- Email: [email protected]
benzoyl peroxide mixture
30 April 1999
Chemical Physics Letters 304 Ž1999. 202–206
Laser vibrational overtone activation of ethyl acrylaterbenzoyl peroxide mixture Oleg Grinevich, D.L. Snavely
)
Center for Photochemical Sciences, Bowling Green State UniÕersity, Bowling Green, OH 43402, USA Received 14 September 1998; in final form 23 February 1999
Abstract Intra- and extracavity laser vibrational overtone polymerization of ethyl acrylaterbenzoyl peroxide mixture has been demonstrated. Five photolysis wavenumbers on and near the fifth CH stretch overtone absorption of benzoyl peroxide was investigated. The polymer yield was monitored by observing the decrease in the intensity ratio of the olefinicrmethyl and methylenic first CH stretch overtone absorptions of ethyl acrylate. The rate of the polymerization did not depend on the photolysis wavenumber. Molecular weights of the overtone initiated polymers were an order of magnitude larger than those obtained by thermal polymerization. The polymerization rate is compared to the intracavity laser vibrational overtone polymerization of methyl methacrylate. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Vibrational overtone excitation has been demonstrated as a means of activation of chemical reactions in the vapor phase w1–5x. Examples of reactions initiated by vibrational excitation in liquids are few and those which have been observed exhibit low product yields. Recently, it was demonstrated that absorption into highly excited vibrational overtone states could be used to initiate polymerization of a monomer mixture w6x. The radical initiated polymerization amplified the product yield by chain propagation. The polymerization system, benzoyl peroxide in methyl methacrylate in a 1:9 mass ratio, was excited by absorption of light into the fifth CH stretch overtone transition of benzoyl peroxide at ) Corresponding author. Fax: q1 419 372 9809; e-mail: [email protected]
604 nm during an intracavity laser photolysis. The polymer yield, as measured by near-IR spectroscopy, was enhanced when the photolysis wavelength corresponded to the peak wavelength for the fifth CH stretch vibrational overtone transition as opposed to an off-peak photolysis. If the polymerization of methyl methacrylate occurs with vibrational overtone excitation, other radical initiated polymerizations should be possible with other monomers and radical precursors. In the methyl methacrylate case, with a 2 h photolysis duration the sample continued to polymerize over a 20 h period. Ethyl acrylate is known to be more reactive than methyl methacrylate w7,8x so a faster rate of polymerization should be possible with an ethyl acrylaterbenzoyl peroxide mixture. This work reports the vibrational overtone initiation of the polymerization of ethyl acrylate wherein the polymerization yield is high enough that extracavity photolysis is possible.
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 2 8 4 - 5
O. GrineÕich, D.L. SnaÕelyr Chemical Physics Letters 304 (1999) 202–206
The progress of polymerization was monitored by near-IR spectroscopy ratioing the intensities of olefinicrnon-olefinic CH first overtone absorptions. A comparison of the molecular weights of the overtone-initiated polymers to those obtained thermally under similar conditions reveals that the overtone-initiated polymers are a factor of 10 larger than those for thermal initiation. These large molecular weights are a result of the low radical concentrations created by the vibrational overtone excitation.
