Macromolecules 1997, 30, 5643-5648 5643 Polymerization of Substituted Styrenes by Atom Transfer Radical Polymerization Jian Qiu and Krzysztof Matyjaszewski* Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213 Received March 27, 1997; Revised Manuscript Received June 16, 1997X ABSTRACT: A series of substituted styrenes were polymerized by controlled/“living” radical polymeri- zation using atom transfer radical polymerization (ATRP), in order to correlate monomer structures with polymerization rate. The effect of substituents is discussed with regard to the Hammett equation. The results show that most of the monomers can polymerize in a controlled way by ATRP; i.e., the apparent polymerization rate is first order with respect to monomer concentration, and molecular weight increases linearly with monomer conversion. The molecular weights obtained fit the theoretical values and polydispersities are relatively low (Mw/Mn < 1.5). Monomers with electron-withdrawing (EW) substituents result in better polymerization control and polymerize faster than those with electron-donating (ED) substituents. The apparent polymerization rate constants follow the Hammett equation with F)1.5. Further study indicates that the difference of polymerization rates for different monomers can be attributed • to both propagation constant, kp , and the equilibrium constant, Keq, for atom transfer. Monomers with • EW substituents have larger kp and Keq values than those with ED substituents; therefore, EW substituents increase the monomer reactivity and decrease the stability of dormant species, while ED substituents have the opposite effects. Introduction dispersity as narrow as Mw/Mn 1.04 have been 7,8 ≈ Substituents play a significant role in chain polym- obtained. This method has proved successful for monomers such as styrene, (meth)acrylates, and acrylo- erization, because polymerizability of a vinyl monomer 9-13 by a radical, anionic, or cationic mechanism is depend- nitrile. From the point of view of both theoretical ent upon the steric and electronic properties of the study and industrial application, it is intriguing to apply substituents. The electronic effect of the substituent ATRP to various substituted styrenes to investigate the manifests itself by altering the electron density of the correlation between polymerization rate and monomer double bond through inductive and resonance effects structure. Furthermore, it is also hoped that such a and its ability to stabilize the active species, whether it study might give us a better understanding of the is a radical, anion, or cation. Although substituent mechanism of ATRP. effects are not as important in radical reactions ( F e In this paper, the kinetic studies of the polymerization 1) as they are in ionic reactions ( F G 5),1-3 radical| | of substituted styrenes with a homogeneous and het- polymerization is affected by substituents| | due to the erogeneous initiating systems are reported, and the participation of polar effects. The relationship between effect of substituents is discussed with regard to the • • the polymerization rate and substituents has been well Hammett equation: log(kpx /kpH ) )Fσ. studied. For example, almost 50 years ago, Walling and co-workers4 concluded from an analysis of free radical Experimental Section copolymerizations that the polymerization rate in- Materials. All the monomers (Aldrich) were purified by creases when the alkenes bear electron-withdrawing passing them through an alumina column to remove stabilizers substituents. Later, Imoto et al.5 reported that a and then purging them with argon for 15-20 min before Hammett relationship could be established in the ho- polymerization. Diphenyl ether, used as solvent, was also • mopolymerization of p-substituted styrenes, i.e. log(kpx / purged with argon for 15-20 min before polymerization. CuBr k •) )Fσ,(F)0.6), in which k • and k • represent was purified according to the literature procedure;14 2,2′- pH px pH ′ ′ the absolute propagation rate constant of substituted bipyridine (bipy) was used as received. 4,4 -Di-(5-nonyl)-2,2 - bipyridine (dNbipy) was synthesized by dimerization of 4-(5- styrene and unsubstituted styrene, respectively, σ is the 15 nonyl) pyridine with 5% Pd/C. CuBr2 was used as received. Hammett constant of the substituent, and F is the Initiator 1-phenylethyl bromide (1-PEBr) was obtained from reaction constant. Aldrich and used without further purification. With the recent development of “living” radical po- Polymerization. The general procedure for the polymer- lymerization, the study of substituent effects in “living” ization was as follows: to a round bottom flask with CuBr and polymerization systems again attracts great interest. ligand were added the degassed solvent and monomer, followed Kazmaier et al.6 showed that, in the “living” free radical by the addition of the initiator. A certain volume of sample polymerization based on the TEMPO/BPO initiating was taken from the flask using a syringe and then dissolved