Photoinitiated Polymerization - ACS Publications - American Chemical

Chapter 38

Pulsed Laser Polymerization of Methyl Methacrylate UsingWilkinson'sCatalyst as a Photoinitiator M . Sakhawat Hussain , Shafique Ahmad Awan , M . A . K h a n , and Haleem Hamid 1

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Department of Chemistry, Center for Applied Physical Sciences, and Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia 3

Pulsed laser polymerization (PLP) of methyl methacrylate (MMA) with [(C H ) P]3RhCl (Wilkinson's catalyst), with and without AIBN was investigated at room temperature. PLP of a MMA sample without Wilkinson's catalyst produced 0.107% PMMA, while the sample having 1.3 millimolar Wilkinson's's catalyst yielded 0.565%. The Wilkinson's catalyst was found to act as a mild photoinitiator promoting the process. The GPC revealed that the values of the polydispersity index became narrower in the presence of Wilkinson's catalyst. The H NMR spectra indicated the atactic nature of the resulting PMMA. 6

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© 2003 American Chemical Society

In Photoinitiated Polymerization; Belfield, Kevin D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction Photoinitiated polymerization occurs when radicals are produced by ultraviolet and visible light irradiation of a reaction system (1). Photoinitiated polymerization offers significant practical advantages such as control of the initiation rates by a combination of the source of radicals, light intensity and temperature. Extensive use of these advantages has been made in the printing and coating industries (2,3). Photopolymerizations are of special interest in applications where economic and/or environmental considerations require the use of solvent-free systems. Due to the assiduous work of Olaj and co-workers (4-6) a technique has developed which utilizes a pulsed laser light source to evaluate the individual kinetic rate constants infree-radicalpolymerization. Controlled synthesis of low molecular weight polymers is assuming great importance as their applications in the detergents, coatings, and water treatment industries continue to develop. Poly(methyl methacrylate) (PMMA) and its copolymers have found a great range of applications because of their good thermo-mechanical and chemical properties in addition to their excellent weatherability, unique transparency and biocompatibility (7). Catalytic chain transfer polymerization was discovered in 1975 but reported in the literature in early 1980s (8). The use of metal-chelates in catalytic chain transfer polymerization is now well known and is considered as a very highly effective method for producing oligomers (9). The chelates of cobalt(ll) have been reported as catalytic chain transfer agents (CCTAs) for the controlled thermal polymerization of MMA, styrene, and other monomers (10, 11). Some transition metal complexes have also been used as catalysts for the polymerization of M M A and styrene. Kato (12) and coworkers achieved wellcontrolled polymerization of M M A using a ternary initiating system consisting ,of CC1 , RuCl (PPh )3, and MeAl(ODBP)2. In 1997, Sawamoto and coworkers, reported two new catalysts, FeCl (PPh ) (13) and NiBr (PPh ) (14) for the atom transfer radical polymerization (ATRP) of MMA. Percec and coworkers (15) reported the ATRP of styrene by using Wilkinson's catalyst but no rigorous treatment of the role of WilkinsonVs catalyst in the polymerization was given. Similarly, Jerome and coworkers (16) reported the polymerization of M M A initiated by 2,2'dichloroacetophenone in the presence of Wilkinson's catalyst plus 7 equivalent of PPh , in THF at 60 °C. They concluded that Wilkinson's catalyst indeed facilitates M M A polymerization initiated by 2,2'4

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D

hv

* * D — ( D * , A ) (I)

+

-2- *

**(D, A )

radicals

(Π)


( ) 1

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dichloroacetophenone at a temperature as low as 60 °C. In the PLP of MMA, the role of Wilkinson's catalyst can be summarized by the formation of exciplex (77) as shown in Equation 1 where D is the donor specie (Wilkinson's catalyst) and "A" is an acceptor, which is M M A (monomer) and/or AIBN (initiator). In (I) the excited donor and acceptor atoms come close to each other providing free radicals via intermediate (Π). In the present paper we report, the bulk polymerization of M M A using an excimer laser (XeCl @ 308 nm) in the presence of Wilkinson's catalyst with and without AIBN. The role of Wilkinson's catalyst as a CCTA or photoinitiator has been investigated. To the best of our knowledge, the PLP of M M A with Wilkinson's catalyst has not so far been reported.