2. Experimental The photolysis sample contained benzoyl peroxiderethyl acrylate Ž10:90% by weight., similar to the concentrations used previously w6x. All samples were deaerated prior to photolyses by bubbling Ar through the monomer mixture for 20 min periods. Photolyses were carried out in a 1 mm sealed cuvette inside and outside the cavity of a Rhodamine 6G dye laser pumped by an Ar ion laser. The dye laser was tuned to the desired wavelength by a birefringent filter. The laser wavelength was determined by a 0.35 m monochromator. The dye laser extracavity power was about 0.1 W for the intracavity photolyses and 0.7 W for the extracavity photolyses as measured by a calibrated Newport 815 digital power meter. Although the rate of polymerization is significantly faster for the intracavity photolysis, extracavity photolyses provided more complete control of the laser power. In the intracavity experiments the laser power changes with the change of the refractive index of the polymerizing mixture. Wavelength selectivity experiments were carried out extracavity since it was crucial to maintain the same power at all wavelengths. The progress of polymerization was monitored by a Mattson near-IR spectrometer equipped with a tungsten lamp, a quartz beamsplitter and a PbSe detector. The first overtone CH stretch region Ž5600–6300 cmy1 . was recorded for this purpose. These measurements were performed in the quartz photolysis cell which transmits in both the first CH overtone and the visible photolysis regions. A complete disappearance of the olefinic absorption indicates 100% polymerization, thus, quantitative analysis can be performed for partially polymerized sam-
203
ples. The peak integration was performed identically for each measurement using the Mattson First operating software. The peaks were integrated to the corrected baseline over the 6225–6100 cmy1 and 6100–5610 cmy1 intervals. The polymer molecular weight was determined using gel permeation chromatography ŽGPC. with a Hewlett–Packard 1050 HPLC system equipped with a Hewlett–Packard Lgel 5 m Mixed-C GPC column. Standards of polymethyl methacrylate with molecular weights ranging from 33500 to 2193000 were used for calibration. Tetrahydrofuran was used as a solvent. Thermal polymerization experiments were performed in a flask kept under dry N2 . The thermolysis temperature was controlled in an oil bath and the monomerrinitiator mixture was vigorously mixed during the polymerization. Prior to thermolyses, the samples were deaerated by bubbling Ar through the sample mixtures for 20 min periods.
3. Results and discussion The vibrational overtone spectrum of ethyl acrylate ŽFig. 1. displays overtone progressions through the fifth overtone of the CH stretch. The assignments are given in Table 1. Ethyl acrylate possesses one weak and two strong absorptions in the region of 15800–16900 cmy1 . This absorption spectrum can
Fig. 1. Vibrational overtone spectrum of ethyl acrylate. Neat ethyl acrylate was used through the fourth CH stretch overtone. The fifth CH stretch overtone is for gaseous ethyl acrylate with the photoacoustic signal intensity in arbitrary units.
204
O. GrineÕich, D.L. SnaÕelyr Chemical Physics Letters 304 (1999) 202–206
Table 1 Transition wavenumbers of ethyl acrylate Peak wavenumber
Assignment
5798, 5958 6168 8470 8789 11065 11517 13546 14116 15933 16375 16730
Õ s 2, methyl, methylenic Õ s 2, olefinic Õ s 3, methyl, methylenic Õ s 3, olefinic Õ s 4, methyl, methylenic Õ s 4, olefinic Õ s 5, methyl, methylenic Õ s 5, olefinic Õ s6, methyl, methylenic Õ s6, combination Õ s6, olefinic
be compared to the spectrum of benzoyl peroxide w2x in this same wavenumber range. The absorption for the fifth CH stretch vibrational overtone Ž16556 cmy1 , 604 nm. lies between the most intense Ž16730 cmy1 , 597.7 nm. and the least intense Ž16382 cmy1 , 610.4 nm. peaks of ethyl acrylate. This fact suggests that no significant amount of light was absorbed by the monomer during the on-peak photolysis Ž604 nm. of the monomerrradical precursor mixture. The spectrum of the first overtone of the CH stretch of the photolysis mixture is presented in Fig. 2. The monomer conversion was measured by ratioing the first overtone olefinic CH stretch Žat 6168 cmy1 . to that of the methylenic and methyl CH
Fig. 2. The first CH stretch vibrational overtone spectrum of the photolysis Ž30 min, 0.15 W. mixture Ža. before photolysis, Žb. after 30 min, Žc. after 75 min. The peaks at 6168 and 5958 cmy1 were integrated to follow the progress of the polymerization process.
