bk-2003-0847.ch005

Chapter 5

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Photoinitiated Polymerization of Selected Thiol-ene Systems Charles E. Hoyle , Μ. Cole , M . Bachemin , W . K u a n g , Viswanathan Kalyanaraman , and Sonny Jönsson 1

1

1

2

1

2

1

School of Polymers and High Performance Materials, Department of Polymer Science, University of Southern Mississippi, 2609 West 4 Street, Hattiesburg, MS 39406 Fusion UV-Curing Systems, Inc., 910 Clopper Road, Gaithersburg, MD 20878-1357 th

2

Thiol-ene based photocurable systems exhibit very rapid rates of polymerization in nitrogen that are comparable to those achieved with traditional nitrogen saturated acrylates. In the presence of air, rates of thiol-ene copolymerization are much greater than are attained by traditional multifunctional acrylates. Even in the absence of an added photoinitiator, rapid copolymerization (thiol-acrylate) and homopolymerization (acrylate) rates of a multifunctional thiol and a difunctional acrylate are recorded in air.

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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.

53


Introduction Details of the photolysis of a mixture of a difunctional thiol and a difunctional ene to produce a linear polymer were published in the late 1940s (7). In that early report, in which no photoinitiator was used, the polymerization probably proceeded via a free-radical polymerization process initiated by a primary sulfur-hydrogen bond cleavage to yield a thiyl and a hydrogen atom radical pair. In the mid 1970s, Morgan and Ketley (2,3) showed that equimolar mixtures of multifunctional thiols and multifunctional enes with benzophenone as a hydrogen abstracting type photoinitiator formed crosslinked networks characterized by high functional group conversion upon exposure to a high intensity lamp source. It was this benzophenone initiated photocuring that was the heart of the early thiol-ene commercial systems. The polymerization was proposed (2) to involve a two step sequence: the first proposed step is the addition of a thiyl radical into the ene to generate a carbon centered radical, and the second step involves a subsequent hydrogen abstraction by the carbon centered radical produced in the first step. A general kinetic depiction forfree-radicalcuring of thiol-enes built around this two step propagation process is shown in Scheme I.

RS* + Other Products

RSH + PI (if used)

Initiation

Propagation 1

RS* + H C=CHR'

Propagation 2

RSH C-CHR' + RSH -> RSH C-CH R' + RS*

2

2

Termination

->

RSH C-CHR' 2

2

RS* + RS*

2

-> RSSR RS

RS*

+ RSH C-CHR' 2

I -> RSH C-CHR' 2

RSH C-CHR' 2

• RSH C-CHR' 2

· I + RSH C-CHR' -> RSH C-CHR' 2

2

Scheme L General thiol-ene photopolymerization process. It is important to notefromScheme I that after the generation of an initial thiyl radical species the thiol-ene propagation basically involves a free-radical addition step (propagation 1) followed by a chain transfer step (propagation 2) that produces addition of a thiol (RSH) across a carbon-carbon double bond.


54


Simply put, the thiol acts like a chain transfer agent that limits the chain propagation to a single step. A high molecular weight crosslinked polymer network is only produced when monomers with functionality of three or greater are used. In die thiol-enefree-radicalpolymerization, the gel point (the point at which an infinite network is produced) is predicted to occur (2,4) according to the standard Eq. 1 for gelation by a step growth mechanism,

