Article pubs.acs.org/Macromolecules
Dimerization of Poly(methyl methacrylate) Chains Using Radical Trap-Assisted Atom Transfer Radical Coupling Christopher J. Valente, Autumn M. Schellenberger, and Eric S. Tillman* Department of Chemistry, Bucknell University, Lewisburg, Pennsylvania 17837, United States
ABSTRACT: End-brominated poly(methyl methacrylate) (PMMABr) was prepared by atom transfer radical polymerization (ATRP) and employed in a series of atom transfer radical coupling (ATRC) and radical trap-assisted ATRC (RTA−ATRC) reactions. When coupling reactions were performed in the absence of a nitroso radical traptraditional ATRC conditionvery little coupling of the PMMA chains was observed, consistent with disproportionation as the major termination pathway for two PMMA chain-end radicals in our reactions. When 2-methyl-2-nitrosopropane (MNP) was used as the radical trap, coupling of the PMMA chains in this attempted RTA−ATRC reaction was again unsuccessful, owing to capping of the PMMA chains with a bulky nitroxide and preventing further coupling. Analogous reactions performed using nitrosobenzene (NBz) as the radical trap showed significant dimerization, as observed by gel permeation chromatography (GPC) by a shift in the apparent molecular weight compared to the PMMABr precursors. The extent of coupling was found to depend on the concentrion of NBz compared to the PMMABr chain ends, as well as the temperature and time of the coupling reaction. To a lesser extent, the concentrations of copper(I) bromide (CuBr), nitrogen ligand (N,N,N′,N′,N″-pentamethyldiethylenetriamine = PMDETA), and elemental copper (Cu) were also found to play a role in the success of the RTA−ATRC reaction. The highest levels of dimerization were observed when the coupling reaction was carried out at 80 °C for 0.5h, with ratio of 1:4:2.5:8:1 equiv of NBz: CuBr:Cu:PMDETA:PMMABr.
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INTRODUCTION Atom transfer radical polymerization (ATRP) has seen an explosion in popularity since its inception in the mid 1990s due to its ability to add control and thus precision to a staggering number of polymerizations.1−12 By manipulating the redox potential of the ligand-bound metal catalyst, the experimenter can achieve an equilibrium between an active, propagating polymer radical and its dormant, halogen-capped form. Conceptually similar, atom transfer radical coupling (ATRC) also involves the same redox chemistry and the same endhalogenated polymers13−17 but its utility has lagged far behind ATRP. ATRC, which is a postpolymerization reaction, simply changes the available reaction partners for the polymer radical and creates a scenario where bimolecular radical−radical termination becomes favored (Scheme 1ATRC vs ATRP). A quick inspection reveals multiple limitations of this method. First, the termination pathway must favor radical−radical coupling (kt), which is the case for polystyrene (PSt) radicals,18,19 and not disproportionation (kd), which may be the case for poly(methyl methacrylate) (PMMA) radicals.20−23 © 2014 American Chemical Society
Second, the concentration of polymer radicals in solution must be at a level that a bimolecular reaction is feasible, meaning that the amount of metal catalyst and (typically) metal reducing agent must be present in relatively high amounts compared to an ATRP reaction, to effectively push the equilibrium further toward the polymer radical.24,25 The inclusion of a radical trap, in the form of either a nitrone26,27 or nitroso compound,28−30 can alter the mechanistic pathway of the coupling reaction so that dimerization occurs in a stepwise sequence. In this so-called radical trapassisted ATRC (RTA−ATRC), the rate of each step becomes first order with respect to the polymer radical, with the k1 being rate limiting.31,32 RTA−ATRC is mechanistically and kinetically summarized, and compared to ATRC, in Scheme 2. Previous work in our group has shown that altering the pathway of the coupling reaction from ATRC to RTA−ATRC allows for more Received: January 22, 2014 Revised: March 10, 2014 Published: March 21, 2014 2226
dx.doi.org/10.1021/ma5001805 | Macromolecules 2014, 47, 2226−2232
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Scheme 1. ATRC Pathway Compared to Propogation Step of ATRP Pathway
Scheme 2. Mechanistic Comparison of ATRC to RTA−ATRC
produced diblock copolymers because of differing KATRP values.34 Bimolecular coupling of PMMA chains has remained elusive due to the steric bulkiness around the chain ends, suppressing radical−radical coupling and favoring disproportionation under ATRC conditions (Scheme 3).20,35 Styrene-assisted and isoprene-assisted coupling reactions have been employed by others as means to join polymer chain ends that are resistant to the head-to-head termination pathway,36 simply by adding a few styrene or isoprene units to the chain ends during the
leniency in the coupling of brominated PSt chains to form dimers26 and in the intramolecular coupling of telechelic PSt chains to form macrocyclic polymers.33 In addition to simply reducing the amount of metal, reaction time, or temperature, we have shown that the ATRP of styrene can be directly converted into an RTA−ATRC reaction with no purification of the PStBr and no removal of the residual monomer.28 Furthermore, two separate brominated polymer chains, PStBr and PMABr, were activated in a single pot and selectively 2227