stretch Ž5958 cmy1 . absorptions. The relative areas of two vibrational overtone peaks at 6168 and 5958 cmy1 were compared as described in Section 2. While the polymerization proceeds the higher energy peak which belongs to olefinic stretches of the monomer loses its intensity. In a standard polyethyl acrylate sample this higher energy peak is absent. A typical monomer conversion versus time plot is shown in Fig. 3 for a 10% mixture of benzoyl peroxide and ethyl acrylate photolyzed extracavity. As can be seen, the polymerization reaction proceeds significantly faster than that for methyl methacrylate. No autoacceleration was observed with ethyl acrylate as was the case with methyl methacrylate w6x where the rate of polymerization greatly increased after reaching 22% conversion. This phenomenon is known as the gel ŽTromsdorff. effect and is not observed for ethyl acrylate since polyethyl acrylate is less viscous than polymethyl methacrylate. This higher viscosity maintains a high radical termination rate constant throughout the polymerization so the conversion versus time plot remains linear throughout the entire polymerization. Ethyl acrylate, itself, did not polymerize if benzoyl peroxide was absent even when photolysed for 2 h at the most intense absorption peak. Taking into account the initial slopes of the polymerization curves and the photolysis durations, the rate of ethyl acrylate polymerization was at least 40 times faster than that of methyl methacrylate. For
Fig. 3. Percent conversion versus time Žmin. for vibrational overtone polymerization Žat 604 nm. of the photolyzed methyl methacrylate Ž2 h, 0.06 W intracavity. w2x and ethyl acrylate Ž0.5 h, 0.7 W extracavity. mixed with 10% benzoyl peroxide.
O. GrineÕich, D.L. SnaÕelyr Chemical Physics Letters 304 (1999) 202–206
this comparison, the methyl methacrylate polymerization rate before the autoacceleration region was used. Although the laser power for the methyl methacrylate polymerization was higher it was not taken into account to estimate the difference in polymerization rates because no clear rate versus power dependance was established. With a 30 min extracavity photolysis at 0.7 W at 604 nm more than 50% of the monomer was converted to polymer after 45 min in the dark. Wavelength selectivity experiments were carried out in the same manner as those for methyl methacrylate w6x. The results are shown in Fig. 4. Five photolysis wavelengths were chosen. Two of those Ž611 and 598 nm. were just outside the benzoyl peroxide fifth overtone transition Žsee wavelength notations on Fig. 2.. Two others Ž617 and 593 nm. were chosen to completely avoid any significant absorption by either benzoyl peroxide and ethyl acrylate. In all cases the polymerization rate was the same indicating no wavelength selectivity. As mentioned above, the benzoyl peroxide absorption does not overlap any ethyl acrylate absorption peaks. Therefore, in the on-peak photolysis Ž604 nm. the monomer does not absorb any significant amount of laser light. In the methyl methacrylate polymerization, the on-peak excitation wavelength coincides with the shoulder of an intense methyl methacrylate absorption whereas the off peak photol-
Fig. 4. Percent conversion versus time Žmin. for vibrational overtone polymerization of ethyl acrylate mixed with 10% benzoyl peroxide at different extracavity photolysis Ž0.7 W, 30 min. wavelengths. 604 nm corresponds to the fifth overtone transition of benzoyl peroxide.
205
Fig. 5. Percent conversion versus time Žmin. for vibrational overtone polymerization of ethyl acrylate mixed with different percentages of benzoyl peroxide photolyzed at 604 nm Ž0.7 W, 30 min..
