Photoinitiated Polymerization - American Chemical Society

Chapter 11

Effect of Preorganization Due to Hydrogen Bonding on the Rate of Photoinitiated Acrylate Polymerization

Downloaded by CORNELL UNIV on July 29, 2016 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch011

Johan F. G. A. Jansen*, Aylvin A. Dias, Marko Dorschu, and Betty Coussens DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands *Corresponding author: email: [email protected]

The maximum rate of photo-initiated polymerization of acrylates is strongly influenced by the ability to form hydrogen bonds. In cases were the hydrogen bonds are in the vicinity of the acrylate moiety higher maximum rates of photo-initiated polymerization are observed. Pre-organization via hydrogen bonding is the mechanism by which this rate enhancement occurs as further proven by the following observations. There is a critical distance beyond which the propensity to form hydrogen bonds does not result in a rate enhancement. Temperature dependent RT-FTIR profiling demonstrated that in cases were hydrogen bonds are present anti-Arrhenius behavior is found. An increased amount of iso-tactic triads were found in the polyacrylates based on monomers capable of hydrogen bonding.

© 2003 American Chemical Society

Belfield and Crivello; Photoinitiated Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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128


Introduction Photo-initiated polymerization of methacrylates and acrylates, being one of the most efficient processes for the rapid production of polymeric materials with well-defined properties, is widely employed especially in the coatings area focused on decorative and protective coatings, stereolithography, dental restorative fillers and the preparation of contact lenses (1,2). In terms of improving the reactivity of the whole system, generally the photo-initiator (or mixtures thereof) has been adapted for obtaining higher reaction rates, which is by far the easiest method. The kinetics of the photoinitiated acrylate polymerization, which is a bulk polymerization has received less attention. Although photo-DSC (3) was already employed for the first time in 1979 by Tryson and Schultz in order to for monitor in real-time acrylate conversions, based on their enthalpy, true real-time kinetic data could only be obtained after the pioneering work of C. Decker who developed Real-Time InfraRed spectroscopy (RT-IR) in which the assumed acrylate enthalpy was eliminated (4). Currently both techniques are well established and employedfrequently(5) although new techniques are also emerging. Moreover, based on these kinetic results a wide variety of models has evolved (6) for instance bi-molecular termination combined with reaction diffusion control (7), mono-molecular termination in the glassy region (8), primary radical termination (9), chain length dependent termination (10), random-walk for heterogeneity (11), and primary cyclization (12) have all been proposed in order to describe cure profiles and network topology. However, none of the proposed models is capable of fully describing the kinetic profiles. The elucidation of different acrylate reactivities especially in relation to their molecular structure, however, received almost no attention. Andrzejewska et al. reported a hetero-atom effect (13). Guymon et al. found rate enhancements in liquid crystalline phases (14). Whereas Decker et al reported about new acrylates with a very high intrinsic reactivity (75). Although no explanation was given on a molecular level for the observed high intrinsic reactivities, the work of C. Decker triggered our research to evaluate different acrylates reactivities in terms of hydrogen bonding capability, being one of natures dominating organizational tools. The results are presented in this paper.

Results and Discussion In Figure 1 some of the fast reacting monomers initially prepared by C. Decker are shown. Although we can offer no explanation for the high reactivity of the oxazolidone and cyclic carbonate functional acrylate yet, the urethane ethyl acrylates suggest however, that the high reactivity of the urethanes could be attributed to hydrogen bonding. In order to test this concept, we performed some


