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Vibrational overtone enhancement of methyl methacrylate polymerization initiated by benzoyl peroxide decomposition
21 March 1997
:
CHEMICAL PHYSICS LETTERS
i ~- /
ELSEVIER
Chemical Physics Letters 267 (1997) 313-317
Vibrational overtone enhancement of methyl methacrylate polymerization initiated by benzoyl peroxide decomposition Oleg Grinevich, D.L. Snavely Department of Chemistry, Bowling Green State University, Bowling Green, OH 43403, USA Received 8 August 1996; in final form 1 December 1996
Abstract
Vibrational overtone initiated polymerization has been demonstrated using intracavity photolysis of a benzoyl peroxide/methyl methacrylate mixture. Excitation of the 6VCH overtone transition of the ground electronic state of benzoyl peroxide creates radicals which subsequently begin the polymerization process. Polymer yield was monitored by comparison of the 2 VcH overtone absorptions for the methyl, methylenic and olefinic CH stretches at 5946 and 6170 cm-~, respectively. Plots of polymer yield versus time demonstrate an autoacceleration of the polymerization rate commencing many hours after the photolysis period. The delay before autoacceleration depends on the duration of the photolysis.
I. Introduction
In vibrational photochemistry laser light initiates chemical reactions by creating vibrationally excited states in the ground electronic state [1]. Although these overtone absorptions are weak, many different gaseous chemical reactions, including ring openings, isomerizations, sigmatropic shifts and dissociations, have been initiated using this method. These reactions involve excitation of the fourth, fifth or sixth CH stretch vibrational overtone state. While the initiation of gaseous reactions by vibrational photochemistry is facile, overtone-initiated reactions in the liquid phase are impeded by fast collisional deactivation. Overtone excitation of the unimolecular dissociation of t-butylhydroperoxide formed t-butoxyl (tB u O ) and hydroxyl ( H O ' ) radicals in the gaseous phase [2]. The 5Vor~ transition (absorption cross section of about 7.5 × 10 - 2 5 c m 2) used for this reaction occurs at 619.0 nm. Radicals have also been
formed by overtone excitation of NH 3 and HOOH [3 -5]. The vibrational overtone spectra of hydrocarbons are characterized by simple progressions corresponding to light atom-heavy atom stretches (called local modes) with peak widths around 100 to 300 c m (about 10 nm wide at 600 nm) [6,7]. Progressions for CH, NH, and OH stretches have been identified. In addition, olefinic, methylenic and methyl CH stretches can often be distinguished in overtone spectra [8-10]. This Letter outlines a method of overtone excitation used to form radicals which subsequently polymerize a monomer in the liquid phase. Vibrational overtone activation in liquids is hindered by fast collisional deactivation; however, in radical polymerization the product is amplified by radical propagation boosting the final yield in the overtone induced reaction. The radical precursor, benzoyl peroxide, possesses an overtone spectrum similar to that of benzene with its aryl local modes. The vibrational
0009-2614/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. •all S 0 0 0 9 - 2 6 1 4 ( 9 7 ) 0 0 1 0 5 - X
314
O. Grinevich, D.L. Snavely / Chemical Physics Letters 267 (1997) 313-317
overtone features of benzoyl peroxide are distinguished from those of the monomer, methyl methacrylate, which possesses predominately olefinic and methyl type local modes. Furthermore, the ratio of the intensities of the methyl, methylenic and olefinic CH overtone absorptions in the polymerizing monomer provides a suitable method to monitor the progress of polymerization in these experiments.
