The Photoinitiated Cationic Polymerization of 3,4-Epoxy-1-butene

Chapter 23

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The Photoinitiated Cationic Polymerization of 3,4-Epoxy-1-butene Marco Sangermano , Stephen N. Falling , and James V . 1

2

Crivello

1,*

1

Department of Chemistry, New York Center for Polymer Synthesis, Rensselaer Polytechnic Institute, 110 8 Street, Troy, NY 12180 Research Laboratories, Eastman Chemical Company, P.O. Box 1972, Kingsport, TN 37662 th

2

The photopolymerization of 3,4-epoxy-1-butene (1) was investigated using Fourier transform real-time infrared spectroscopy. The effects of photoinitiator structure and concentration and light intensity on the photopolymerization were investigated. Monomer 1 was found to be more reactive than its saturated analog, 1,2-epoxybutene, and its halogenated derivatives, 3,4-dibromo-1,2-epoxybutane and 3,4-dichloro-1,2-epoxybutane.

Introduction 3,4-Epoxy-l-butene (vinyl oxirane, 1) is a molecule with considerable potential for the production of new monomers and polymers as well as fine, specialty and commodity chemicals (/). An efficient, continuous process for 1 by the selective, vapor-phase oxidation of 1,3-butadiene (equation 1) has been developed by Monnier and co-workers (2-5). This development has renewed interest in 1 as a difunctional monomer with reactive epoxy and vinyl groups.

266

© 2003 American Chemical Society

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

267 o

2

Ag/Al 0 225 °C


2

ο

3

1

(D

One rapidly emerging technology upon which this monomer may have an impact is cationic U V curing, a process which involves the rapid, photoinduced polymerization of liquid, multifiinctional monomers to give solid, crosslinked films useful for coatings, printing inks and adhesives. Typically, onium salt photoinitiators such as diaryliodonium and triarylsulfonium salts are employed in this process (6). Polymerization of the monomer results from attack by a strong protic acid which is generated during photolysis. Monomers containing the epoxide group undergo facile ring-opening polymerization by this process. In the case of 1, polyethers bearing pendant vinyl groups may result by simple ringopening polymerization. In addition, there was the possibility that the double bond could also interact with the neighboring epoxy group during polymerization to produce polymers in which all four-carbon atoms of the monomer are incorporated into the polymer backbone (7,8).

Experimental 3,4-Epoxy-l-butene (1), 3,4-dibromo-l,2-epoxybutane (3) and 3,4-dichloro1,2-epoxybutane (4) were used as received from Eastman Chemical Company (Kingsport, TN). 1,2-Epoxybutane was used as receivedfromAldrich Chemical Company (Milwaukee, WI). The diaryliodonium salt photoinitiators employed in this investigation were prepared as described previously (9). H-NMR spectra were obtained using a Varian, Inova 500 MHz Spectrometer. !

Bulk Photopolymerization of 3,4-Epoxy-l-butene (1) A 3 mL solution of 1 containing 1.0 mol% (4-n-decyloxyphenyl)phenyliodonium hexafluoroantimonate (IOC 10) was prepared. The solution was transferred to a 15 mm diameter quartz reaction tube, sealed with a rubber cap then placed in an ice bath and irradiated for two minutes in a Rayonet Photochemical Reactor. Chloroform was added to the reaction mixture and the polymer solution poured into methanol. The precipitated polymer was isolated byfiltrationthen dissolved in chloroform and reprecipitated into methanol. This process was repeated once more. The resulting polymer was dried in a vacuum oven at 50°C then the H-NMR spectrum recorded in CDC1 . l