129


Figure 1. Some of the reactive monomers prepared by C. Decker

initial work with undecyl amide N-ethyl-acrylate, being capable of hydrogen bonding and pentyl amide N-methyl N-ethyl-acrylate as non hydrogen bonding control compound. Employing Real Time Fourier Transform Infra Red spectroscopy (RT-FTIR) we determined the maximum rate of polymerization of these compounds (16). In good agreement with our expectations, based on the requirement of hydrogen bonding for high rates of polymerization, were the maximum rates of polymerization (undecyl amide N-ethyl-acrylate 9 mol/1 sec; pentyl amide N-methyl N-ethyl-acrylate 1.9 mol/1 sec). These encouraging initial results triggered us to systematically vary the nature of the moiety capable of hydrogen bonding i.e. amides (17), urethane, "inverted urethanes" and ureas and determine the consequences on reactivity. Analogous esters and carbonates were prepared and evaluated as control compounds which pocess no capability for hydrogen bonding. The synthesis of these compounds is depicted in Figure 2. Although the preparation of the amide ethyl-acrylates via the oxazoline route is very facile, purification of the end products however is very difficult due to the fact that during the synthesis approximately 1% of the corresponding ester ethyl-acrylamides are formed. As we were unable to remove this by product by either chromatography or distillation we abandoned this facile synthetic methodology. In order to obtain the compounds with purity's exceeding 99% (based on GC) we performed all the synthesis via the acid chloride, chloro formate or isocyanate routes. All the syntheses gave the crude compounds in yields between 70 and 90%. Most of the urea ethyl acrylates (except ethyl urea ethyl-acrylate) were solids at room temperature. As a consequence, the rates of polymerization of the ureas were determined at 50°C when all the samples were in their liquid state. In Table I the maximum rates of polymerization are shown. It is evident from Table I that all the monomers capable of forming hydrogen bonds exhibit 3-6 times higher polymerization rates when compared with their non-hydrogen bonding analogues, demonstrating that hydrogen bonding plays an important role in these polymerizations. The explanation for these higher rates of polymerization is that the hydrogen bonding of the acrylates facilitates a kind of pre-organization that forces the acrylate double bonds in close vicinity of each other, and consequently enhances the rate of



Figure 2.Synthetic routes to various acrylates


ο

131 Table I. Maximum Rates (mol/1 sec) of Photo-Initiated Polymerization of Monomers Capable of Hydrogen Bonding and their Counterparts, Measured by RT-FTIR using a Transflection Cell at Room-Temperature Employing a Light Intensity of 135mW /cm (medium pressure mercury lamp) 2

CH 23.5

CH 19.1

13.6

9.3

Ο

16.1

9.7

5

4.5

0

15.9

15.2

14.5

14.0

22.2*

18.2*

18.0*

17.9*

4.4

3.8

3.5

3.2

6.5

5.7

5.5

5.4

3

ο


Η

4

9

Ο

Η

Ο Ο

Η

S

25.2 Η

0

0

0

0

*Measured at 50°C due to their melting points

polymerization. However, an explanation based on pre-organisation via hydrogen bonding has several consequences: • • •

There should be a critical distance between the acrylate group and the hydrogen bonding moiety beyond which there is no effect on the cure speed. In contrast to normal Arrhenius type behavior, lower maximum rates of polymerization should be found at elevated temperatures. The tacticity of the polymer formed should be altered as the monomers are organized towards each other.

The effect on tacticity is also the final discriminator between inter- and intra-molecular hydrogen bonding. Only when inter-molecular hydrogen bonding is present, an effect on tacticity can be expected (18).


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Effect of Bridge Length A consequence of pre-organization is that when the hydrogen bonding moiety is put at a larger distance from the reactive acrylate group, there will be a critical distance beyond which the pre-organization, due to the hydrogen bonding will have little or no effect on the rate of polymerization. In order to evaluate this effect, we prepared several ethyl amide N-alkyl-acrylates in which the alkyl chain was varied in length i.e. from ethyl to hexyl. The results of the RT-FTIR study are shown in Table II. The rates of photo-initiated polymerization of these ethyl amid N-alkylacrylates are gradually reduced going from ethyl to pentyl. The ethyl amide Nhexyl-acrylate exhibits a rate of polymerization similar to the monomers not capable of forming hydrogen bonds i.e. ester and carbonate acrylates, reenforcing the pre-organization via hydrogen bonding theory.

Table II: Effect of Bridge Length on the Maximum Rates (mol/1 sec) of Photo-Initiated Polymerization of Monomers Capable of Hydrogen Bonding, as Measured by RT-FTIR /

0

23.5

3 18.5

2 21.8

4 16.8

5 5.0

Effect of Temperature Generally, if reactions are performed at higher temperatures, higher rates are observed, this is the so called Arrhenius behavior. However, if the high rate of polymerization is mainly due to pre-organization via hydrogen bonding, the rate will be reduced at higher temperatures, as it is well known that the level of hydrogen bonding is reduced at elevated temperatures. Therefore, we studied the cure behavior of undecyl amide N-ethyl-acrylate, at elevated temperatures. We chose this monomer in order to avoid any problems related to volatility. The results are shown in Table 3 and Figure III.