o x2ol
°
1 x
,s
o
200 xSO
2. Experimental Mixtures of peroxide and monomer were prepared and deaerated by bubbling dry N 2 through the monomer-initiator mixture for 30 min periods. The sample was then placed inside a 0.1 cm pathlength quartz cuvette and aligned within the cavity of a continuous wave dye laser (operating with Rhodamin 6G dye) pumped by an Ar ÷ laser. The photolysis laser was tuned to the desired wavelength by an intracavity birefringent filter. The dye laser wavelength was determined by a 0.35 meter monochromator. In this way the mixture was exposed to the full circulating power of the dye laser. The dye laser output typically ranged from 0.1 to 0.5 W measured extracavity with a calibrated Newport 815 digital power meter. The intracavity circulating power was more than 50 times higher. Vibrational overtone spectra of benzoyl peroxide and methyl methacrylate were recorded on a Mattson near-IR spectrometer with a tungsten lamp, a quartz beamsplitter and either a PbSe (3000 to 9000 c m - ~ ) or Si (9000 to 15000 cm -j ) detector. For benzoyl peroxide the spectral region from 2700 to 4000 c m - 1 was recorded in a KBr pellet while the region from 4000 up through 14000 cm-~ was recorded in a C C I 4 solution in a quartz cuvette. Neat methyl methacrylate was used to record the overtone spectrum in the 4000-14000 cm-1 region. Intracavity photoacoustic spectroscopy recorded the gaseous spectrum of methyl methacrylate from 15500 to 17500 c m - i.
3. Results The vibrational overtone spectrum of benzoyl peroxide, shown in Fig. 1, displays overtone transitions
14000
J
i
12000
10000
80'00
60'00
Wavenumbers
Fig. 1. Vibrationalovertonespectrumof benzoylperoxide. A KBr pellet was used to record the spectrumfrom2000 to 4000 cm- ~at 4 cm-z resolution. The region from 4000 to 14500 cm-~ was recorded using a saturated CCI4 of benzoyl peroxide. The intensity scales are presented in the figure. through the fourth overtone of the CH stretch. The spectrum is similar to that of benzene [11] since the dominant features arise from the CH stretches on the benzyl ring. The activation barrier to the formation of radicals from benzoyl peroxide is 30 kcal/mol [12], indicating that the fourth vibrational overtone, at 14100 cm -~, is the lowest overtone required to initiate polymerization. The wavelength for the 5 ~'CH falls in the falloff of the laser gain curve for the DCM dye and TiSa laser. For this reason, the 6 Vc. transition, which lies near the peak of the rhodamin 6G gain curve, was selected for the photolysis wavelength. Another advantage to the 6VcH transition is that a more energetic reactant was created increasing the rate of reaction. The transition wavenumbers for the first (5991 cm-~), second (8808 c m - l ) , third (11518 cm -~) and fourth (14100 cm - l ) overtones were used in a Birge-Sponer plot to extrapolate to the fifth overtone (6t, CH at 16556 c m - J) required for the photolysis. Although there was some uncertainty in using this extrapolation technique to determine the photolysis wavelength, the width of the overtone peaks at these energies guaranteed that our selected photolysis wavelength would excite the 6~,cH stretching motion. The mechanical frequency and anharmonicity for the Birge-Sponer plot were 3055 and 59 cm -~, respectively. The vibrational overtone spectrum of methyl methacrylate is displayed in Fig. 2 and the transition
O. Grineuich, D.L. Snaoely / Chemical Physics Letters 267 (1997) 313-317
¢6
it 16()00
14000
12(~00 10000 Wavenurnbers
BOO0
000
Fig. 2. Vibrational overtone spectrum of methyl methacrylate. Neat methyl methacrylate was used to record the spectrum from 4000 to 14000 c m - 1 at 4 c m - 1 resolution. The intensity scale is in absorbance units with the spectrum from 10500 cm-~ expanded by a factor of 100. The region from 15500 to 17500 c m - i was recorded by intracavity photoacoustic spectroscopy of gaseous methyl methacrylate. The intensity label for this portion of the spectrum is the arbitrary units of the photoacoustic signal.