system, electron-withdrawing groups increase the po- in THF as a point at zero conversion (t ) 0). The flask was lymerization rate for substituted styrenes, which is then immersed in an oil bath thermostated at 110 °C. At consistent with the conventional radical polymerization. timed intervals, the same amount of sample was withdrawn In the last 2 years, we developed a novel “living” from the flask and dissolved in THF for further analysis. radical polymerization method, atom transfer radical Characterization. Monomer conversion was determined by GC using a Shimadzu GC-14A with DB-WAX column. polymerization (ATRP), in which polymers with prede- 5 Molecular weight and molecular weight distribution were termined molecular weight up to Mn 10 and poly- ≈ measured using phenogel GPC columns (guard, linear, 1000 and 100 Å). Polystyrene standards were used to calibrate the X Abstract published in Advance ACS Abstracts, September 1, columns. 1H-NMR study was performed on a 300 MHz Bruker 1997. NMR spectrometer using CDCl3 as a solvent, and GC-MS S0024-9297(97)00422-1 CCC: $14.00 © 1997 American Chemical Society 5644 Qiu and Matyjaszewski Macromolecules, Vol. 30, No. 19, 1997 Figure 2. Molecular weights vs monomer conversion for Figure 1. Kinetic plots for ATRP of substituted styrenes in ATRP of substituted styrenes in diphenyl ether at 110 °C. [M]0 diphenyl ether at 110 °C. [M]0 ) 4.37 M and [M]0:[1-PEBr]0: ) 4.37 M and [M]0:[1-PEBr]0:[CuBr]0:[bipy]0 ) 100:1:1:3. [CuBr]0:[bipy]0 ) 100:1:1:3. Table 1. Various Substituted Styrenes and their σ deceleration of the polymerization rate. For monomers Values16 having smaller propagation constants, the effect of side electron electron reactions is more obvious than for monomers having withdrawing (EW) donating (ED) larger propagation constants, and the apparent rate monomer σ monomer σ coefficients for these polymerizations were computed from the initial slopes of the kinetic plots. Figure 2 4-CF3 sty 0.54 3-Me sty -0.07 shows the evolution of molecular weights with monomer 3-CF3 sty 0.43 4-Me sty -0.17 conversion. Molecular weights increase linearly with 4-Br sty 0.23 4-CMe3 sty -0.20 4-Cl sty 0.23 4-OMe sty -0.27 monomer conversion in all cases and match with the 4-F sty 0.06 theoretical values. This indicates fast initiation and a small contribution of transfer under these conditions. measurements were performed on HP 59970 MS ChemStation The molecular weight distributions are relatively nar- using THF as the solvent. row (Mw/Mn < 1.5) compared to those of conventional radical polymerizations, as shown in Figure 3. 4-OMe Results and Discussion styrene is the only exception, from which no high The substituted styrenes used in this study are listed polymer can be obtained under conventional ATRP in Table 1. reaction conditions. General Features of Polymerization. The kinetic Substituent Effects on Polymerization Rate. As plots of the polymerizations are shown in Figure 1. The can be seen from Figure 1, monomers with electron- apparent polymerization rate is first order with respect withdrawing substituents polymerize faster than those to monomer concentration in most cases, which means bearing electron-donating substituents in the order of the concentrations of growing radicals are constant. The 3-CF3, 4-CF3 > 4-Br, 4-Cl > 4-F, 4-H > 3-Me > 4-OMe app apparent rate coefficients (kp )-d(ln[M])/dt) can be > 4-Me > 4-CMe3. The apparent rate coefficients for obtained from the slopes of the straight kinetic plots. each monomer are listed in Table 2. For some monomers such as styrene, 4-F styrene, 4-Me The results are in agreement with those from con- 5 styrene, and 4-CMe3 styrene, deviation from linear ventional radical polymerization as well as previously kinetics occurs at long reaction times due to some side reported “living” radical polymerization initiated by reactions including elimination and termination.17 Both TEMPO/BPO.6 In order to get quantitative information, elimination and termination remove R-X from the a Hammett plot was made (Figure 4). A linear correla- reaction. In addition, termination also reduces [CuI] tion was obtained between the apparent rate coefficient II app and increases [Cu ]. The overall effect results in the kp and Hammett constants σ for different substitu- Macromolecules, Vol. 30, No. 19, 1997 Polymerization of Substituted Styrenes 5645 established with regard to the absolute propagation rate • constant kp , rather than the apparent rate coefficient app kp which additionally comprises the concentration of growing radicals. The rate law for ATRP can be derived from eq 1, in • which growing radicals, Pn , are reversibly generated from the dormant alkyl halides, Pn-X. The activation process requires catalysis by a CuI species, whereas X-CuII deactivates reversibly the growing radicals. An equilibrium is established between the dormant species and the growing radicals, and the equilibrium constant Keq can be expressed by eq 2. In eq 2, since the k act I P • X–CuII (1) Pn–X + Cu n + kdeact • kp + M Figure 3.