, which kept an argon atmosphere outside the reaction system. The automated synthesizer was connected to an online size exclusion chromatography (SEC) and an off-line gas chromatography (GC). A Gilson liquid handling system was used in the automated synthesizer.

Figure 1. Visualization of the automated synthesizer set-up.

The monomer conversion was determined from the concentration of the residual monomer using an off-line Interscience Trace GC with an auto-sampler, equipped with a Rtx-5 (Crossbond 5% diphenyl-95% dimethyl polysiloxane) capillary column (30 m χ 0.25 mm ID x 0.25 urn df) with polymerization solvents (toluene, /^-xylene, and w-butylbenzene) as internal references. After optimization, the GC measurement of one sample required 5 min. Molecular weights and molecular weight distributions (MWDs) were measured with an online SEC set-up (Shimadzu gel permeation chromatography (GPC) equipped with a LC-10AD VP pump and a RID-6A differential refractometer) at ambient temperature. Tetrahydrofuran (THF) was used as the eluent at a flow rate of l.OmL/min. A linear column (PLgel 5 um Mixed-D, Polymer Laboratories, 30 cm) was used. The calibration curve was prepared with poly(methyl meth acrylate) (PMMA) standards. The GPC measurement of one sample required 15 min.

Polymerization Procedures A typical ATRP was carried out in the automated synthesizer as follows; CuBr (0.0596 g, 0.42 mmol) was manually added to the reaction vessels (75 mL, three parallel reactions). Inertization (three cycles of vacuum (15 min)/argon


197 filling) of these reaction vessels was conducted at 120 °C to remove the oxygen and moisture. The temperature of the oil bath was then lowered to 25 °C. A degassed stock solution of M M A (6.2407 g, 62.33 mmol) in /?-xylene (5.7734 g) and a degassed stock solution of NHPMI (0.2372 g, 1.25 mmol) in p-xylene (2.8867 g) were added subsequently. The reaction temperature was increased to 90 °C and at the same time the reflux liquid was cooled to -5 °C. After the reaction mixtures were vortexed at 600 rpm for 15 min at 90 °C, a degassed solution of initiator EBIB (0.0811 g, 0.42 mmol) in p-xylene (2.8867 g) was added during 2 min. The reaction mixtures were then vortexed at 600 rpm, and the polymerizations were sampled at suitable time periods throughout the reactions. The samples were diluted with THF, and parts of them were used for GC measurements in order to determine the monomer conversion. The rest was purified automatically by passing through aluminium oxide columns in the solid phase extraction (SPE) set-up prior to the SEC measurements. The ATRP in the conventional set-up was carried out according to the procedure as previously reported (15).

Results and Discussion The ATRP of M M A mediated by CuBr/NHPMI (Scheme 2) was investigated with an automated synthesizer. The volume of the solvent used was always twice that of M M A in each ATRP system and a molar ratio of initiator to CuBr to NHPMI of 1:1:3 was utilized. A homogeneous dark brown solution was obtained when the reaction mixture was heated to 90 °C. The fast screening of the reaction conditions for ATRP by using an automated synthesizer can significantly speed up the research. However, it should be evaluated in advance whether the automated synthesizer can provide reproducible results as well as comparable results with those obtained from the conventional laboratory experiments. Therefore, selected experiments were carried out both in the automated synthesizer and in a conventional set-up in the laboratory in order to allow a detailed comparison of the obtained results. The reproducibility of the ATRP carried out in the automated synthesizer as well as the comparability of the results from the automated synthesizer and the conventional experiments has been confirmed and described elsewhere (16). In addition, an automated purification procedure was developed to purify the polymers prepared via ATRP for the SEC measurements. Hand-made deactivated aluminium oxide columns (0.5 cm) in SPE cartridges (length = 5.6 cm, diameter = 0.6 cm) including porous polyethylene frits and ASPEC caps together with 2 mL of THF (as the eluent) were utilized to purify the polymers (16).

198 C H

RX

+

ι CH =C 2

J

Solvent, 90 °C

I

C=0 ι OCH

•

CuBr/NHPMI

Λ

I

3

OCH

(RX « EBIB, B£B,TSC) Advances in Controlled/Living Radical Polymerization Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/28/16. For personal use only.