wavenumbers are tabulated in Table 1. The overtone spectrum is more complicated displaying contributions from both the olefinic and methyl CH stretches. The methyl CH stretch overtone transition wavenumbers fit a Birge-Sponer plot with a mechanical frequency and anharmonicity of 2954 and 56 cm - j , respectively. In this plot, the average of the four methyl CH stretch wavenumbers, listed in Table 1, was used for the first overtone. The necessity of taking this average indicates some mixing of the methyl and, possibly olefinic CH stretches, at the first overtone. The transition wavenumbers for the second and higher overtones fit to a Birge-Sponer straight line. The second through fifth overtone transitions for the olefinic CH stretches fit a BirgeSponer plot with a mechanical frequency and anharmonicitiy of 3023 and 51 cm -~, respectively. Although the benzoyl peroxide 6VCH absorption does overlap with the methyl methacrylate overtone absorption, a control experiment with only methyl methacrylate present did not polymerize. The effect of the overlapping absorptions in methyl methacrylate and benzoyl peroxide serves to spoil the intracavity laser power at the photolysis wavelength but does not initiate the polymerization reaction.
315
The first successful polymerization detected visually occurred with a mixture of 10% benzoyl peroxide in methyl methacrylate photolyzed at 604 nm for about 14 h (overnight) with extracavity laser powers of about 0.2 W. Photolysis at the same power and duration of methyl methacrylate alone does not polymerize [13]. In subsequent experiments the photolysis duration was limited to 2 to 4 h. This was done to limit the polymer yield so that growth studies could be undertaken. For a quantitative measure of polymerization yields, the photolyzed mixture was diluted in CDC13 and placed in an NMR tube. The percent polymerization was monitored by carbon NMR by ratioing the area of the carbonyl carbon peak in the polymer (shifted downfield by ~ 10 ppm in the polymer) to that in the monomer. After a four hour photolysis at 0.1 W laser output power, the polymerization had proceeded in the dark to about 25% by the following day according to this NMR technique. An alternative technique was developed to measure polymer yield which did not require diluting the sample in CDCI 3. In this technique the relative areas of two vibrational overtone peaks at 6170 c m - ~ and 5946 cm-~ were compared. Fig. 3 demonstrates that as time increases after a two hour intracavity photolysis the spectrum of the sample mixture continues to change over the next 24 h period. The decrease in the peak at 6170 c m - l corresponds to the decrease in the monomer and the increase in the peak at 5946 c m - t indicates the increase in the polymer component of the mixture. The integrations of the peak areas were performed in an identical fashion for each spectrum. First the baseline was corrected using the manual baseline correction available in the Mattson First operating software. Then the peaks were integrated to this corrected baseline over the 6250 to Table 1 Transition wavenumbers ( c m - t ) and assignments for methyl methacrylate
methyl
olefmic
2Vcn
3VCH
4VCH
5VCn
6PCH
5668 5790 5890 5945 6170
8502
11107
13592
16142
8768 9019
11466
14151
16599
316
O. Grinevich, D.L. Snavely/ Chemical Physics Letters 267 (1997) 313-317
>.
6200
6000
5800
56(~0
Wavenumbers
Fig. 3. Spectrum of the photolysed (2 h, 0.06 W) mixture (a) before photolysis, (b) after 4.75 h, and (c) after 23.5 h. The peaks at 6170 and 5946 cm-~ were integrated to follow the progress of the polymerization reaction.
6125 cm- l and 6125 to 5500 cm- 1 intervals. These spectroscopic changes provide a convenient analysis of the polymer yield which can be undertaken directly in the original photolysis cell. The 0.1 cm quartz cell is simply removed from the laser cavity and aligned inside a near-IR spectrometer. The wavelength dependence of the polymerization was confirmed by photolyzing the benzoyl peroxide/methyl methacrylate mixture at 598 (above the peak) and 610 (below the peak) nm [12]. After six hour intracavity photolyses the mixtures photolysed at the off peak wavelengths exhibited at least three times less polymer formation when checked the following day by the near-IR technique described above. These off peak photolysis samples never exceeded 10% polymerization even after a 24 h period. Furthermore control experiments on mixtures which were not photolyzed were checked after a 24 h period. The polymerization was less than 3% (the limit of the near-IR detection) for these samples. The average power of the laser in these experiments was difficult to control. In early experiments the laser was seen to flicker after many hours of photolysis due to the presence of polymer in solution. For quantitative analysis, the photolysis time was limited to 2 h, eliminating the possibility of laser fluctuation. Even with this correction, the intracavity laser power does fluctuate due to the presence of the liquid sample. Typical extracavity laser powers range from 0.05 to 0.20 W.