ysis wavelengths lay outside the monomer peaks. Since the monomer is 90% of the mixture, this ‘double’ absorption may favor the polymerization of methyl methacrylate with the on-peak photolysis. The ethyl acrylate polymerization rate did not change for all five photolysis wavelengths. It may be that the reactivity of the ethyl acrylaterbenzoyl peroxide mixture is so high, even weak absorption into the vibrational continuum forms a sufficiently high concentration of radicals to initiate polymerization. To reduce the reactivity of the ethyl acrylaterbenzoyl peroxide mixture, we photolyzed monomer mixtures containing 2%, 1% and 0.5% initiator were photolyzed at the five photolysis wavelengths. Although the rate of polymerization decreased with initiator concentration, it was still significantly higher than the rate of methyl methacrylate overtone polymerization ŽFig. 5.. No differences in the rate of polymerization with wavelength were observed. Molecular weight measurements of the vibrational overtone polymers were performed. The overtone results were compared to those obtained from thermal experiments at various temperatures using the same 10% concentration of initiator. The weight average molecular weights, presented in Table 2, differ by an order of magnitude for overtone and thermal experiments. The average weight molecular
O. GrineÕich, D.L. SnaÕelyr Chemical Physics Letters 304 (1999) 202–206
206
Table 2 Average number molecular weights Ž M w . and polydispersity indexes ŽPI. for vibrational overtone and thermal experiments Žmonomer mixed with 10% benzoyl peroxide. Ethyl acrylate thermal activation T Ž8C. 60 80 100
MW
PI
415000 308000 176000
2.12 2.46 3.90
Ethyl acrylate vibrational overtone activation Power ŽW.
Photolysis time Žmin.
Dark time Žmin.
MW
PI
0.1 0.1 0.1 0.1 0.1 0.9 0.7
15 15 15 15 15 30 30
20 35 50 65 80 60 60
2720000 3360000 2940000 3160000 2860000 2020000 2090000
1.30 1.31 1.58 1.47 1.38 2.27 1.99
T Ž8C.
MW
PI
60 80
85500 60700
1.76 2.72
Methyl methacrylate thermal activation
Methyl methacrylate vibrational overtone activation Power ŽW.
Photolysis time Žmin.
Dark time Žmin.
MW
PI
0.5
120
450
827000
1.79
at all times resulting in few radical–radical termination steps. The lack of radical–radical termination steps, in turn, results in higher molecular weights. In thermal polymerization and photopolymerization with electronic state excitation, the rate of initiation is significantly higher and the polymer chains are terminated faster as a result of high free radical concentration. The measurement of polyethyl acrylate molecular weights with various degrees of conversion were undertaken. The polymerizing sample was checked at 20, 35, 50, 65, and 80 min after the 15 min intracavity photolysis Ž0.1 W power.. Molecular weights of polyethyl acrylate prepared with different laser powers were also compared. All changes in the measured molecular weights fall within the experimental error of the GPC technique, indicating that neither percent conversion nor laser power influence the molecular weights of overtone initiated polymers.
Acknowledgements Authors would like to thank D.C. Neckers for valuable discussions and the McMaster Endowment for financial support.
References weights in overtone activation experiments ranged from 2.0 to 3.4 = 10 6 for polyethyl acrylate and did not exceed 1.0 = 10 6 for polymethyl methacrylate. In contrast, thermally obtained polymers possessed lower weights which did not exceed 5.0 = 10 5 for polyethyl acrylate and 1.0 = 10 5 for polymethyl methacrylate at temperatures ranging from 60 to 1008C. Overtone polymerization is a slower process if compared to thermal polymerization at 608C and higher temperatures. The polymerization is controlled by the rate of radical formation controlled by the rate of visible light absorption. Since the initiation proceeds slowly the radical concentration is low
w1x S. Leytner, D.L. Snavely, O. Grinevich, Chem. Phys. Lett. 277 Ž1997. 443. w2x I. Ouporov, O. Grinevich, D.L. Snavely, J. Chem. Phys. 104 Ž15. Ž1996. 5852. w3x S. Hassoon, N. Rajapakse, D.L. Snavely, J. Phys. Chem. 96 Ž1991. 2576. w4x D.L. Snavely, O. Grinevich, S. Hassoon, G. Snavely, J. Chem. Phys. 104 Ž1996. 5845. w5x F.F. Crim, Annu. Rev. Phys. Chem. 35 Ž1984. 657. w6x O. Grinevich, D.L. Snavely, Chem. Phys. Lett. 267 Ž1997. 313. w7x G. Odian, Principles of Polymerization, 3rd edn., Wiley, New York, 1991. w8x M. Stevens, Polymer Chemistry, 2nd edn., Oxford University Press, Oxford, New York, 1990.
