Rev. 2001, 101, 2921. (3) (a) Wayland, B. B.; Poszmik, G.; Mukerjee, S. L. J. Am. Chem. Soc. 1994, 116, 7943. (b) Wayland, B. B.; Basickes, L.; Mukerjee, S.; Wei,.
LIVING RADICAL POLYMERIZATION OF STYRENE CATALYZED BY SILVER (II) AND COPPER (II) TETRAPHENYL PORPHYRINS Alexandru D. Asandei,1,2 Isaac W. Moran 1 and Christian Brückner2 1
Institute of Materials Science, Polymer Program, 2 Department of Chemistry, University of Connecticut, Storrs, CT 06269.
Introduction Molecular weight (Mn) and polydispersity (Mw/Mn) control in living radical polymerization (LRP) is achieved via the persistent radical effect1a or degenerative processes.1b In metal catalyzed LRP, a paramagnetic transition metal complex either mediates the reversible termination of a growing polymeric radical with a halide as in the copper catalyzed atom transfer radical polymerization (ATRP),2 or reversibly terminates the growing chains via the formation of a metal-carbon bond as in the case of cobalt porphyrins3 or molybdenum derivatives.4 Various metal porphyrins were previously investigated as potential catalytic chain transfer (CCT)5 agents in free radical polymerizations, but complexes of Fe, Ni, V, Sn, Cu, Zn, Mg, Cr, Pd, Pt, and Mn demonstrated no activity.6a,b Retardation of the polymerization was observed for Zn ~ Ni < Pd < Cu ~ Mn ~ Cr porphyrins7a and Fe protoporphyrin IX dimethyl esters.7b A Rh(II) porphyrin was able to promote photopolymerization of acrylates with partial living character.8 Moreover, Al porphyrins are well established catalysts for living anionic/immortal polymerizations.9 While Cu is heavily used in ATRP, the different reactivity and poor solubility of Ag derivatives in conjunction with the instability of Ag(II) salts made their use in radical polymer chemistry very scarce and limited just to conventional redox radical initiations.10 Ag(II) and Cu(II) porphyrins have been described11 and the electronic properties of these paramagnetic d9 complexes are well understood.12 Recently there is increased interest in their use in photo/magnetoactive polymers.13 We are presenting herein investigations on the scope and limitations of these metal porphyrins as persistent radicals for the LRP of styrene. Experimental Materials. Silver tetraphenylporphyrin (Ag(II)TPP) and copper tetraphenylporphyrin (Cu(II)TPP) were synthesized as described in the literature.11 2,2’-Azobisisobutyronitrile (AIBN, Aldrich; 98%) was recrystallized from methanol. Diphenyl ether (Ph2O, Alfa, 99%) was used as received. Styrene (Aldrich, 99+%) was passed through a basic Al2O3 column. Techniques. 1H-NMR (500 MHz) spectra were recorded on a Bruker DRX500 at 24 °C in CDCl3 (Aldrich; 1% v/v tetramethylsilane (TMS) as internal standard). GPC analyses were performed at 34 °C on a Waters 150-C Plus gel permeation chromatograph equipped with a Waters 410 differential refractometer, a Waters 2487 dual wavelength absorbance UV-VIS detector set at 254 nm, a Polymer Laboratories PL-ELS 1000 evaporative light scattering (ELS) detector and with a Jordi Gel DVB 105 Å, a PL Gel 104 Å, a Jordi Gel DVB 100 Å, and a Waters Ultrastyragel 500 Å column setup. Tetrahydrofuran (Fisher; 99.9 % high performance liquid chromatography grade) was used as an eluent at a flow rate of 1 mL/min. Number-average (Mn) and weight-average molecular weights (Mw) were determined from calibration plots constructed with polystyrene standards. Polymerization. Monomer (styrene, 1 mL, 8.7 mmol), solvent (Ph2O, 1 mL), initiator (AIBN, 7.2 mg, 0.044 mmol), and catalyst (Ag(II)TPP, 62.9 mg, 0.087 mmol) were added to a 25-mL Schlenk tube. The tube was degassed by several freeze-pump-thaw cycles, filled with Ar, and the reaction mixture was heated at 90 °C in an oil bath. The side arm of the tube was flushed with a continuous stream of Ar before and during sampling. Samples were taken using an airtight syringe, were divided in two and used for conversion and molecular weight determination by NMR and respectively by GPC. Results and Discussion We have performed experiments with various reagent ratios and at different temperatures using AIBN as initiator and phenyl ether as solvent. Figure 1 presents the evolution of Mn and Mw/Mn with conversion while Figure 2 shows the corresponding first order kinetic plots. The results are summarized in Table 1.