The time evolution of the polymerization after a 2 h photolysis at an extracavity power of 0.06 W is shown in Fig. 4. This polymerization curve is typical of the gel effect [14] which occurs when polymerization has proceeded to the point where diffusion limits the radical chain termination steps. When the radical chain termination is no longer facile the polymerization rate is seen to accelerate. This autoacceleration occurs when the small concentration of radicals formed by overtone activation has grown into larger trapped radicals which cannot diffuse toward one another. Autoacceleration has been observed in neat methyl methacrylate [14]. In these experiments, the length of time before the autoacceleration occurs depends on the intracavity laser power. The higher the power the earlier the autoacceleration occurs. This is a general trend of the data, however, due to the difficulty in controlling the average intracavity power during the polymerization the behavior was difficult to quantify. When the photolysis period is limited to only 15 min the same overall behavior is observed but the period before autoacceleration lengthens by at least l0 h (18 h period for 2 h, 0.06 W photolysis and 24 h period for 15 min, 0.15 W photolysis). Samples photolyzed off the vibrational overtone absorption peak never reached this autoacceleration step due to low overall conversion.
100
D 80 ¸
El El
60'
40-
13
# D
20'
D
m
El
13 i
i
10
20 lime,
30
hrs.
Fig. 4. Percent conversion versus time (h) for vibrational overtone initiated polymerization of methyl methacrylate.
O. Grinevich, D.L. Snavely / Chemical Physics Letters 267 (1997) 313-317
4. Conclusion
Excitation into the fifth vibrational overtone of the CH stretch of benzoyl peroxide initiates polymerization in a liquid mixture of benzoyl peroxide and methyl methacrylate. The photolysis was carried out inside a quartz cuvette aligned within a dye laser cavity. The polymerization is specific to the absorption wavenumber of the vibrational overtone transition. The percent polymerization was monitored by ratioing the areas of the olefinic to methyl and methylenic peak absorptions for the first vibrational CH stretch overtones. The polymerization autoaccelerates after a period of several hours.
References [1] F.F. Crim, Ann. Rev. Phys. Chem. 35 (1984) 657. [2] D.W. Chandler, W.E. Farneth and R.N. Zare, J. Chem. Phys. 77 (1982) 4447.
317
[3] B.R. Foy, M.P. Casassa, J.C. Stephenson and D.S. King, J. Chem. Phys. 92 (1990) 2782. [4] L.J. Butler, T.M. Ticich, M.D. Likar and F.F. Crim, J. Chem. Phys. 85 (1986) 2231. [5] X. Luo and T.R. Rizzo, J. Chem. Phys. 96 (1992) 5129. [6] B.R. Henry, Vibr. Spectra Struct. 10 (1981) 269. [7] M.S. Child, Acc. Chem. Res. 18 (1985) 45. [8] B. Henry, Acc. Chem. Res. 29(12) (1987) 430. [9] J.S. Wong, R.A. MacPhail, C.B. Moore and H.L. Strauss, J. Phys. Chem. 86 (1982) 1478. [10] C.B, Moore and J.S. Wong, J. Chem. Phys. 77(2) (1982) 603. Ill] R.G. Bray and M.J. Berry, J. Chem. Phys. 71(12) (1979) 4909. [12] W.A. Pryor, Introduction to free radical chemistry, (Prentice-Hall, Englewood Cliffs, NJ, 1966). [13] D.L. Snavely and O. Grinevich, Imag. Sci. Tech. 49th Ann. Conf. Proc. (1996). [14] G. Odian, Principles of polymerization, 3rd Ed. (Wiley, New York, 1991).