3

» , R-+CH2~C-iirX ' C=0 r

3

(PMMA)

(EBIB)

(BEB)

(TSC)

(NHPMI)

Scheme 2

A T R P of M M A with Different Initiators Figure 2 shows the effect of the initiators used on the ATRP of M M A at 90 °C in p-xylene with CuBr/NHPMI as the catalyst and [MMAyfinitiatory [CuBryfNHPMIJo^ 150:1:1:3. Three different kinds of initiators were used in this study, including ethyl 2-bromoisobutyrate (EBIB), (1-bromoethyl)benzene(BEB), and /j-toluenesulfonyl chloride (TSC). Two parallel reactions for each ATRP system were carried out in the automated synthesizer and good reproducibility of the reactions can be clearly seen in Figure 2. In all cases, almost a linear relationship between ln([M] /[M]) and reaction time t was obtained, indicating that no significant radical termination was present in the polymerization processes. The initiators revealed only a little influence on the kinetics of the polymerization and the reaction rates. In addition, an induction period of 19 to 86 min was observed in these systems. The origin of this induction period is not very clear yet, and further investigation is going on at present. The effects of the initiators on the molecular weights and polydispersities of the polymers were also studied (Figure 3). A l l the polymers were purified by using the above-mentioned deactivated aluminium oxide columns automatically before the SEC measurements. The ATRP initiated by EBIB was well-controlled in terms of the molecular weights and MWDs of the polymers. The numberaverage molecular weights determined by SEC, M Ec» increased linearly with increasing monomer conversion and were close to the theoretical values (i.e., M ) calculated according to eq 1, indicating that the polymerization was living and controlled. 0

n>S

n>th

199


1.4

t (min)

Figure 2. Plots of ln([M]Λ

A T R P of M M A in Different Solvents The ATRP of M M A with CuBr/NHPMI as the catalyst and EBIB as the initiator was carried out at 90 °C in a series of solvents such as toluene, p-xylene, and w-butylbenzene to study the effect of solvents on the polymerization (two parallel reactions for each ATRP system in /^-xylene and w-butylbenzene). The kinetic results revealed that ln([M]o/[M]) increased almost linearly with reaction time / throughout the reactions when toluene and p-xylene were used as solvents, demonstrating that the radical concentrations remained constant during the reactions (Figure 4). However, a curved kinetic plot was found for the ATRP in w-butylbenzene, suggesting the occurrence of radical termination during the ATRP process (15). The polymerization rate in toluene was almost the same as that in /7-xylene, while the reaction in w-butylbenzene proceeded much faster. The differences in the polymerization rates in different solvents were also observed from the different viscosities of the reactions. The reaction mixture in w-butylbenzene was so viscous at the end of the reaction (about 7 h) that stirring became impossible, while the reaction mixtures in toluene and p-xylene were still low viscous solutions. The effect of the polarity of the solvents on the ATRP of M M A catalyzed by CuBr/Af-alkyl-2-pyridylmethanimine has been investigated and faster reactions were observed in more polar solvents (20). However, the reactions in different apolar solvents have hardly been compared, largely due to the opinion that the

201

2 1.6


I cf 0.8

0.4 0 0

100

200

300

400

500

t (min)

Figure 4. Plots ofln([M](/[MJ) versus reaction time tfor the ATRP of MMA at 90 °Cwith [MMA]o/[EBIB](/[CuBr]o/[NHPMI] = 150:1:1:3 and different solvents such as n-butylbenzene (φ,Ο), p-xylene (M£3), and toluene (A). 0

C MA(%) M

Figure 5. Dependence of M c andPDIs of the polymers on the monomer conversion of the ATRP of MMA at 90 °C in toluene (Δ), p-xylene (M£2)> * n-butylbenzene (%,Q). [MMA] (/[EBIB] (/[CuBr] (/[NHPMI] = 150:1:1:3. n t S E

αηα

0

different apolar solvents with close polarity, such as toluene and p-xylene, have no influence on the polymerization rate. This is consistent with our experiments in toluene and p-xylene but not in the case of w-butylbenzene. The cause of the remarkable increase of the polymerization rate in w-butylbenzene is not clear yet, and further investigations are ongoing at present. Figure 4 also revealed the slow initiation at the beginning of the reactions.