e = [ l « W l X W ) r

Eq.l

where α is thefractionalconversion required to attain an infinite gel network, r is the stoichoimetric thiol/ene molar functionality ratio, fMoi is the fuctionality of the thiol, and f is the functionality of the ene. The gel formation for thiol-enes according to Eq. 1 is quite different from polymerization of multifunctional acrylate monomers where microgelation, which results in a heterogeneous polymerization medium and reaction diffusion controlled termination kinetics, occurs in the early stages of polymerization. Basically, the thiol-ene step growth polymerization proceeds to relatively high conversion in a medium that has quite a low viscosity before gelation. According to Jacobine (4% this allows the shrinkage which accompanies freeradical polymerization to take place in a homogeneous medium of low viscosity and low gel content. Hence, the shrinkage in a thiol-ene polymerization does not result in delamination of films to the same extent as occurs with traditional acrylate free-radical polymerization where shrinkage occurs during/after the crosslinked network has begun to form extensively. Literature reports (4) indicate little oxygen inhibition of thefree-radicalpolymerization of thiol-enes since the peroxy radicals that are formed by reaction of oxygen with the carbon centered radicals readily abstract a hydrogen from a thiol to continue the freeradical chain process: oxygen inhibition occurs with acrylate based radical polymerization processes since there is only one propagation step (reaction of a carbon centered radical chain end with the double bond of a monomer) and no facile hydrogen abstraction process as occurs in the second propagation step in Scheme I. A particularly interesting and important aspect of the polymerization process outlined in Scheme I is the wide variation in ene structures which can participate in thiol-ene polymerization. Both in a review article by Jacobine (4) and a crucial paper by Morgan and Ketley (2) relative rates are reported for the addition of thiols to a several enes including simple alkenes (such as 1-heptene, 2-pentene, and 1,6-hexadiene), allyl ethers, vinyl acetate, alkyl vinyl ethers, conjugated dienes, styrene, and even acrylates. Work in other labs, including ours, indicates that virtually any monomer with an ene functionality will participate in a radical polymerization process with thiols. This, of course, makes thiol-ene chemistry very versatile since coating resins that contain one or more of the ene monomer types can be readily formulated from a wide t

ene



55

variety of monomers which impart different physical properties into the final cured films. In summary, there has been a large body of information published in the past dealing with thiol-ene photopolymerization: most of it during the late 1970s and early 1980s as chronicled in great detail in a review in 1993 (4). The polymerization process and the subsequent polymers produced offer many advantages including fast rates, low oxygen inhibition during curing, low film shrinkage, and flexible films with good adhesion. The versatility of thiol-ene photocuring warrants consideration in solving some of today's most pressing problems in the industry dealing with polymerization in air and cure of thin/thick films. In this paper, we present recent results from our laboratory that clearly point out the salient features of thiol-ene photoinitiated polymerization. In addition to illustrating the kinetic rates for thiol-ene polymerization in air and nitrogen systems, we give quantitative results that show how oxygen inhibition in traditional acrylate systems can be reduced by incorporation of thiol concentrations that are less than stochiometric. The results reported herein are inspired by the tremendous effort in this field by Ketley and Morgan in the 1970s. Hopefully quantitative examples presented herein of previously defined thiol-ene systems and polymerization characteristics will inspire new practitioners in the field to consider this old technology and its relevance to photocuring today and in the future.

Experimental All monomers used were either obtained form Aldrich Chemical Co. or UCB. The photo-DSC and real-time infrared (RTIR) measurements were obtained using either a 450-Watt medium pressure mercury lamp or a high pressure 75-Watt xenon lamp, respectively. The photo-DSC exotherms were recorded on a modified Perkin-Elmer 7 while the real-time infrared results were recorded on a modified Bruker 88IR spectrometer.

Results and Discussion Results in this section will be presented in order to illustrate the features of thiol-ene photopolymerization that make it attractive as an alternative to traditional photocurable systems based strictly upon acrylates and mixtures of acrylates. As indicated in the Introduction Section, the free-radical chain reaction involved in the photopolymerization of thiol-ene mixtures has been reported in the literature to be a rapid process that is relatively uninhibited by the presence of oxygen. Herein, we will illustrate that the polymerization is indeed rapid by a direct comparison with acrylate polymerization in the


56

CH OCCH CH SH 2

2

2

HS(CH ) SH

I V CH CH —CH OCCH CH SH 3

2

2

2

2

6

2


I Î CH OCCH CH SH 2

2

2

Trimethylolpropane tris$-mercaptopropionate)

1,6-Hexanedi thiol

?