:
CHEMICAL PHYSICS LETTERS
i ~- /
ELSEVIER
Chemical Physics Letters 267 (1997) 313-317
Vibrational overtone enhancement of methyl methacrylate polymerization initiated by benzoyl peroxide decomposition Oleg Grinevich, D.L. Snavely Department of Chemistry, Bowling Green State University, Bowling Green, OH 43403, USA Received 8 August 1996; in final form 1 December 1996
Abstract
Vibrational overtone initiated polymerization has been demonstrated using intracavity photolysis of a benzoyl peroxide/methyl methacrylate mixture. Excitation of the 6VCH overtone transition of the ground electronic state of benzoyl peroxide creates radicals which subsequently begin the polymerization process. Polymer yield was monitored by comparison of the 2 VcH overtone absorptions for the methyl, methylenic and olefinic CH stretches at 5946 and 6170 cm-~, respectively. Plots of polymer yield versus time demonstrate an autoacceleration of the polymerization rate commencing many hours after the photolysis period. The delay before autoacceleration depends on the duration of the photolysis.
I. Introduction
In vibrational photochemistry laser light initiates chemical reactions by creating vibrationally excited states in the ground electronic state [1]. Although these overtone absorptions are weak, many different gaseous chemical reactions, including ring openings, isomerizations, sigmatropic shifts and dissociations, have been initiated using this method. These reactions involve excitation of the fourth, fifth or sixth CH stretch vibrational overtone state. While the initiation of gaseous reactions by vibrational photochemistry is facile, overtone-initiated reactions in the liquid phase are impeded by fast collisional deactivation. Overtone excitation of the unimolecular dissociation of t-butylhydroperoxide formed t-butoxyl (tB u O ) and hydroxyl ( H O ' ) radicals in the gaseous phase [2]. The 5Vor~ transition (absorption cross section of about 7.5 × 10 - 2 5 c m 2) used for this reaction occurs at 619.0 nm. Radicals have also been
formed by overtone excitation of NH 3 and HOOH [3 -5]. The vibrational overtone spectra of hydrocarbons are characterized by simple progressions corresponding to light atom-heavy atom stretches (called local modes) with peak widths around 100 to 300 c m (about 10 nm wide at 600 nm) [6,7]. Progressions for CH, NH, and OH stretches have been identified. In addition, olefinic, methylenic and methyl CH stretches can often be distinguished in overtone spectra [8-10]. This Letter outlines a method of overtone excitation used to form radicals which subsequently polymerize a monomer in the liquid phase. Vibrational overtone activation in liquids is hindered by fast collisional deactivation; however, in radical polymerization the product is amplified by radical propagation boosting the final yield in the overtone induced reaction. The radical precursor, benzoyl peroxide, possesses an overtone spectrum similar to that of benzene with its aryl local modes. The vibrational
0009-2614/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. •all S 0 0 0 9 - 2 6 1 4 ( 9 7 ) 0 0 1 0 5 - X
314
O. Grinevich, D.L. Snavely / Chemical Physics Letters 267 (1997) 313-317
overtone features of benzoyl peroxide are distinguished from those of the monomer, methyl methacrylate, which possesses predominately olefinic and methyl type local modes. Furthermore, the ratio of the intensities of the methyl, methylenic and olefinic CH overtone absorptions in the polymerizing monomer provides a suitable method to monitor the progress of polymerization in these experiments.