3


268


Photopolymerization Studies The thin film photopolymerizations were monitored using Fourier transform real-time infrared spectroscopy (FT-RTIR). A Midac M-1300 FTIR spectrometer equipped with a liquid nitrogen-cooled mercury-cadmium-telluride detector was used. The instrument was fitted with a UVEXS Model SCU-110 mercury arc lamp equipped with a flexible liquid optic wand. The end of this wand was placed at a distance of 4-20 cm and directed at an incident angle of 45° onto the sample window. UV light intensities were measured with the aid of a UV radiometer at the sample window. Photopolymerizations were carried out at room temperature using unfiltered UV light from a Hg arc source with bulk monomers containing various concentrations of the indicated photoinitiator. The monomer/photoinitiator solutions were coated onto 12 μηι oriented and corona-treated polypropylene films, covered with identical polypropylene films, then mounted in 5 cm χ 5 cm slide frames. The thickness of the liquid monomer films was estimated at 10-25 μιη. Infrared spectra were collected at a rate of one spectrum per second using LabCalc data acquisition software and processed using GRAMS-386 software (Galactic Industries Corp.). During irradiation, the decrease of the IR absorbance due to the vinyl group at 1640 cm' and/or the epoxy group at 820 cm" was monitored. In all cases, 3 to 5 runs were recorded and the results averaged. The kinetic parameter, Rp/[M ], was determined from the initial slopes of the irradiation time-conversion curves according to equation 2, where Rp and [M ] are respectively the rate of polymerization and the initial monomer concentration and the conversions are as determined from the curves at irradiation times t\ and t21

1

0

0

M

Rp/[ ol

=

([conversion]t2 - [conversion^i)/(t2-ti)

(2)

Results and Discussion The diaryliodonium salt photoinitiators selected for this study are (4alkoxyphenyl)phenyliodonium salts 2. These were selected for their good solubility in 1 and other epoxide monomers. Furthermore, it has been previously shown that these photoinitiators possess high quantum yields of photolysis and are extraordinarily efficient photoinitiators of both vinyl ether and epoxide cationic polymerizations (10,11). On photolysis (equation 3), diaryliodonium salts 2 undergo irreversiblefragmentationto give a variety of organic products together with a protic acid, HMtXm, derived by hydrogen abstraction reactionfromthe monomer or solvent (6). The strong acid HMtX then catalyzes ring opening polymerization of the epoxide. m



269

!

Figure 1 shows the H-NMR spectrum of the polymer of 1 prepared by bulk cationic photopolymerization using (4-n-decyloxyphenyl)phenyIiodonium hexafluoroantimonate (IOC10,2 where C„H i = CioH i and MtX * = SbF ") as the photoinitiator. The *H-NMR assignments were made on the basis of model compounds (12) and published spectra of structurally similar polymers (13). The spectrum indicates that about 80% of the repeat units are derived from direct epoxide ring-opening polymerization. Equation 4 illustrates an example of such a direct epoxide ring-opening step. The spectrum also indicates the incorporated into the polymer backbone. Equation 5 illustrates an example of this mode of addition which can be rationalized as proceeding via an S 2' conjugate addition mechanism. Previous work with alcohol-initiated, acidcatalyzed polymerizations of 1 gave polymers with similar compositions (7,5). 2n+

2

m

6

N

The polymer formed in this polymerization was also found to be lightly crosslinked. Crosslinking took place whether the polymerization was carried out in air or nitrogen. In contrast, the alcohol-initiated, cationic polymerization of 1 with catalytic trifluoromethanesulfonic acid yields soluble, uncrosslinked polymers (7). Photolysis of diaryliodonium salt photoinitiators also yields free radicals that can lead to crosslinking either by hydrogen abstraction-coupling reactions or by direct addition polymerization of the pendant double bonds. The aliphatic absorbance in the H-NMR spectrum is evidence of such crosslinking. A brief, systematic investigation of the effects of various experimental parameters on the rate of the photoinitiated cationic polymerization of 1 was carried out. In this study we employed FT-RTIR {14J5) to monitor the rates of !