Chemical Physics Letters 304 Ž1999. 202–206
Laser vibrational overtone activation of ethyl acrylaterbenzoyl peroxide mixture Oleg Grinevich, D.L. Snavely
)
Center for Photochemical Sciences, Bowling Green State UniÕersity, Bowling Green, OH 43402, USA Received 14 September 1998; in final form 23 February 1999
Abstract Intra- and extracavity laser vibrational overtone polymerization of ethyl acrylaterbenzoyl peroxide mixture has been demonstrated. Five photolysis wavenumbers on and near the fifth CH stretch overtone absorption of benzoyl peroxide was investigated. The polymer yield was monitored by observing the decrease in the intensity ratio of the olefinicrmethyl and methylenic first CH stretch overtone absorptions of ethyl acrylate. The rate of the polymerization did not depend on the photolysis wavenumber. Molecular weights of the overtone initiated polymers were an order of magnitude larger than those obtained by thermal polymerization. The polymerization rate is compared to the intracavity laser vibrational overtone polymerization of methyl methacrylate. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Vibrational overtone excitation has been demonstrated as a means of activation of chemical reactions in the vapor phase w1–5x. Examples of reactions initiated by vibrational excitation in liquids are few and those which have been observed exhibit low product yields. Recently, it was demonstrated that absorption into highly excited vibrational overtone states could be used to initiate polymerization of a monomer mixture w6x. The radical initiated polymerization amplified the product yield by chain propagation. The polymerization system, benzoyl peroxide in methyl methacrylate in a 1:9 mass ratio, was excited by absorption of light into the fifth CH stretch overtone transition of benzoyl peroxide at ) Corresponding author. Fax: q1 419 372 9809; e-mail: [email protected]
604 nm during an intracavity laser photolysis. The polymer yield, as measured by near-IR spectroscopy, was enhanced when the photolysis wavelength corresponded to the peak wavelength for the fifth CH stretch vibrational overtone transition as opposed to an off-peak photolysis. If the polymerization of methyl methacrylate occurs with vibrational overtone excitation, other radical initiated polymerizations should be possible with other monomers and radical precursors. In the methyl methacrylate case, with a 2 h photolysis duration the sample continued to polymerize over a 20 h period. Ethyl acrylate is known to be more reactive than methyl methacrylate w7,8x so a faster rate of polymerization should be possible with an ethyl acrylaterbenzoyl peroxide mixture. This work reports the vibrational overtone initiation of the polymerization of ethyl acrylate wherein the polymerization yield is high enough that extracavity photolysis is possible.
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 2 8 4 - 5
O. GrineÕich, D.L. SnaÕelyr Chemical Physics Letters 304 (1999) 202–206
The progress of polymerization was monitored by near-IR spectroscopy ratioing the intensities of olefinicrnon-olefinic CH first overtone absorptions. A comparison of the molecular weights of the overtone-initiated polymers to those obtained thermally under similar conditions reveals that the overtone-initiated polymers are a factor of 10 larger than those for thermal initiation. These large molecular weights are a result of the low radical concentrations created by the vibrational overtone excitation.