o x2ol
°
1 x
,s
o
200 xSO
2. Experimental Mixtures of peroxide and monomer were prepared and deaerated by bubbling dry N 2 through the monomer-initiator mixture for 30 min periods. The sample was then placed inside a 0.1 cm pathlength quartz cuvette and aligned within the cavity of a continuous wave dye laser (operating with Rhodamin 6G dye) pumped by an Ar ÷ laser. The photolysis laser was tuned to the desired wavelength by an intracavity birefringent filter. The dye laser wavelength was determined by a 0.35 meter monochromator. In this way the mixture was exposed to the full circulating power of the dye laser. The dye laser output typically ranged from 0.1 to 0.5 W measured extracavity with a calibrated Newport 815 digital power meter. The intracavity circulating power was more than 50 times higher. Vibrational overtone spectra of benzoyl peroxide and methyl methacrylate were recorded on a Mattson near-IR spectrometer with a tungsten lamp, a quartz beamsplitter and either a PbSe (3000 to 9000 c m - ~ ) or Si (9000 to 15000 cm -j ) detector. For benzoyl peroxide the spectral region from 2700 to 4000 c m - 1 was recorded in a KBr pellet while the region from 4000 up through 14000 cm-~ was recorded in a C C I 4 solution in a quartz cuvette. Neat methyl methacrylate was used to record the overtone spectrum in the 4000-14000 cm-1 region. Intracavity photoacoustic spectroscopy recorded the gaseous spectrum of methyl methacrylate from 15500 to 17500 c m - i.
3. Results The vibrational overtone spectrum of benzoyl peroxide, shown in Fig. 1, displays overtone transitions
14000
J
i
12000
10000
80'00
60'00
Wavenumbers
Fig. 1. Vibrationalovertonespectrumof benzoylperoxide. A KBr pellet was used to record the spectrumfrom2000 to 4000 cm- ~at 4 cm-z resolution. The region from 4000 to 14500 cm-~ was recorded using a saturated CCI4 of benzoyl peroxide. The intensity scales are presented in the figure. through the fourth overtone of the CH stretch. The spectrum is similar to that of benzene [11] since the dominant features arise from the CH stretches on the benzyl ring. The activation barrier to the formation of radicals from benzoyl peroxide is 30 kcal/mol [12], indicating that the fourth vibrational overtone, at 14100 cm -~, is the lowest overtone required to initiate polymerization. The wavelength for the 5 ~'CH falls in the falloff of the laser gain curve for the DCM dye and TiSa laser. For this reason, the 6 Vc. transition, which lies near the peak of the rhodamin 6G gain curve, was selected for the photolysis wavelength. Another advantage to the 6VcH transition is that a more energetic reactant was created increasing the rate of reaction. The transition wavenumbers for the first (5991 cm-~), second (8808 c m - l ) , third (11518 cm -~) and fourth (14100 cm - l ) overtones were used in a Birge-Sponer plot to extrapolate to the fifth overtone (6t, CH at 16556 c m - J) required for the photolysis. Although there was some uncertainty in using this extrapolation technique to determine the photolysis wavelength, the width of the overtone peaks at these energies guaranteed that our selected photolysis wavelength would excite the 6~,cH stretching motion. The mechanical frequency and anharmonicity for the Birge-Sponer plot were 3055 and 59 cm -~, respectively. The vibrational overtone spectrum of methyl methacrylate is displayed in Fig. 2 and the transition
O. Grineuich, D.L. Snaoely / Chemical Physics Letters 267 (1997) 313-317
¢6
it 16()00
14000
12(~00 10000 Wavenurnbers
BOO0
000
Fig. 2. Vibrational overtone spectrum of methyl methacrylate. Neat methyl methacrylate was used to record the spectrum from 4000 to 14000 c m - 1 at 4 c m - 1 resolution. The intensity scale is in absorbance units with the spectrum from 10500 cm-~ expanded by a factor of 100. The region from 15500 to 17500 c m - i was recorded by intracavity photoacoustic spectroscopy of gaseous methyl methacrylate. The intensity label for this portion of the spectrum is the arbitrary units of the photoacoustic signal.