270

5

«*

3

2

-η

1

Figure 1. *H-NMR spectrum ofpoly(3,4-epoxy-l-butene) produced by bulk photopolymerization of 1 using 1 mol% IOC10 as photoinitiator. the photopolymerization. We have described the technique and the configuration of our apparatus in previous publications from this laboratory {10,16). Figure 2 shows an FT-RTIR study of the polymerization of bulk 1 containing 2.0 mol% of IOC 10 as photoinitiator. Depicted in this figure are two conversion versus time curves: the epoxy group at 820 cm" and the vinyl group at 1640 cm" . These results support the conclusion drawn from the *H-NMR spectrum (Figure 1) concerning crosslinking. While the conversion of the epoxy two conversion versus time curves: the epoxy group at 820 cm" and the vinyl group at 1640 cm' . These results support the conclusion drawn from the H NMR spectrum (Figure 1) concerning crosslinking. While the conversion of the epoxy groups is nearly quantitative, only about 30% of the vinyl groups are consumed during the course of the polymerization reaction (250 seconds irradiation). 1

1

1

1

!

Effect of Light Intensity The effect of light intensity on the photopolymerization of bulk 1 containing 1.0 mol% of IOC 10 as photoinitiator was investigated using FTRTIR. The kinetic curves (Figure 3) were obtained at UV light intensities of 10 and 5 mW/cm . The kinetic parameters R«/[Mo] takenfromthe initial slopes of these two curves are respectively, 12 χ 10 s" and 1.1 x ÎOV . A considerably faster polymerization rate was observed at the higher light intensity. These results suggest that at the lower light intensity, the rate of polymerization is light limited. 2

1

1



0

!

150

2

Figure 2. FT-RTIR study of the photopolymerization of 1 with 2 mol% IOC10. Epoxy groups (±); vinyl groups (m). Light intensity 15 mW/cm .

Irradiation Time (sec)

\

100

1

50


1

200

272 Photoinitiator Concentration The effect of the photoinitiator concentration on the rate of the cationic ring-opening polymerization of 1 was investigated at a U V light intensity of 5 mW/cm using FT-RTIR (Figure 4). The photopolymerizations were carried out in bulk 1 containing 1.0 mol% or 2.0 mol% of IOC 10. A higher polymerization rate was obtained at the higher photoinitiator concentration. Previously (i7), we have observed that the maximum rate of epoxide ringopening photopolymerization was generally achieved at a photoinitiator concentration of 2-3 mol%.


2

Comparison of Cationic Photoinitiators on 1 Photopolymerization Several different diaryliodonium salts 2 were employed as cationic photoinitiators for the polymerization of 1. In each case, the concentration of the photoinitiator was 2.0 mol% with respect to the monomer. Initially, three related hexafluoroantimonate salts were employed, with different length alkoxy groups. Using IOC8, IOC10 and IOC11 ( C H = C H , CioH C H respectively), no difference in the kinetic behavior of the 1 photopolymerization was observed. However, major differences were observed when the counterion of IOC 10 was varied as shown in Figure 5. As expected (18% the polymerization rate decreases according to the sequence: SbF " > AsF " > P F \ n

2n+l

8

1 7

6

50

100

2b

n

6

150

6

200


Figure 3. Study of the effect of UV light intensity on the rate of photopolymerization of 1 in the presence of 1.0mol%IOC10: 5 mW/crr? (±); lOmW/cm (m). 2


2 3


273

Ο

50

100

ISO

200


Figure 4. Study of the effect of concentration of IOC10 on the rate of photopolymerization of 1: 1.0 mol% (m); 2.0 mol% (k). Light intensity 5 mW/cm

Figure 5. FT-RTIR study of the influence of (4-n-decyloxyphenyl)phenyliodonium counterions on the rate of 1 photopolymerization: SbFi (m); AsF " (A); andPFi (·). Light intensity 10 mW/cm . 6

2


274

Photopolymerization of Related Monomers


Monomer 1 is also a useful intermediate for the production of a variety of epoxide and vinyl monomers. Olefin hydrogénation of 1 (19) gives its saturated analog, 1,2-epoxybutane (3), while olefin halogenation (20) produces 3,4-dibromo-l,2-epoxybutane (4) or 3,4-dichloro-1,2-epoxybutane (5) (equation 6).