Table III: Effect of Temperature on the Polymerization rate of Undecyl amide N-ethyl-acrylate

Temp°C Rate

30 9

40 9.6

50 9.5

60 9

70 8.2

80 7.3

90 6.0



133

20

30

40

50

€0

70

80

90

100

Temperature (C)

Figure 3. Effect of temperature on the maximum polymerization rate of und amide N-ethyl-acrylate

As we are employing RT-FTIR we can monitor the amide shifts simultaneously. The amide II bond shiftsfrom1546 cm' at 30°C to 1539 cm' at 90°C and the N-H shifts from 3312 cm' at 30°C to 3325 cm' at 90°C. Both shifts are indicative of the fact that the amount of hydrogen bonding is reduced at higher temperatures. Both shifts are shown in figure 4. The reduction in polymerization rate, due to the elevated temperature, especially combined with the observed IR-shifts, provides strong additional evidence for the validity of the pre-organization via hydrogen bonding theory. It should be noted that in case of urethanes, we were only able to observe a shift towards non-hydrogen bonding at higher temperatures in the urethane N-H. Unfortunately, additional evidence based on other urethane absorptions could not be obtained due to overlapping signals. We also did not see any shift in the acrylate carbonyl, which would be the case if the hydrogen bonding was an intra-molecular process towards the acrylate carbonyl. However, as this shift could be obscured by non-bonding acrylate carbonyls this result only suggest that the hydrogen bonding occurs in an intermolecular fashion. 1

1

1

1

Effect on Tacticity Finally one can wonder about the impact of such a pre-organization phenomena on the tacticity of the resultant polymer formed. Assuming that the



2

0

3

0

4

0

8

6

0

7

0

8

0

9

0

100

2

0

3

0

4

0

5

0

0

0

70 Temperature fC)

8 0 8 0

Figure 4. Effect of temperature on the IR shifts of the amid moiety

temperature {C>

0


100

135


complete polymerization would occur via an organized structure, mainly isotactic polyaerylate would be formed. The formation of such an iso-tactic polyaerylate is depicted in figure 5. Whereas, generally in the case of polyacrylates prepared radically, predominantly syndio-tactic material would be produced.

Figure 5. Proposed mechanism by which pre-organisation via hydrogen bonding could lead to the formation of isotactic polymer. Although, it is very unlikely that in the case of pre-organization via hydrogen bonding all monomers would be organized, one could expect an enhancement in the amount of isotactic polymer formed when pre-organisation plays a dominant role. In general the tacticity can be better determined if methacrylates are used. Therfore, we performed these experiments with undecyl amid N-ethyl-methacrylate as monomer capable of hydrogen bonding, pentyl amid N-methyl N-ethyl-methacrylate as contol compound and lauryl methacrylate as reference sample. The results are shown in Table IV. The tacticity of poly-(lauryl methacrylate) produced via photo-initiated polymerization is identical to poly-(lauryl methacrylate) prepared with thermal initiators like AIBN (19). The tacticity of poly-(pentyl amide N-methyl N-ethyl methacrylate) is the same as for poly-(lauryl methacrylate) (within the errors of the measurements). This indicates that an amide moiety as such does not alter the tacticity of the polymer. Although in the case of poly-(undecyl amide N-ethyl methacrylate) no pure isotactic polymer is formed, the enhancement of the amount of isotactic triads is significant. This strongly suggests that preorganization via hydrogen bonding plays a role in how the acrylates are oriented towards each other during the polymerization. Moreover, this offers the evidence that the hydrogen bonding is indeed occurring in an inter-molecular fashion.


136 Table IV. Tacticity Data based on 13C-NMR of Poly-Methacrylates formed by Photo-Initiated Polymerization

Tacticity


Syndio-tactic (%) a-tactic (%) Iso-tactic (%)

pentyl amide lauryl Undecyl amide N~ N-methyl N- methacrylate ethylethylmethacrylate methacrylate 58

65

65

28 14

33 2

31 4

Conclusion In conclusion we have demonstrated that the maximum rate of photoinitiated polymerization of acrylates is strongly influenced by the ability to form hydrogen bonds. Pre-organization via hydrogen bonding is the proposed mechanism on a molecular level via which this rate enhancement occurs as further proven by the following findings. There is a critical distance above which the ability to form hydrogen bonds does not result in a rate enhancement. So the mere presence of a hydrogen bonding moiety in the molecule is not sufficient. Temperature dependent RT-FTIR profiling demonstrated that in case hydrogen bonds are present anti-Arrhenius behavior is found i.e. lower rates at higher temperatures in contrast to the observed and expected Arrhenius behavior of lauryl acrylate. Andfinallyan increased amount of isotactic triads are found in the polyacrylates based on monomers capable of hydrogen bonding compared to polyacrylates in which the monomers are not capable of hydrogen bonding.