wavenumbers are tabulated in Table 1. The overtone spectrum is more complicated displaying contributions from both the olefinic and methyl CH stretches. The methyl CH stretch overtone transition wavenumbers fit a Birge-Sponer plot with a mechanical frequency and anharmonicity of 2954 and 56 cm - j , respectively. In this plot, the average of the four methyl CH stretch wavenumbers, listed in Table 1, was used for the first overtone. The necessity of taking this average indicates some mixing of the methyl and, possibly olefinic CH stretches, at the first overtone. The transition wavenumbers for the second and higher overtones fit to a Birge-Sponer straight line. The second through fifth overtone transitions for the olefinic CH stretches fit a BirgeSponer plot with a mechanical frequency and anharmonicitiy of 3023 and 51 cm -~, respectively. Although the benzoyl peroxide 6VCH absorption does overlap with the methyl methacrylate overtone absorption, a control experiment with only methyl methacrylate present did not polymerize. The effect of the overlapping absorptions in methyl methacrylate and benzoyl peroxide serves to spoil the intracavity laser power at the photolysis wavelength but does not initiate the polymerization reaction.
315
The first successful polymerization detected visually occurred with a mixture of 10% benzoyl peroxide in methyl methacrylate photolyzed at 604 nm for about 14 h (overnight) with extracavity laser powers of about 0.2 W. Photolysis at the same power and duration of methyl methacrylate alone does not polymerize [13]. In subsequent experiments the photolysis duration was limited to 2 to 4 h. This was done to limit the polymer yield so that growth studies could be undertaken. For a quantitative measure of polymerization yields, the photolyzed mixture was diluted in CDC13 and placed in an NMR tube. The percent polymerization was monitored by carbon NMR by ratioing the area of the carbonyl carbon peak in the polymer (shifted downfield by ~ 10 ppm in the polymer) to that in the monomer. After a four hour photolysis at 0.1 W laser output power, the polymerization had proceeded in the dark to about 25% by the following day according to this NMR technique. An alternative technique was developed to measure polymer yield which did not require diluting the sample in CDCI 3. In this technique the relative areas of two vibrational overtone peaks at 6170 c m - ~ and 5946 cm-~ were compared. Fig. 3 demonstrates that as time increases after a two hour intracavity photolysis the spectrum of the sample mixture continues to change over the next 24 h period. The decrease in the peak at 6170 c m - l corresponds to the decrease in the monomer and the increase in the peak at 5946 c m - t indicates the increase in the polymer component of the mixture. The integrations of the peak areas were performed in an identical fashion for each spectrum. First the baseline was corrected using the manual baseline correction available in the Mattson First operating software. Then the peaks were integrated to this corrected baseline over the 6250 to Table 1 Transition wavenumbers ( c m - t ) and assignments for methyl methacrylate
methyl
olefmic
2Vcn
3VCH
4VCH
5VCn
6PCH
5668 5790 5890 5945 6170
8502
11107
13592
16142
8768 9019
11466
14151
16599
316
O. Grinevich, D.L. Snavely/ Chemical Physics Letters 267 (1997) 313-317
>.
6200
6000
5800
56(~0
Wavenumbers
Fig. 3. Spectrum of the photolysed (2 h, 0.06 W) mixture (a) before photolysis, (b) after 4.75 h, and (c) after 23.5 h. The peaks at 6170 and 5946 cm-~ were integrated to follow the progress of the polymerization reaction.
6125 cm- l and 6125 to 5500 cm- 1 intervals. These spectroscopic changes provide a convenient analysis of the polymer yield which can be undertaken directly in the original photolysis cell. The 0.1 cm quartz cell is simply removed from the laser cavity and aligned inside a near-IR spectrometer. The wavelength dependence of the polymerization was confirmed by photolyzing the benzoyl peroxide/methyl methacrylate mixture at 598 (above the peak) and 610 (below the peak) nm [12]. After six hour intracavity photolyses the mixtures photolysed at the off peak wavelengths exhibited at least three times less polymer formation when checked the following day by the near-IR technique described above. These off peak photolysis samples never exceeded 10% polymerization even after a 24 h period. Furthermore control experiments on mixtures which were not photolyzed were checked after a 24 h period. The polymerization was less than 3% (the limit of the near-IR detection) for these samples. The average power of the laser in these experiments was difficult to control. In early experiments the laser was seen to flicker after many hours of photolysis due to the presence of polymer in solution. For quantitative analysis, the photolysis time was limited to 2 h, eliminating the possibility of laser fluctuation. Even with this correction, the intracavity laser power does fluctuate due to the presence of the liquid sample. Typical extracavity laser powers range from 0.05 to 0.20 W.