2. Experimental The photolysis sample contained benzoyl peroxiderethyl acrylate Ž10:90% by weight., similar to the concentrations used previously w6x. All samples were deaerated prior to photolyses by bubbling Ar through the monomer mixture for 20 min periods. Photolyses were carried out in a 1 mm sealed cuvette inside and outside the cavity of a Rhodamine 6G dye laser pumped by an Ar ion laser. The dye laser was tuned to the desired wavelength by a birefringent filter. The laser wavelength was determined by a 0.35 m monochromator. The dye laser extracavity power was about 0.1 W for the intracavity photolyses and 0.7 W for the extracavity photolyses as measured by a calibrated Newport 815 digital power meter. Although the rate of polymerization is significantly faster for the intracavity photolysis, extracavity photolyses provided more complete control of the laser power. In the intracavity experiments the laser power changes with the change of the refractive index of the polymerizing mixture. Wavelength selectivity experiments were carried out extracavity since it was crucial to maintain the same power at all wavelengths. The progress of polymerization was monitored by a Mattson near-IR spectrometer equipped with a tungsten lamp, a quartz beamsplitter and a PbSe detector. The first overtone CH stretch region Ž5600–6300 cmy1 . was recorded for this purpose. These measurements were performed in the quartz photolysis cell which transmits in both the first CH overtone and the visible photolysis regions. A complete disappearance of the olefinic absorption indicates 100% polymerization, thus, quantitative analysis can be performed for partially polymerized sam-
203
ples. The peak integration was performed identically for each measurement using the Mattson First operating software. The peaks were integrated to the corrected baseline over the 6225–6100 cmy1 and 6100–5610 cmy1 intervals. The polymer molecular weight was determined using gel permeation chromatography ŽGPC. with a Hewlett–Packard 1050 HPLC system equipped with a Hewlett–Packard Lgel 5 m Mixed-C GPC column. Standards of polymethyl methacrylate with molecular weights ranging from 33500 to 2193000 were used for calibration. Tetrahydrofuran was used as a solvent. Thermal polymerization experiments were performed in a flask kept under dry N2 . The thermolysis temperature was controlled in an oil bath and the monomerrinitiator mixture was vigorously mixed during the polymerization. Prior to thermolyses, the samples were deaerated by bubbling Ar through the sample mixtures for 20 min periods.
3. Results and discussion The vibrational overtone spectrum of ethyl acrylate ŽFig. 1. displays overtone progressions through the fifth overtone of the CH stretch. The assignments are given in Table 1. Ethyl acrylate possesses one weak and two strong absorptions in the region of 15800–16900 cmy1 . This absorption spectrum can
Fig. 1. Vibrational overtone spectrum of ethyl acrylate. Neat ethyl acrylate was used through the fourth CH stretch overtone. The fifth CH stretch overtone is for gaseous ethyl acrylate with the photoacoustic signal intensity in arbitrary units.
204
O. GrineÕich, D.L. SnaÕelyr Chemical Physics Letters 304 (1999) 202–206
Table 1 Transition wavenumbers of ethyl acrylate Peak wavenumber
Assignment
5798, 5958 6168 8470 8789 11065 11517 13546 14116 15933 16375 16730
Õ s 2, methyl, methylenic Õ s 2, olefinic Õ s 3, methyl, methylenic Õ s 3, olefinic Õ s 4, methyl, methylenic Õ s 4, olefinic Õ s 5, methyl, methylenic Õ s 5, olefinic Õ s6, methyl, methylenic Õ s6, combination Õ s6, olefinic
be compared to the spectrum of benzoyl peroxide w2x in this same wavenumber range. The absorption for the fifth CH stretch vibrational overtone Ž16556 cmy1 , 604 nm. lies between the most intense Ž16730 cmy1 , 597.7 nm. and the least intense Ž16382 cmy1 , 610.4 nm. peaks of ethyl acrylate. This fact suggests that no significant amount of light was absorbed by the monomer during the on-peak photolysis Ž604 nm. of the monomerrradical precursor mixture. The spectrum of the first overtone of the CH stretch of the photolysis mixture is presented in Fig. 2. The monomer conversion was measured by ratioing the first overtone olefinic CH stretch Žat 6168 cmy1 . to that of the methylenic and methyl CH
Fig. 2. The first CH stretch vibrational overtone spectrum of the photolysis Ž30 min, 0.15 W. mixture Ža. before photolysis, Žb. after 30 min, Žc. after 75 min. The peaks at 6168 and 5958 cmy1 were integrated to follow the progress of the polymerization process.