Experimental

Materials The MMA (>99 %) obtained from Fluka was mixed with 10% NaOH to remove inhibitor. It was then washed with distilled water. And the residual water was removed by adding anhydrous sodium sulfate. The monomer was distilled under dry nitrogen at 58-60 torr and 33-35 °C (18). AIBN was recrystallized from CH C1 . Since MMA is harmful to respiratory system and skin and can also cause irritation to eyes, all these steps need to be performed under a fume hood Wilkinson's catalyst from Fluka and methanolfromBDH were used as receivedfromthe manufacturer. 2

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Gel Permeation Chromatography The PMMA samples were analyzed on WATERS GPC 150C plus. The 1,2,4-Trichlorobenzene was used as solvent with PL Gel 10 pm column. The flow rate was 1.0 ml/min and temperature was 150°C. Polystyrene standards were used for determining molecular weights and polydispersity of PMMA prepared in this study.

*ΗΝΜ». l

The U NMR spectra were recorded on Jeol JNM-LA 500 NMR spectrometer at 500 MHzfrequencyat 297° Κ in CDC1 . The spectra were 3


454 recorded with the following instrumental conditions: points = 32768, frequency = 10000.0 Hz, pulse delay = 5.361 sec, resolution = 0.31 Hz, scans = 16, Solvent = CDC1 , temperature = 298 °K. 3

U V LASER.


A Lambda Physik Model E M G 203 MSC with XeCl Excimer laser giving 308 nm input at 10 Hz was used. The pulse duration was about 20 ns.

UV/Vis Spectrophotometer. A Perkin Elmer Lambda 5 UV/Vis spectrophotometer was used to record the UV spectra, using quartz cells with the following conditions: slit = 2 nm, response = 0.2 sec, lamp = 332.8 nm, solvent = chloroform.

Pulsed Laser Polymerization Wilkinson's catalyst (2.0 mg) was dissolved by slight warming in 10.0 ml of freshly distilled MMA. Four Schlenk tubes containing 20.0 mg of AIBN were labeled as A, B, C, and D. To the tube A were added 1.0 ml of Wilkinson's catalyst solution and 5.0 ml of MMA; to tube B, 3.0 ml Wilkinson's catalyst solution and 3.0 ml MMA; to tube C, 5.0 ml of Wilkinson's catalyst solution and 1.0 ml of MMA; while to tube D (control sample) only 6.0 ml of pure M M A (without any Wilkinson's catalyst). The dissolved 0 was removed by repeated freeze-thaw technique and the tubes were sealed. Just prior to PLP, the seal was broken and the contents of the tube was transferred to a 4.0 ml capacity quartz cell under dry nitrogen atmosphere. Thus, each sample in the quartz cell contained 13.8 mg (2.03xl0 M) of AIBN, with different concentrations of Wilkinson's catalyst. A Lambda Physik Model E M G 203 MSC was used for excimer laser (XeCl) irradiation (45 minutes for each of the sample) at 308 nm, using the system shown in Figure 1 (19). Typical laser energy per pulse was 150 mJ with a repetition rate of 10 Hz. The laser beam spot size was 10 mm χ 30 mm which covered most of the volume of the sample cell. No focusing or beam expansion was needed in this system. Laser energy was continuously monitored using a Molectron J-50 probe. The discharge voltage on the laser tube was increased in a controlled manner to maintain a constant energy output. The resulting polymer was added to 40 ml of methanol with constant stirring. Subsequently, all methanol and residual MMA was evaporated. For removing any traces of residual MMA, a vacuum drying procedure was applied. 2

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Sample Holder Water Circulation

Thermocouple

Reaction Cell

λ = 308 nm

Laser Light

Figure 1. Schematic diagram showing PLP of MMA at 25 °C.


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Results and Discussion Initially, both AIBN and Wilkinson's catalyst were used on the assumption that the latter will act as a catalytic chain transfer agent and restrict further growth of polymer chains. The data to investigate the effects of the concentration of Wilkinson's catalyst on the molecular weights and polydispersity of PMMA using 2.03 χ 10* M AIBN as initiator in the pulsed laser polymerization of M M A are shown in Table I.

Table L Effect of the Concentration of Wilkinson's catalyst on the Molecular Weights and Polydispersity of P M M A using 2.03 χ 10 M AIBN as Initiator in Pulsed Laser Polymerization of M M A .


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RunU Reference A Β C

M„ Cone. (mM) Conversion (%) 14030 0 12.72 6330 0.0361 15.05 10490 0.1083 15.71 0.180S 17.26 13150