The time evolution of the polymerization after a 2 h photolysis at an extracavity power of 0.06 W is shown in Fig. 4. This polymerization curve is typical of the gel effect [14] which occurs when polymerization has proceeded to the point where diffusion limits the radical chain termination steps. When the radical chain termination is no longer facile the polymerization rate is seen to accelerate. This autoacceleration occurs when the small concentration of radicals formed by overtone activation has grown into larger trapped radicals which cannot diffuse toward one another. Autoacceleration has been observed in neat methyl methacrylate [14]. In these experiments, the length of time before the autoacceleration occurs depends on the intracavity laser power. The higher the power the earlier the autoacceleration occurs. This is a general trend of the data, however, due to the difficulty in controlling the average intracavity power during the polymerization the behavior was difficult to quantify. When the photolysis period is limited to only 15 min the same overall behavior is observed but the period before autoacceleration lengthens by at least l0 h (18 h period for 2 h, 0.06 W photolysis and 24 h period for 15 min, 0.15 W photolysis). Samples photolyzed off the vibrational overtone absorption peak never reached this autoacceleration step due to low overall conversion.
100
D 80 ¸
El El
60'
40-
13
# D
20'
D
m
El
13 i
i
10
20 lime,
30
hrs.
Fig. 4. Percent conversion versus time (h) for vibrational overtone initiated polymerization of methyl methacrylate.
O. Grinevich, D.L. Snavely / Chemical Physics Letters 267 (1997) 313-317
4. Conclusion
Excitation into the fifth vibrational overtone of the CH stretch of benzoyl peroxide initiates polymerization in a liquid mixture of benzoyl peroxide and methyl methacrylate. The photolysis was carried out inside a quartz cuvette aligned within a dye laser cavity. The polymerization is specific to the absorption wavenumber of the vibrational overtone transition. The percent polymerization was monitored by ratioing the areas of the olefinic to methyl and methylenic peak absorptions for the first vibrational CH stretch overtones. The polymerization autoaccelerates after a period of several hours.
References [1] F.F. Crim, Ann. Rev. Phys. Chem. 35 (1984) 657. [2] D.W. Chandler, W.E. Farneth and R.N. Zare, J. Chem. Phys. 77 (1982) 4447.
317
[3] B.R. Foy, M.P. Casassa, J.C. Stephenson and D.S. King, J. Chem. Phys. 92 (1990) 2782. [4] L.J. Butler, T.M. Ticich, M.D. Likar and F.F. Crim, J. Chem. Phys. 85 (1986) 2231. [5] X. Luo and T.R. Rizzo, J. Chem. Phys. 96 (1992) 5129. [6] B.R. Henry, Vibr. Spectra Struct. 10 (1981) 269. [7] M.S. Child, Acc. Chem. Res. 18 (1985) 45. [8] B. Henry, Acc. Chem. Res. 29(12) (1987) 430. [9] J.S. Wong, R.A. MacPhail, C.B. Moore and H.L. Strauss, J. Phys. Chem. 86 (1982) 1478. [10] C.B, Moore and J.S. Wong, J. Chem. Phys. 77(2) (1982) 603. Ill] R.G. Bray and M.J. Berry, J. Chem. Phys. 71(12) (1979) 4909. [12] W.A. Pryor, Introduction to free radical chemistry, (Prentice-Hall, Englewood Cliffs, NJ, 1966). [13] D.L. Snavely and O. Grinevich, Imag. Sci. Tech. 49th Ann. Conf. Proc. (1996). [14] G. Odian, Principles of polymerization, 3rd Ed. (Wiley, New York, 1991).