202


80000 -ι

6

7

8

9

Eiution time (min)

Figure 6. GPC traces of the polymers prepared via ATRP at 90 °C with n-butyl benzene as the solvent C = 10% (a), 55% (b), 73% (c), and87% (d). MMA

The solvents were found to have little influence on the molecular weights of the polymers (Figure 5). The M Ec increased linearly with increasing monomer conversion and were comparable to values. The ATRP systems in toluene and p-xylene resulted in polymers with unimodal GPC traces and PDI values < 1.3 throughout the reactions (Figure 5). The PDI values of the polymers prepared via the ATRP in w-butylbenzene, however, increased with monomer conversion and were larger than 1.5 when the monomer conversion was higher than 80%. This large increase of the PDI values of the polymers might in part be ascribed to the presence of radical termination during the reaction. In addition, the high viscosity of the reaction mixture at the end of the reaction, which was likely to influence the activation and deactivation processes of the equilibrium in ATRP (Scheme 1), could also be partially responsible for this phenomenon. It is known that the ATRP of M M A with a too high radical concentration will mainly result in radical disproportionation (21). Therefore, the M Ec of the polymers prepared via the ATRP of M M A with a too high radical concentration should be comparable with those of the polymers obtained from the well-controlled systems, but the PDI values of the polymers would be larger in the former case. This agrees well with our experimental results (Figure 5). The GPC traces of the polymers obtained from the ATRP in w-butylbenzene are shown in Figure 6. Small shoulders were observed on the high molecular weight sides of the GPC traces of the obtained polymers at relatively high monomer conversions, suggesting that radical coupling was also present in the ATRP with nbutylbenzene as the solvent. Besides, the tailing of the GPC traces was clearly observed, which could be attributed to the dead polymers resulted from the radical termination during the reactions. n>S

nfS

203


Effect of Initiator and Cu(I) Concentrations A series of reactions at different reactant ratios were carried out at 90 °C using the automated synthesizer in order to investigate the effect of the initiator and Cu(I) concentrations on the polymerization (Figure 7, two parallel reactions for each system). The results showed that ln([M](/[M]) increased almost linearly with reaction time t for all the reactions with [MMA]o/[EBIB]o/[CuBr]o/ [NHPMI]o = 150:1:1:3, 100:1:1:3, and 50:1:1:3, revealing that all the polymerizations proceeded in a controlled way and no significant radical termination took place during the reactions. The polymerization rate increased with the increase of the initiator and Cu(I) concentrations, and the average apparent rate constants (k = slope of the kinetic plot) of the three reactions were 0.0054,0.0038,0.0030 min" , respectively. Figure 8 shows that the Μ^ increased linearly with monomer conversion for all the reactions with different reactant ratios. However, they were slightly higher than the A/ values revealing that persistent radical effect took place at the beginning of the reactions and thus lowered the initiation efficiency of the systems (22). The PDI values of the polymers obtained from these systems were below 1.3 and almost identical, indicating the well-controlled reactions. m

1

5Ε0

n>th

Conclusions This paper describes the successful application of an automated synthesizer in the homogeneous ATRP of M M A mediated by CuBr/NHPMI. The polymerizations initiated by EBIB, BEB, and TSC resulted in nearly linear kinetic plots of ln([M] /[M]) versus reaction time / and identical polymerization rates. However, the molecular weights and PDIs of the obtained polymers were significantly influenced by the utilized initiators. The ATRP with EBIB and TSC as initiators provided controlled reactions while an uncontrolled system was obtained for the ATRP with BEB as the initiator. EBIB was proven to be the best initiator for the studied system. The effect of the solvents, including toluene, p-xylene, and w-butylbenzene, on the polymerization was studied. Both the linear kinetic plots of ln([M]o/[M]) versus reaction time t and the linear dependence of the molecular weights on the monomer conversion were obtained for the ATRP systems in toluene and p-xylene. A curved kinetic plot was observed for the ATRP in w-butylbenzene although the molecular weights still increased linearly with the monomer conversion. The polymerization in toluene proceeded as fast as that in p-xylene, while the reaction rate dramatically increased in the case of w-butylbenzene, leading to radical termination reactions (radical disproportionation and coupling) and higher PDI values of the obtained polymers. The molecular weights of the polymers were almost not influenced by the solvents used and they were all comparable to the M h values. An increase of the initiator and Cu(I) concentrations resulted in an increase of the polymerization rate. A l l the studied reactions with different reactant ratios were controlled in terms of the molecular weights and PDI values of the obtained polymers. In addition, an induction period was present in most of the studied systems, which requires further investigation. 0

n>t

204


2.5

t (min)

Figure 7. Plots ofln([M]o/[M]) versus reaction time t for the ATRP of MMA in p-xylene at 90 °C using [MMA]

Living Radical Polymerization - American

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