CH OCCH=CH 2

2

I ? CH CH —CH OCCH=CH

2

CH OCCH=CH

2

3

2

2

2

1,6-hexanedioldiaerylate (HDDA)

Trimethylolpropanetriacrylate (TMPTA)

Trimethylolpropane diallyl ether

.

0

/ N ^ O

x

^

0

Pentaerythritol triallyl ether

^ =

Triethyleneglycol divinylether

V-0(CH ) CH 2

5

Hexyl acrylate


3


57 presence and absence of oxygen. Examples will be drawn from a variety of thiol-ene combinations to illustrate the diversity of combinations which are encompassed by thiol-ene chemistry. Before proceeding it is important to point out that strict copolymerization involving addition of a thiol group across a carbon-carbon double bond does not always occur when the ene is capable of homopolymerization itself. In those cases, L e., where the ene is capable of independent free-radical polymerization, there can actually be two simultaneous polymerization processes involving both the thiol-ene step growth mechanism (as illustrated in Scheme I) and conventional chain growth polymerization kinetics. Such copolymerization processes are quite interesting as will be described herein. The structures of the monomers used in this investigation are given below to serve as a reference throughout the paper.

Thiol-Ene Polymerization Rate in Nitrogen Compared to Acrylate Figure 1 shows photo-DSC exotherms for the photopolymerization of a typical thiol-ene combination consisting of 1,6-hexanedithiol and trimethylolpropane diallyl ether with 1 wt% of a typical photoinitiator, 1,1dimethoxyphenylacetophenone (DMPA), added to initate thefree-radicalstepgrowth polymerization process. Since a linear polymer that is not crosslinked is formed, the exotherm results are compared to the photopolymerization of hexylacrylate which also gives a linear acrylate polymer upon polymerization in the presence of the same photoinitiator. The results in Figure 1 clearly show that this particular thiol-ene polymerization in nitrogen is inherently faster than the acrylate polymerization and certainly illustrates that the basic thiol-ene polymerization (in a nitrogen atmosphere) competes quite favorably with acrylate polymerization. We note that we have chosen to use a particular alkyl dithiol and a difunctional allyl ether as representative of thiol-ene polymerization rates for comparison with the monofunctional acrylate. Different rates would be obtained if different enes or thiols were used. For instance, substantially faster rates would be obtained if the difunctional allyl ether were replaced with a difunctional vinyl ether with electron rich double bonds. Concomitantly, lower rates would be obtained with a difunctional unsaturated ester which is both electron deficient and sterically hindered.

Thiol-Ene Polymerization Rates In Air Compared to Acrylates Photoexotherms in Figures 2 and 3 show that the photopolymerization of typical multifunctional acrylates 1,6-hexanedioldiacrylate (HDDA) and trimethylolpropane triacrylate (ΤΜΡΤΑ) with 1 wt % DMPA photoinitiator is greatly inhibited in air compared to nitrogen.


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70

-τ

0.0

0.5

1.0

1.5

Time, min Figure L Photo-DSC exotherms for the photopolymerization of (a) a 1:1 molar mixture of 1,6-hexanedithiol and trimethylolpropane diallyl ether, and (b) hexylacrylate: both initiated with 1 wt% DMPA. Light intensity from medium pressure mercury lamp was 1.0 mW/cm . 2

100 80

·

Ί

·

A

0.0

0.5

1.0

1.5

2.0

Time, min Figure 2. Photo-DSC exotherms of HDDA with 1 wt% DMPA initiated with a medium pressure mercury lamp with intensity of 1.0 mW/crrf in air/nitrogen. In contrast to the acrylate results, Figure 4 shows that the photopolymerization exotherm of a 1:1 molar mixture of trimethylolpropane tris (β-mercaptopropionate) and pentaerythritol triallyl ether with the same concentration of photoinitiator and light intensity is only marginally diminished in the presence of oxygen. As indicated in previous literature dealing with thiol/acrylate photopolymerization (5), such a result is most likely a consequence of the hydrogen abstraction from a thiol group by peroxy