M 26570 9830 16350 24290 w

PDI 1.89 1.52 1.56 1.85

Table I indicates (hat the polydispersity of the PMMA samples is less than 1.89 which is most likely due to a termination process, that under the prevailing conditions is mainly by disproportionation. There is decrease of the PDI values from 1.89 in the reference sample to 1.52 in the presence of0.0361mM solution (Sample A) of Wilkinson's catalyst. The polydispersity index (PDI) increases for sample Β and sample C. that is apparently due to the increase in percent conversion of M M A resulting in an increase in the molecular weights. It looks as though the autoacceleration is due to a gel effect that might be operating to some extent. Actually, the polymer weight depends on the ratio [M]/[I] . Normally, the initiator concentration [I] decreases faster than [M] and the molecular weight of the polymer produced at any instant increases with conversion. As a result, the molecular weight distribution increases with percent conversion. But it is worthwhile to note that PDI for sample C in which 0.1805 mM Wilkinson's catalyst was used is still less (1.85 with a percent conversion of 17.26 %) as compared to control sample for which PDI is also 1.89 with percent conversion 12.72 %. Therefore, it can be concluded that though the Wilkinson's catalyst does not inhibit polymerization (rather promote it), it still narrows the MWD. The GPC chromatograms are shown in Figure 2. It is clear from these chromatograms that the shape of the chromatogram C is similar to that of the reference sample R. The difference between chromatogram A and reference 1/2


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samples indicates that there is some change in the termination process in the presence of Wilkinson's catalyst, indicating that Wilkinson's catalyst might be responsible for the increase in the percent conversion. Figure 3 shows that percent conversion of M M A increases as a function of the amount of Wilkinson's catalyst used. Since the amount of AIBN is constant in each sample, it is concluded that Wilkinson's catalyst is responsible for the increase in the percent conversion of PMMA.

dwt/d(logM)

logMW-

logMW"

logMW"

logMW"

dwt/d(logM)

Figure 2. Molecular weight distribution Curves for different samples determined by GPC. A, B C and R have the same meaning as in Table I. t

Figure 3: Dependence of the PMMA conversion (%) on the concentration (millimoles per liter) of Wilkinson's catalyst using AIBN as initiator.


458 In order to verify that Wilkinson's catalyst can act as mild photoinitiator, the effect of irradiation time on PLP of M M A at a constant concentration of Wilkinson's catalyst without adding AIBN, was investigated. The results are presented in Table II.


Table Π. Effect off the irradiation time on Pulsed Laser Polymerization of M M A at constant concentration (1.3mM) of Wilkinson's catalyst without AIBN. Sample # 1 2 3 Control

Time (min.) 25 35 45 35

Conversion (%) 0.122 0.565 0.710 0.107

A control experiment (having no Wilkinson's catalyst) was irradiated by the laser for 35 minutes, the percent conversion was found to be 0.107 %, while for sample #2 having Wilkinson's catalyst for the same duration of laser irradiation, the percent conversion was higher (0.565 %). The UV/Vis spectrum for Wilkinson's catalyst indicates strong absorption bands at 276,268, and 260 nm. There is also significant absorption around 308 nm for Wilkinson's catalyst, while for AIBN and MMA there is no significant absorption at this wavelength. All these observations suggest that Wilkinson's catalyst either dissociates to give free radicals, which in turn produce free radicals of MMA directly or enhances the total number offreeradicals of AIBN when it is present. In this case, photochemical activation has accelerated the ligand dissociation from Wilkinson's catalyst. Absorption of UV photons of the proper wavelength can excite an electron which is due to a strong δ-antibonding effect would favor the loss of the more strongly δ-donating ligand (20). In the present case, P(Ph) is a stronger sigma donating ligand (21) compared to CI", therefore, the Rh-P(Ph)3 bond is most probably broken. Another possibility is that Wilkinson's catalyst absorbs energy generates an excited state and from this excited state it loses it's extra energy to AIBN or directly to the MMA monomer, and thus helps to form free radicals as indicated in equation 3


459 1. Thus, Wilkinson's catalyst behaves like a photoinitiator or photosensitizer under excimer laser irradiation. !

The H NMR spectra of PMMA formed in our study are shown in Figure 4. The peaks around 3.60 ppm are due to OCH , whereas the peak around 3.76 3

OCH


OCT,

4.00

3.50 1.50 1.25

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0.75

Figure 4. 500MHZ *HNMR of PMMA in CDCl at room temperature. The A, B, C and R have the same meaning as in Table I. 3

ppm is either from unreacted residual monomer or due to the terminal ester group, which is adjacent to a chlorine atom (16). Unfortunately, the C H signals can not be analyzed unambiguously. Figure 4 suggests that in all cases in our experiments, atactic PMMA was formed i.e. polymer incorporates simultaneously both type of C-CH groups i.e. syndiotactic and isotactic. Both C-CH and 0-CH peaks for the samples C, B, and A are shifted to some extent with respect to C-CH and O C H peaks in the reference sample. For the analysis of tactiecity by H NMR spectroscopy, not only the nearest, but also the next nearest neighbors, should also be included i.e. we usually need to look at the triad, tetrad sequences as compared to diad sequence (22). The atactic PMMA formed in this work (Figure 4), is similar to this polymer prepared by other methods where C-CH groups produce three signals of very different intensities in the region 0.80-1.18 ppm. The signal at 0.80 2

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ppm indicates syndiotactic arrangement of C-CH groups in the atactic PMMA sample, while the signal around 0.98 ppm corresponds to an isotactic arrangement of C-CH groups in the same atactic PMMA sample. The signal at 1.18 ppm belongs to methyl groups at changeover positions between isotactic and syndiotactic C-CH groups in the atactic PMMA sample. Integration ratios for syndiotactic to isotactic C-CH groups are shown in Table ΙΠ. 3

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Table Ι Π . Intensity Ratios of Syndiotactic to Isotactic C - C H groups i n Ή N M R of atactic P M M A


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Sample # Intensity ratio Ref.