TABLE 1. POLYMERIZATION OF STYRENE MEDIATED BY AG(II)TPP AND CU(II)TPP.
200/1/1
Temp. °C 60
kpexp h-1 0.0453
Init. Effic. -
Ag(II)TPP
200/1/1
90
0.2924
-
Ag(II)TPP
200/1/2
90
0.3528
-
4
Ag(II)TPP
200/1/1
130
0.7158
0.26
5
Ag(II)TPP
200/1/2
130
0.8442
0.35
6
Ag(II)TPP
200/1/4
130
0.8638
0.40
7
Cu(II)TPP
200/1/1
110
0.2967
0.27
8
Cu(II)TPP
200/1/2
130
0.4255
0.26
Exp
Catalyst
[M]/[I]/[C]
1
Ag(II)TPP
2 3
As observed in Figure 1, at 60 °C, with Ag(II)TPP, Mn is about 80,000 and is approximately constant with Mn,theor (i.e. conversion) while Mw/Mn is ~ 1.42. An increase in the temperature to 90 °C leads to a decrease in Mn to values in-between 20,000 to 30,000 and a more pronounced dependence of Mn on conversion (especially at a lower AIBN/Ag(II)TPP ratio) while Mw/Mn is still in-between 1.3 and 1.4. At 110 °C, a linear dependence of Mn on conversion is observed using Cu(II)TPP. However, polydispersities increase with conversion from 1.4 to 2.2. At 130 °C all polymerizations show a linear increase of Mn on conversion indicative of the fact that unlike the case for lower temperatures, the metal porphyrin is able to reversibly endcap the growing chain from the early stages of conversion. Nontheless, the Mw/Mn values increase from 2 to 2.5. The initiator efficiency of AIBN increases from 0.26 to 0.40 and increases with [Ag(II)TPP].
As seen in Figure 2, the polymerizations follow first order kinetics and the concentration of growing chains remains constant in time. Under the same conditions, the Cu(II)TPP polymerization is slower than the corresponding AgTPP experiment and provides somewhat lower polydispersity. Variations in [Ag(II)TPP] do not affect the rate of polymerization as significantly as decreasing the temperature which has a much stronger effect.
While detailed mechanistic investigations are in progress, we believe that the polymerization occurs as outlined in Scheme 1. The low initiator efficiency may be explained by side reactions between metal porphyrin and AIBN (eqs. 2 and 3) similar to the known adverse effects of peroxides in Co mediated processes.5a Depending on temperature and the [M]/[I]/[C] ratio, the polymerization occurs most likely via a combination of Ag(II)TPP or Cu(II)TPP-catalyzed CCT (eqs. 5 and 6) and LRP (eq. 7). At higher temperatures (110 °C - 130 °C), a living radical process is observed and it is controlled by the reversible formation of C-Ag or C-Cu bonds. At low temperatures (60 °C), CCT is probably stronger, while at 90 °C, both processes are of comparable importance with CCT being favored by an increase in [Ag(II)TPP]. A related behavior was observed in the polymerization of styrene at 60 °C with Co derivatives.5c While CCT is typically observed for methacrylates5 and LRP for acrylates,3 styrene may provide an unique example where the mechanism may be selected simply by a change in temperature. Polymerization control via reversible termination of the growing chain with the porphyrins is supported by the fact that the Ag(III) oxidation state is accessible, and that by contrast with Ag(I) halides, Ag(II) porphyrins are soluble and readily oxidizeable to the metal centered Ag(III)TPP metalloradical and not to porphyrin-cation radicals.11,12 However, porphyrin incorporation in the chain may also occur via copolymerization.5
Scheme 1. Mechanism of the Ag(II)TPP-catalyzed polymerization of styrene.