stretch Ž5958 cmy1 . absorptions. The relative areas of two vibrational overtone peaks at 6168 and 5958 cmy1 were compared as described in Section 2. While the polymerization proceeds the higher energy peak which belongs to olefinic stretches of the monomer loses its intensity. In a standard polyethyl acrylate sample this higher energy peak is absent. A typical monomer conversion versus time plot is shown in Fig. 3 for a 10% mixture of benzoyl peroxide and ethyl acrylate photolyzed extracavity. As can be seen, the polymerization reaction proceeds significantly faster than that for methyl methacrylate. No autoacceleration was observed with ethyl acrylate as was the case with methyl methacrylate w6x where the rate of polymerization greatly increased after reaching 22% conversion. This phenomenon is known as the gel ŽTromsdorff. effect and is not observed for ethyl acrylate since polyethyl acrylate is less viscous than polymethyl methacrylate. This higher viscosity maintains a high radical termination rate constant throughout the polymerization so the conversion versus time plot remains linear throughout the entire polymerization. Ethyl acrylate, itself, did not polymerize if benzoyl peroxide was absent even when photolysed for 2 h at the most intense absorption peak. Taking into account the initial slopes of the polymerization curves and the photolysis durations, the rate of ethyl acrylate polymerization was at least 40 times faster than that of methyl methacrylate. For
Fig. 3. Percent conversion versus time Žmin. for vibrational overtone polymerization Žat 604 nm. of the photolyzed methyl methacrylate Ž2 h, 0.06 W intracavity. w2x and ethyl acrylate Ž0.5 h, 0.7 W extracavity. mixed with 10% benzoyl peroxide.
O. GrineÕich, D.L. SnaÕelyr Chemical Physics Letters 304 (1999) 202–206
this comparison, the methyl methacrylate polymerization rate before the autoacceleration region was used. Although the laser power for the methyl methacrylate polymerization was higher it was not taken into account to estimate the difference in polymerization rates because no clear rate versus power dependance was established. With a 30 min extracavity photolysis at 0.7 W at 604 nm more than 50% of the monomer was converted to polymer after 45 min in the dark. Wavelength selectivity experiments were carried out in the same manner as those for methyl methacrylate w6x. The results are shown in Fig. 4. Five photolysis wavelengths were chosen. Two of those Ž611 and 598 nm. were just outside the benzoyl peroxide fifth overtone transition Žsee wavelength notations on Fig. 2.. Two others Ž617 and 593 nm. were chosen to completely avoid any significant absorption by either benzoyl peroxide and ethyl acrylate. In all cases the polymerization rate was the same indicating no wavelength selectivity. As mentioned above, the benzoyl peroxide absorption does not overlap any ethyl acrylate absorption peaks. Therefore, in the on-peak photolysis Ž604 nm. the monomer does not absorb any significant amount of laser light. In the methyl methacrylate polymerization, the on-peak excitation wavelength coincides with the shoulder of an intense methyl methacrylate absorption whereas the off peak photol-
Fig. 4. Percent conversion versus time Žmin. for vibrational overtone polymerization of ethyl acrylate mixed with 10% benzoyl peroxide at different extracavity photolysis Ž0.7 W, 30 min. wavelengths. 604 nm corresponds to the fifth overtone transition of benzoyl peroxide.
205
Fig. 5. Percent conversion versus time Žmin. for vibrational overtone polymerization of ethyl acrylate mixed with different percentages of benzoyl peroxide photolyzed at 604 nm Ž0.7 W, 30 min..