59


100 -,

T i m e min 9

Figure 3. Photo-DSC exotherms of TMPTA with 1 wt% DMPA initiated with medium pressure mercury lamp with intensity of 1.0 mW/cm in air/nitrogen. 2

100

-τ

80

-

60

-

40

-

20

-

0

-

Β Ο

C

Ctf S

0.0

Time, min Figure 4. Photo-DSC exotherms of 1:1 molar mixtures of trimethylolpropane tris (β-mercaptopropionate) and pentaerythritol triallyl ether in air and nitrogen initiated by medium pressure mercury lamp with intensity of 1.0 mW/cm . 2


60 radicals (see Scheme II) to generate a hydrogen peroxide and a thiyl radical which feeds back into the radical chain process described in Scheme L In other words, the peroxy terminating radical does not lead to chain termination as found in traditional acrylate radical polymerizations.


Addition of Multifunctional Thiols to Multifunctional Acrylates: With and Without added Cleavage Type Photoinitiator Varying amounts of trimethylolpropane tris(P-mercaptopropionate) were added to HDDA and the photopolymerizations conducted in air. In Figure 5, exotherms for HDDA polymerization in air with 0, 5, and 50 mole percent of the trithiol are shown for the case where 0.1 wt% DMPA is used as the photoinitiator. Surprisingly, addition of only 5 mol% of trimethylolpropane tris (β-mercaptopropionate) results in a large acceleration of the polymerization compared to the case for HDDA with only the photoinitiator present. Apparently, addition of only a small amount of the thiol greatly enhances the polymerization rate of HDDA in the presence of air in accordance with the oxygen scavenging Scheme II (5).

RS*

+

CHrCHR

RSCHr-CHaR

1

* RSCHr-CHR'

1

+

RS*

^ RS-CHj-èHR

1

RSH

RS*

+

OOH RSCHi-èHR'

Scheme II. Basic oxygen scavenging mechanism for thiol-enes.



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Time (sec)

Figure 5. Photo-DSC exotherms of HDDA with OJ wt%DMPA with increasing concentration ^/trimethylolpropane tris(P-mercaptopropionate) in air initiated by a medium pressure mercury lamp with intensity of 60 mW/cnf: (a) HDDA; (b) 5:95 TrithioUHDDA; (c) 50:50 TrithiolHDDA.

As exemplified by the RTIR results in Figure 6, a 50:50 molar trimethylolpropane tris(P-mercaptopropionate) and pentaerythritol triallyl ether mixture readily copolymerizes without the need for an added photoinitiator. Photo-DSC exotherm results (not given herein) also show the same relative rapid polymerization for this system in air and nitrogen saturated samples in the absence of an added photoinitiator. The self-initiated process (see reference 4 and references listed therein) probably occurs via direct excitation of the thiol to cleave the sulfur hydrogen bond and generate hydrogen and thiyl radicals capable of initiating thefree-radicalchain process. Although the rate of HDDA polymerization is substantial in air saturated systems with added thiol (Figure 7), there is still a discernable difference between the rate of polymerization of mixtures of trimethylolpropane tris (β-mercaptopropionate) and HDDA in air versus nitrogen purged samples with no added photoinitiator present. For example, in Figure 8 for a 25:75 trimethylolpropane tris (βmercaptopropionate) and HDDA mixture, the exotherm rate is substantially reduced in the presence of oxygen. Although, as pointed out already, the acrylate polymerization exhibits much less oxygen inhibition in the presence of the thiol (see Scheme II) than acrylate polymerization in air with no thiol added. [Incidentally, an interesting aspect of the thiol mixtures with multifunctional enes without added photoinitiator is the enhanced photostability in simulated outdoor weathering tests].


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100τ

Time (s)

Figure 6. Percent allyl ether and thiol conversion from RT-IR analysis as function of exposure time in a 50:50 trimethylolpropane tris (βmercaptopropionate)/pentaerythritol triallyl ether molar mixture sandwiched between salt plates with no added photoinitiator.