dx.doi.org/10.1021/ma5001805 | Macromolecules 2014, 47, 2226−2232
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Scheme 3. Fate of PMMA Radicals: (Top) Termination by Disproportionation; (Bottom) Termination by Radical Coupling (ATRC)
Scheme 4. ATRP of MMA to form PMMABr and Subsequent RTA−ATRC of PMMABr
ice bath and opening the flask to the atmosphere. GPC analysis was performed on the crude reaction mixture (Mn = 8400, PDI = 1.21, Mp = 12700). ATRC of PMMABr was performed with analogous procedures and reagents, in the absence of nitrosobenzene. RTA− ATRC of PMMABr was performed with analogous procedures and reagents, replacing NBz with MNP. Thermolysis of PMMABr Dimers Prepared by RTA−ATRC. To an oven-dried round-bottom flask was added PMMA dimers formed from RTA−ATRC (Mn = 6800, Mp = 11400, PDI = 1.30), which were run through an alumina column and concentrated into a 1 mL THF solution. Dimethylformamide (4.0 mL) and a dried magnetic stir bar were also added to the flask. The solution was submerged in an oil bath at 125 °C and allowed to reflux for 6 h. GPC analysis was performed on the crude reaction mixture (Mn = 4800, Mp = 5950, PDI = 1.14). Measurements. Polymer samples were analyzed on an Eco SEC GPC system (Tosoh Biosciences LLC) connected to a PC running EcoSEC data analysis software. The equipment was temperature controlled at 40 °C, and equipped with a UV detector, a dual flow RI detector and two TSK gel super Multipore H-ZM columns. A six-point calibration was obtained with polystyrene standards (TOSOH, Mp range: 2.6 × 102 to 7.06 × 105 g/mol) and was used to obtain molecular weight characteristics and polydispersity index (PDI) values of all polymers. All measurements were using THF as the eluent. UV− vis analysis was carried out in THF solvent at 5 × 10−3 M for PMMABr, MNP, and nitroxide-capped PMMA using a Thermo Scientific Evolution 600 UV−vis spectrophotometer.
coupling reaction and essentially performing an ATRC on diblocks with PSt at the ends. Also, if the halogen-capped polymer has a prohibitively low KATRP value, as is the case for poly(acrylate)s,37 styrene-assisted ATRC has also proven an effective means to dimerize the chains. Here, we show that brominated PMMA (PMMABr) chains can be directly employed in RTA−ATRC reactions and undergo dimerization to an extent similar to PSt radicals in ATRC reactions.
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MATERIALS
Copper(I) bromide (CuBr, Aldrich, 98%), copper(II) bromide (CuBr2, Aldrich, 99%), and copper nanopowder (Cu0, Acros, 99.8%) were used as received. Methyl methacrylate (MMA, Aldrich, 99%), ethyl α-bromoisobutyrate (EbiB, Aldrich, 98%), chloroform-d (99.8 atom % D, 1% v/v TMS) and N,N,N′,N′,N″-pentamethyldiethylene triamine (PMDETA, Aldrich, 99%) were used as received and stored at 5 °C. 2-Methyl-2-nitrosopropane dimer (MNP, Aldrich, 98%), and nitrosobenzene (NBz, Aldrich, ≥97%) were used as received and stored at −20 °C. Tetrahydrofuran (THF) was collected from an Innovative Technology PureSolv solvent purification system. Synthesis of PMMABr Precursor by ATRP. To a flame-dried two-neck round-bottom flask was added methyl methacrylate (9.0 mL, 84 mmol), CuBr2 (94 mg, 0.42 mmol), ethyl 2-bromoisobutyrate (63 μL, 0.42 mmol), 9.0 mL THF and a dried magnetic stir bar. The flask was attached to a Schlenk line and was subjected to three cycles of freeze−pump−thaw before placing the flask in an oil bath at 80 °C. Once equilibrated, PMDETA (88 μL, 0.43 mmol) was added via an argon flushed syringe to initiate the reaction and the solution was allowed to stir for 2 h. The reaction was terminated by placing the flask in an ice bath and opening the flask to the atmosphere. Copper was removed from the solution by column chromatography and the resulting concentrated solution was precipitated in hexanes to form a white powder. The product was dried under vacuum for 24 h. Mn = 4900, Mp = 6750, PDI = 1.22. RTA−ATRC of PMMABr. To an oven-dried two-neck roundbottom flask was added monobrominated poly(methyl methacrylate) (Mn = 4800, PDI = 1.22, Mp = 6150, 75.0 mg, 15.6 μmol), CuBr (9.0 mg, 62.5 μmol), Cu(0) (2.5 mg, 39 μmol), nitrosobenzene (1.7 mg, 15 μmol), 3.0 mL of THF and a dried magnetic stir bar. The solution was attached to a Schlenk line and was subjected to three cycles of freeze− pump−thaw before placing the flask in an oil bath at 80 °C. Once equilibrated, PMDETA (26 μL, 125 μmol) was added via an argon flushed syringe to initiate the reaction and the solution was allowed to stir for 30 min. The reaction was terminated by placing the flask in an
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RESULTS AND DISCUSSION Atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) was carried out in THF using ethyl αbromoisobutyrate (EbiB) as the initiator, CuBr2 as the catalyst, and PMDETA as the ligand with ratios of ([MMA]:[EbiB]: [CuBr2]:[PMDETA]) = (200:1:1:1), with characteristics of these PMMABr. Polymerizations were run for 2 h resulting in number-average molecular weight (Mn) values ranging from 5000 to 11000 Da, with polydispersity (PDI) values