ysis wavelengths lay outside the monomer peaks. Since the monomer is 90% of the mixture, this ‘double’ absorption may favor the polymerization of methyl methacrylate with the on-peak photolysis. The ethyl acrylate polymerization rate did not change for all five photolysis wavelengths. It may be that the reactivity of the ethyl acrylaterbenzoyl peroxide mixture is so high, even weak absorption into the vibrational continuum forms a sufficiently high concentration of radicals to initiate polymerization. To reduce the reactivity of the ethyl acrylaterbenzoyl peroxide mixture, we photolyzed monomer mixtures containing 2%, 1% and 0.5% initiator were photolyzed at the five photolysis wavelengths. Although the rate of polymerization decreased with initiator concentration, it was still significantly higher than the rate of methyl methacrylate overtone polymerization ŽFig. 5.. No differences in the rate of polymerization with wavelength were observed. Molecular weight measurements of the vibrational overtone polymers were performed. The overtone results were compared to those obtained from thermal experiments at various temperatures using the same 10% concentration of initiator. The weight average molecular weights, presented in Table 2, differ by an order of magnitude for overtone and thermal experiments. The average weight molecular
O. GrineÕich, D.L. SnaÕelyr Chemical Physics Letters 304 (1999) 202–206
206
Table 2 Average number molecular weights Ž M w . and polydispersity indexes ŽPI. for vibrational overtone and thermal experiments Žmonomer mixed with 10% benzoyl peroxide. Ethyl acrylate thermal activation T Ž8C. 60 80 100
MW
PI
415000 308000 176000
2.12 2.46 3.90
Ethyl acrylate vibrational overtone activation Power ŽW.
Photolysis time Žmin.
Dark time Žmin.
MW
PI
0.1 0.1 0.1 0.1 0.1 0.9 0.7
15 15 15 15 15 30 30
20 35 50 65 80 60 60
2720000 3360000 2940000 3160000 2860000 2020000 2090000
1.30 1.31 1.58 1.47 1.38 2.27 1.99
T Ž8C.
MW
PI
60 80
85500 60700
1.76 2.72
Methyl methacrylate thermal activation
Methyl methacrylate vibrational overtone activation Power ŽW.
Photolysis time Žmin.
Dark time Žmin.
MW
PI
0.5
120
450
827000
1.79
at all times resulting in few radical–radical termination steps. The lack of radical–radical termination steps, in turn, results in higher molecular weights. In thermal polymerization and photopolymerization with electronic state excitation, the rate of initiation is significantly higher and the polymer chains are terminated faster as a result of high free radical concentration. The measurement of polyethyl acrylate molecular weights with various degrees of conversion were undertaken. The polymerizing sample was checked at 20, 35, 50, 65, and 80 min after the 15 min intracavity photolysis Ž0.1 W power.. Molecular weights of polyethyl acrylate prepared with different laser powers were also compared. All changes in the measured molecular weights fall within the experimental error of the GPC technique, indicating that neither percent conversion nor laser power influence the molecular weights of overtone initiated polymers.
Acknowledgements Authors would like to thank D.C. Neckers for valuable discussions and the McMaster Endowment for financial support.
References weights in overtone activation experiments ranged from 2.0 to 3.4 = 10 6 for polyethyl acrylate and did not exceed 1.0 = 10 6 for polymethyl methacrylate. In contrast, thermally obtained polymers possessed lower weights which did not exceed 5.0 = 10 5 for polyethyl acrylate and 1.0 = 10 5 for polymethyl methacrylate at temperatures ranging from 60 to 1008C. Overtone polymerization is a slower process if compared to thermal polymerization at 608C and higher temperatures. The polymerization is controlled by the rate of radical formation controlled by the rate of visible light absorption. Since the initiation proceeds slowly the radical concentration is low
w1x S. Leytner, D.L. Snavely, O. Grinevich, Chem. Phys. Lett. 277 Ž1997. 443. w2x I. Ouporov, O. Grinevich, D.L. Snavely, J. Chem. Phys. 104 Ž15. Ž1996. 5852. w3x S. Hassoon, N. Rajapakse, D.L. Snavely, J. Phys. Chem. 96 Ž1991. 2576. w4x D.L. Snavely, O. Grinevich, S. Hassoon, G. Snavely, J. Chem. Phys. 104 Ž1996. 5845. w5x F.F. Crim, Annu. Rev. Phys. Chem. 35 Ž1984. 657. w6x O. Grinevich, D.L. Snavely, Chem. Phys. Lett. 267 Ž1997. 313. w7x G. Odian, Principles of Polymerization, 3rd edn., Wiley, New York, 1991. w8x M. Stevens, Polymer Chemistry, 2nd edn., Oxford University Press, Oxford, New York, 1990.