Ο

3

1

4,X = Br S,X = C1

(

6

)

Figure 6 shows a comparison of the photopolymerizations of 1 and derivatives 3-5. Monomer 1 exhibits considerably higher reactivity in cationic photopolymerization than its saturated analog 3. This suggests that the neighboring vinyl group of 1 exerts an activating influence on the epoxide group. One way in which this may occur is through resonance stabilization of the propagating oxonium ion end group of the polymer chain as depicted in equation 7.

^ΟΝΑΑΛ

Ο^ΛΛΛ

o^/S/V*

^

As shown in Figure 6, monomer 1 is also more reactive than its brominated and chlorinated derivatives: 4 and 5. These monomers can be viewed as analogs of the industrially-important monomer—epichlorohydrin. Additionally, due to the high reactivity of monomer 1, it may have utility as a reactive diluent for less reactive epoxide monomers (21). However the low molecular weight, volatility and toxicity of epoxide 1 may greatly limit its applications in this regard.


275

100 90 80

•

70 60 ε

50


40 30

•

20 10 0•* J L J

,

,

,

20

40

m

100

Irradiation Time (sec) Figure 6. Comparison of the reactivity of 3,4-epoxy-l-butene (1, m) with 1,2epoxybutane (3,

5,4-dibromo-l,2-epoxybutane (4, A); and3,4-dichloro2

1,2-epoxybutane (5 ·). Light intensity 10 mW/cm , photoinitiator 2.0 mol% IOC10, f

Conclusions 3,4-Epoxy-1 -butène (1) is a very reactive monomer in photoinitiated cationic polymerization. Using typical diaryliodonium salt cationic photoinitiators, rapid ring-opening epoxide polymerization of 1 takes place. We have observed that the polymerization of this monomer is subject to the same experimental parameters as other epoxy monomers and that there is an optimum light intensity and photoinitiator concentration necessary to achieve the highest rate. Diaryliodonium salt photoinitiators bearing the hexafluoroantimonate anion are the most reactive for the photopolymerization of 1.

Acknowledgement Financial support of this work was gratefully received from Eastman Chemical Company, Kingsport, Tennessee.


276

References


1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Denton, D.; Falling, S.; Monnier, J.; Stavinoha, Jr., J.; Watkins, W. Chemica Oggi May 1996, p 17. Monnier, J. R. Applied Catalysis A: General 2001, 221, 73. Monnier, J. R.; Muehlbauer, P. J. U.S. Patent 4,897,498, 1990. Monnier, J. R.; Muehlbauer, P. J. U.S. Patent 4,950,773, 1990. Stavinoha, J. L; Tolleson, J. D. U.S. Patent 5,117,012, 1992. Crivello, J. V. In Ring-Opening Polymerization, Brunelle, D. J., Ed., Hanser, Munich, 1993, p 157. Matayabas, Jr., J. C.; Falling, S. N. U.S. Patent 5,434,314, 1995. Matayabas, Jr., J. C.; Falling, S. N. U.S. Patent 5,393,867, 1995. Crivello, J. V.; Lee, J. L. J. Polym. Sci., Polym. Polym. Part A: Chem. Ed. 1989, 27, 3951. Rajaraman, S. K.; Mowers, W. Α.; Crivello, J. V. J. Polym. Sci.,Part A: Polym., Part A: Polym. Chem. Ed. 1999, 37, 4007. Mowers, W. Α.; Crivello, J. V.; Rajaraman, S. K. RadTech Report, March/April, 2000. Godleski, S. A. U.S. Patent 5,189,199, 1993. Wagener, Κ. B.; Brzezinska, K.; Bauch, C. G. Makromol. Chem., Rapid Commun. 1992, 13, 75. Decker, C.; Moussa, Κ. Makromol. Chem., 1990, 191, 963.


The Photoinitiated Cationic Polymerization of 3,4-Epoxy-1-butene

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