I-I
∆
2I
(1)
I-I + Ag(II)TPP
I + I-Ag(III)TPP
(2)
I + Ag(II)TPP
I-Ag(III)TPP
(3)
I + nM ~Pn + Ag(II)TPP
~Pn
(4)
~Pn= + H-Ag(III)TPP (5)
H-Ag(III)TPP + mM
~Pm
(6)
~Pn + Ag(II)TPP
~Pn-Ag(III)TPP
(7)
kp, M
Conclusions Ag(II)TPP and Cu(II)TPP were tested as persistent metalloradicals in the polymerization of styrene initiated with AIBN. At 60 °C, polydispersities are in the range of 1.3 to 1.4, but Mn is independent of conversion. In contrast, at 110 °C to 130 °C, molecular weight increases linearly with conversion indicating a living radical polymerization, but polydispersities also increase from 2.0 to 2.5. An intermediate situation is seen at 90 °C. The polymerization occurs most likely by a temperature controlled combination of CCT and LRP. Acknowledgments. Financial support from the University of Connecticut Research Foundation is gratefully acknowledged. References. (1) Fischer, H. J. Polym. Sci.; Part A: Polym. Chem. 1999, 37, 1885. (b) Fukuda, T.; Goto, A.; Ohno, K. Macromol. Rapid Commun. 2000, 21, 151. (2) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921. (3) (a) Wayland, B. B.; Poszmik, G.; Mukerjee, S. L. J. Am. Chem. Soc. 1994, 116, 7943. (b) Wayland, B. B.; Basickes, L.; Mukerjee, S.; Wei, M. Macromolecules 1997, 30, 8109. (4) Le Grognec, E.; Claverie, J.; Poli, R. J. Am. Chem. Soc. 2001, 123, 9513. (5) (a) Gridnev, A. A.; Ittel, S. D. Chem. Rev. 2001, 101, 3611. (b) Gridnev, A. J. Polym. Sci.: Part A: Polym. Chem. 2000, 38, 1753. (c) Heuts, J. P. A.; Forster, D. J.; Davis, T. P.; Yamada, B.; Yamazoe, H.; Azukizawa, M. Macromolecules 1999, 32, 2511. (6) Enikolopyan, N. S.; Smirnov, B. R.; Ponomarev, G. V.; Bel’govskii, I. M. J. Polym. Sci., Chem. Ed. 1981, 19, 879. (b) Karmilova, L. V.; Ponomarev, G. V.; Smirnov, B. R.; Bel’govskii, I. M. Russ. Chem. Rev. 1984, 53, 132. (7) (a) Golikov, I. V.; Mironicev, V. E.; Golubchikov, O. A.; Smirnov, B. R. Izv. Vyssh. Uchebn. Zaved., Khi. Khim. Tekhnol. 1983, 26, 1118 (Russian); Chem. Abstr. 1984, 100, 68782v. (b) Finkenaur, A. L.; Dickinson, C. L.; Chien J. C. W. Macromolecules 1983, 16, 728. (8) Wayland, B. B.; Poszmik, G.; Fryd, M. Organometallics 1992, 11, 3534. (9) (a) Aida, T.; Inoue, S. Acc. Chem. Res. 1996, 29, 39. (b) Inoue, S. J. Polym. Sci.: Part A: Poly. Chem. 2000, 38, 2861. (10) Basha, P. G.; Ariff, M.; Jainudeen, M. D.; Gopalan, V.; Venkatarao, K. J. Macromol. Sci., Chem. 1986, A23, 473. (11) (a) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour, J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476. (b) Eaton, S. S.; Eaton, G. R. J. Am. Chem. Soc. 1975, 97, 3660. (c) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem. 1970, 32, 2443. (d) Scheidt, W. R.; Mondal, J. U.; Eigenbrot, C. W.; Adler, A.; Radonovich, L. J.; Hoard, J. L. Inorg. Chem. 1986, 25, 795. (12) (a) Kadish, K. M.; Lin, X. Q.; Ding, J. Q.; Wu, Y. T.; Araullo, C. Inorg. Chem. 1986, 25, 3236. (b) Godziela. G. M.; Goff, H. M. J. Am. Chem. Soc. 1986, 108, 2237. (13) (a) Kajiwara, A; Takamizawa, S.; Yamaguchi, T.; Mori, W.; Yamaguchi, K.; Kamachi, M. Molec. Cryst. Liq. Cryst. 1997, 306, 25. (b) Aramata, K.; Kajiwara, A.; Kamachi, M.; Umemura, Y.; Yamagishi, A. Macromolecules 1998, 31, 3397. (c) Aramata, K.; Kamachi, M.; Takahashi, M.; Yamagishi, A. Langmuir 1997, 13, 5161.