2.495

A Β C

2.522 2.474 2.416

The intensity ratio is around 2.5, which indicates that the number of syndiotactic type C-CH groups are approximately 2.5 times greater than the isotactic C-CH groups in the experimentally formed atactic PMMA in this study. 3

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Conclusion Our results indicate that Wilkinson's catalyst allows M M A to be polymerized at room temperature using excimer laser irradiation in the presence as well as absence of AIBN as initiator acting as a mild photoinitiator. The PDI values become narrower in the presence of Wilkinson's catalyst, but on the other hand there is increase of PDI values with percent conversion. The resulting PMMA is atactic in nature. The mechanism by which Wilkinson's catalyst takes part in the polymerization process needs further investigation.

Acknowledgement This research was supported by KFUPM through SABIC grant (Project # SABIC/1999-5). We are also grateful to the Research Institute KFUPM for GPC analysis.


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References 1. Oster, G.; Yang, N. L. Chem. Rev. 1968, 68, 125. 2. Pappas, S. P; " Photopolymerization," in Encyclopedia of Polymer Science and Engineering, Wiley Interscience, New York, 1988, 11, 186-212. 3. Pelgrims, J. J Oil Col. Chem. Assoc. 1978, 61, 114. 4. Olaj, O. F; Bitai, I.; Gleixner, G. Makromol. Chem. 1985, 186, 2569. 5. Olaj, O. F; Bitai, I.; Hinkelmann, F. Makromol. Chem. 1987, 188, 1689. 6. Olaj, O. F; Bitai, I. Angew. Makromol. Chem. 1987, 155, 117. 7. Janeczek, H.; Jedlinski, Z.; Bosek, I. Macromolecules, 1999, 32, 4503. 8. Gridnev, A. J. Polymer.Science, Part A, Polymer Chemistry, 2000, 38(10), 1753. 9. Suddaby, K. G.; Sanayei, R. Amin; Rudin, R.; O'Driscoll, K.F., J. Applied Polymer Acience, 1991, 43, 1565. 10. Burczyk, A . F.; O'Driscoll, K. F.; Rempel, G. L . Journal of Polymer Science. Polymer Chemistry Edition, 1984, 22, 3255-3262. 11. Sanayei, R. Α.; O'Driscoll, K. F. J. Macromol. Sci-Chem. A 26(8), 1989, 1137-1149. 12. Kato, M . ; Kamigaito, M . ; Sawamoto, M . ; Higashimura, T. Macromolecules, 1995, 28, 1721-1723. 13. Ando, T. ; Kamigaito, M.; Sawamoto, M . Macromolecule, 1997, 30, 16, 4507. 14. Uegaki, H.; Kotani, Y.; Kamigaito, M.; Sawamoto, M . Macromolecules, 1997, 30, 2249. 15. Percec, V.; Marboiu, B.; Neumann, Α.; Ronda, J. C; Zhao. M . Macromolecules, 1996, 29, 3665. 16. Moineau, G.; Granel, C.; Dubois, Ph.; Jerome, R.; Teyssie, Ph. Macromolecules, 1998, 31, 542-544. 17. Odian, G. Principles of Polymerization, Third Edition, A Wiley -Interscience Publication, 1991, 222-226. 18. Billmeyer, F. W.; Collins, Ε. Α.; Bares, J. Experiments in Polymer Science, John Wiley & Sons, 1973, 333-335. 19. Davis, T. P.; O'Driscoll, K. F.; Piton, M . C.; Winnik, Μ. Α. Macromolecules, 1989, 22, 2785-2788. 20. Lukehart, C. M . Fundamental Transition Metal Organometallic Chemistry, Brooks/ Cole Publishing Company, California, 1985, 67. 21. Miesseler G. L . ; Tarr, D. A . Inorganic Chemistry, Second Edition, Prentice- Hall, Inc. 1999, 408. 22. Friebolin, H. Basic One and Two - Dimensional NMR Spectroscopy, New York : VCH, 1991, 305-307


Photoinitiated Polymerization - ACS Publications - American Chemical

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