Acknowledgement The management of DSM Research, DSM Coating Resins and DSM Desotech are kindly acknowledged for their permission to publish this work

Experimental section Most of the syntheses are rather straight-forward organic procedures, therefore, only one example is given. All the chemicals are used as obtained from Aldrich or other sources, except THF, which was distilled over Na^enzophenone, and pyridine, which was freshly distilled prior to use. The


137 RT-FTIR setup is described in detail elsewhere (16). A medium pressure mercury lamp was used as source or irradiation resulting in an intensity of 135 mW /cm at the sample, as measured with a Solatel Sola-scope 1. It should be noted that in contrast to most published RT-FTIR measurements the samples were not measured as laminates but under an inert atmosphere. Nitrogen was purged for at least 5 min before any RT-FTIR measurement. All RT-FTIR measurements were performed in triplicate including one with a purge time of 15 minutes. As the results were identical to those obtained with a 5 min purge, it was concluded that 5 min purging was sufficient for removing the oxygen. All RT-FTIR measurements were performed employing 1 weight % Irgacure 184 (2hydroxy-cyclohexyl phenyl ketone, HCPK, Ciba) as the photo-initiator. Curing the methacrylate samples for the determination of the tacticity was performed under nitrogen employing a Fusion curing rig equipped with F600 D bulbs. The tacticity was determined by C-NMR using a Bruker ARX 400 in DMSO-d6 at 80°C


2

,3

Synthesis of ethyl-O-urethane-N-ethyl acrylate: To a stirred cooled solution at 0 °C, purged with dry air, of 14.22g (107 mmol) N-2-hydroxy ethyl-O-ethyl urethane and 8.45g (107 mmol) pyridine in 300ml THF, was slowly added 10.1 g (112 mmol, 1.05 eq) acryloyl chloride whilst maintaining the temperature below 5 °C. After the addition was complete the reaction mixture was allowed to warm slowly to room temperature and was stirred at room temperature for an additional 6 hours. Work-up: The pyridinium salt was filtered off, followed by a quick wash with 20ml water, 0.1 Ν hydrochloric acid, saturated NaHC0 solution and brine (in case the separation was tedious diethyl ether was added). After drying over Na S0 . The THF was removed in vacuo yielding 6.1 g ( 33mmol, 31 %) ethylO-urethane-N-ethyl acrylate as crude product with a purity based on NMR of 97%. 3

2

4

Preferred alternative work-up: The pyridinium salt was filtered off, followed by removal of the THF at a vacuum of 10 mBar, yielding 18,2 g ( 97mmol, 90%) crude product with a purity based on NMR of 85%. Distillation procedure: 1 gram of the crude product was used to determine the boiling point of the acrylate by slowly raising the temperature at a vacuum of 2 mBar until all remaining THF, pyridine and acryloyl chloride was distilled of. At 135°C the boiling temperature remained constant for approximately 10 min before the acrylate started to polymerize in the distillation flask due to the prolonged heating. Next 15 g of the remaining crude product was distilled by quickly raising the temperature to 150°C, taking a large first run, followed by the product. As soon as any polymerization was noticed or suspected the distillation was stopped yielding 9 g product, which still contained some trace impurities presumably due to the stabilizer used in the acryloyl chloride. In order to obtain pure product, i.e. no other signals detected on GC, the distillation had to be repeated three times after which only 2 gram (10% yield) of pure ethyl-Ourethane-N-ethyl acrylate was obtained.


138 H-NMR (CDCI3, 200MHz): δ (ppm) 6.44 (dd; 1H; J=17.22; J=1.48), 6.15 (dd; 1H; J=10.33; J=17.22), 5.88 (dd; 1H; J=10.33; J=1.48), 5.72 (s br; 1H; NH), 4.23 (m; 4H), 3.21 (quintet; 2H; J=6.77), 1.14 (t; 3H; J=6.85); C-NMR !

13

(CDCI3,50MHz): δ (ppm) 166.7(s), 157.0 (s), 132.0 (t), 128.8 (d), 63.6 (t), 63.0 1


(t), 36.5 (t), 15.8 (q) ; IR (neat): abs. cm , 3345 (N-H), 1728 (C=0), 1534 (NHdef),810(C=C-H). All liquid compounds were distilled at least three times before they were obtained in a pure form, except the ureas, which were recrystalized three times. In case a bulb-to-bulb distillation is used approximately 5 distillations are required, however the overall yields are significantly higher as lower temperatures are employed (for ethyl-O-urethane-N-ethyl acrylate 23% overall yield).