Time (sec)

Figure 7. Photo-DSC exotherms of HDDA with added trithiol (concentration as indicated) initiated by output of medium pressure mercury lamp with intensity of 60 mW/cm in air: (a) HDDA with 0.1wt% DMPA; (b) 30:70 TrithiolHDDA with no initiator; (c) 50:50 TrithioUHDDA with no initiator. 2



63

ι 20.0

40.0

60.0

•

80.0

ι

• fc

100.0 120.0

Time (sec) Figure 8. Photo-DSC exotherms of 25:75 trimethylolpropane tris (βmercaptopropionate)/HDDA mixture initiated by output of medium pressure mercury lamp with intensity of 60 mW/cm in (a) nitrogen and (b) air. 2

In contrast to the results in Figure 8 for the thiol/HDDA mixture, photopolymerization of a 25:75 molar mixture of trimethylolpropane tris (βmercaptopropionate) and triethyleneglycol divinylether experiences relatively little reduction in the exotherm rate in air compared to nitrogen (Figure 9). As determined by RTIR (not shown herein), there is essentially no vinyl ether group homopolymerization and oxygen does not quench the thiol/vinyl ether copolymerization to an appreciable extent. 160η

10.0

12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0

28.0

Time (sec) Figure 9. Photo-DSC exotherms of a 25:75 trimethylolpropane tris (βmercaptopropionate) mixture with triethyleneglycol divinylether initiated by output of medium pressure mercury lamp with intensity of 60 mW/cn? in (a) nitrogen and (b) air. In Photoinitiated Polymerization; Belfield, Kevin D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

64


Practical Examples of Thiol-Ene Photocuring Before concluding, several practical examples from our lab that illustrate how thiol-ene photopolymerization can be used to extend the field of photocuring to new areas are briefly mentioned. First, thick optically clear samples of up to several centimeters thick were made by using various combinations of multifunctional acrylates, multifunctional triallyl ethers and a triallyltriazine with trimethylolpropane tris (β-mercaptopropionate). The trithiol/acrylate formed a very flexible elastic material with excellent energy storage while the trithiol/triallyltriazine formed a hard glass. Second, we found that a tetrafunctional thiol and a trifunctional vinyl ether sample (about 200 microns thick) with a small concentration of photoinitiator cured in air upon exposure to sunlight to give a highly crosslinked network in a few seconds. Third, we found that several thiol-ene mixtures with low concentrations of photoinitiator photocured in air to give highly crosslinked thin films (on the order of 1 micron) with relatively small total light intensities. Details of these practical systems will be forthcoming in future publications.

Conclusions Thiol-ene photopolymerization with/without added photoinitiator proceeds rapidly in both nitrogen and air. The results herein provide quantitative comparisons for the rapid polymerization of both stochiometric and nonstochiometric acrylate/thiol mixtures and a vinyl ether/thiol mixture in air and nitrogen. The acrylate/thiol mixtures are more susceptible to oxygen inhibition probably due, at least in part, to selective oxygen inhibition of the acrylate homopolymerization.

References 1. Marvel, C. S.; Chambers, R. R. J. Am. Chem. Soc. 1948, 70, 993. 2. Morgan, C. R.; Magnotta, F.; Ketley, A. D. J. Polym. Sci.: Polym. Chem. Ed. 1977, 15, 627. 3. Morgan, C.R.; Ketley, A .D. J. Polym. Sci.: Polym. Lett. Ed. 1978, 16, 75. 4. Jacobine, A. T. In Polymerization Mechanisms; Fouassier, J. P.; Rabek, J. F., Eds.; Radiation Curing in Polymer Science and Technology; Elsevier Science Publishers Ltd.; New York, 1993, pp. 219-268. 5. Gush, D. P.; Ketley, A. D. Mod. Paint. Coat. 1978, 68, 61.


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