References and Notes: 1.

a) Nie, J.; Rabek, J. F.; Linden, L.-A. Polym. Int. 1999, 48, 129.; b)

Narayanan, V.; Scranton, A. B. Trends in Polym. Sci. 1997, 5, 415.; c) Lovell, L. G.; Stansburry, J. W.; Syrpes, D. C.; Bowman, C. N.

Macromolecules 1999, 32, 3913. and references cited therein. 2. Kloosterboer, J. G. Adv. Polym. Sci. 1988, 84, 1. 3. 4.

Tryson G.R. , Schultz A.R, J. Polym.Sci.,Polym. Phys. Edn. 1979, 17, 2059. See for instance: Decker C., Elzaouk B., Decker D., J. Macromol Sci; Pure

Appl. Chem. 1996, A33, 173. and references cited therein. 5.

a) Doornkamp, A. T.; Tan, Y. Y. Polym. Commun. 1990, 31, 362.; b) Lecamp, L.; Youssef, B.; Bunel, C.; Lebaudy, P. Polymer 1999, 40, 1403.; c) Khudyakov, I. V.; Legg, J. C.; Purvis, M. M.; Overton, B. J. Ind. Eng.

Chem. Res. 1999, 38, 3353.; d) Selli, E.; Bellobono, I. R. in "Radiation

Curing in Polymer Science and Technology", Fouassier, J. P.; Rabek J. F. eds, Elsevier , London, volume 3, pl. e) Decker, C.; Masson, F.; Bianchi, C. Polym. Prepr. 2001, 42, 304.; f) Decker, C.; Alzaouk, B.; Decker, D. J. Macromol. Sci.A Pure Appl. Chem. 1996, A33, 173.; g) Yamada, B.; Azukizawa, M.; Yamazoe, H.; Hill, D. J. T.; Pomery, P. J. Polymer, 2000, 41, 5611.; h) Seno, M.; Fukui, T.; Hirano, T.; Sato, T. J. Polym. Sci., A Polym. Chem. 2000, 38, 4264.; i) Doetschman, D. C.; Mehlenbach, R. C.; Cywar, D. Macromolecules 1996, 29, 1807. 6. Panke, D. Macomol. Theory Simul. 1995, 4, 759. 7. Anseth, K. S.; Bowman, C. N. Polym. React. Eng. 1993, 1,499.; b) Anseth, K. S.; Wang, C. M.; Bowman, C. N. Macromolecules 1994, 27, 650. 8. Wen, M.; McCormick, Α. V. Macromolecules 2000, 33, 9247. 9. Goodner, M. D.; Bowman, C. N.Macromolecules 1999, 32, 6552. 10. Berchtholt, K. A. ; Lovell, L. G.; Nie, J.; Hacioglu, B.; Bowman, C. N.

Polymer 2001, 42, 4925 11. Hutchinson, J. B.; Anseth, K. S. Polymer Preprints 2000, 41, 1326.


139 12. Elliot, J. Ε., Bowman, C. N.

Macromolecules 1999, 32, 8621.

13. Andrzejewska, E.; Andrzejewski, M. J. Polym. Sci. A, Polym. Chem. 1998,

36, 665. 14. a) Guymon, C. Α.; Hoggan, Ε. N.; Clark, Ν. Α.; Rieker, T. P. Walba, D. M.; Bowman, C. N. Science 1997, 275, 57 ; b) Guymon, C. Α.; Bowman, C. N.


Macromolecules 1997, 30, 5271 15. Decker, C.; Moussa, Κ. Makromol. Chem. Rapid Commun. 1990, 11, 159. 16. For a detailed description about the RT-FTIR equipment used see: Dias Α.Α., Hartwig H., Jansen J.F.G.A., Surface Coatings International, JOCCA, 2000, 83, 382. 17. Some data about amid ester acrylates, see for instance Jansen J.F.G.A. WO patent 98/33855, 1998 and Jansen J.F.G.A. European Patent 0970977A1 1997. 18. Based on molecular modeling using Insight II with AM1 as semi-empirical force field it was calculated that the intra-molecular hydrogen bonding structures posses a higher energy level compared to their inter-molecular hydrogen bonding analogues, reducing the possibility that the effect on the rate is caused by intra-molecular hydrogen bonding. 19 Pham,Q.-T.; Petiaud, R.; Waton, H.; Llauro, M.-F.; Proton and Carbon NMR spectra of Polymers; J. Wiley & Sons: New York, 1984; vol 3. and references cited.


Photoinitiated Polymerization - American Chemical Society

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