Quaternary Ammonium N,N-Dimethyldithiocarbamates as Photobase

Chapter 30 Quaternary Ammonium N,N

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- D i m e t h y l d i t h i o c a r b a m a t e s as P h o t o b a s e G e n e r a t o r s

Masahiro Tsunooka, Hideki Tachi, Takayuki Yamamoto, and Masamitsu Shirai Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan

Photo-chemical behavior of quaternary ammonium dithiocarbamates ( Q A salts) as photobase generators (PBGs) and photo-initiated thermal isolubilization o f epoxides with the Q A salts were investigated. Stability of Q A salts in various solutions was also examined. They were stable in acetonitrile, D M F , D M S O and aq. alkaline solution, though their structure affected their stability. Q A salts acted as effective photo-initiated thermal crosslinker for poly(glycidyl methacrylate). 4Chlorothioxanthone, phenanthrene, and benzophenone were found to be sensitizers for the photolysis of Q A salts.

Recently, photobase generators (PBGs) which generate basic compounds such as amines upon U V irradiation have become very attractive compounds similar to well accepted photoacid generators (PAGs) in the fields o f U V airing and microlithography (1,2). The papers on PBGs are, however, very few compared with those on PAGs (3-7). Tertiary amines are much stronger bases than primary and secondary amines, and photo-generated tertiary amines are expected to be very promising in their use as catalysts in the above fields. Most papers on PBGs, however, are those on photo-generation o f primary or secondary amines from them, and only a few papers on PBGs which generate tertiary amines were published. NJ/JN-

© 2003 American Chemical Society

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

351


352 trimethylbenzhydrylanunonium iodide (8), quaternary ammonium carboxylic acid and quaternary ammonium borates (9,10) were reported to generate tertiary amines photochemically. In previous papers (11-15), we have already reported that the quaternary ammonium ^^-(hmethyldithiocarbamates (QA salts) act as PBGs which produce tertiary amines such as 1,4-diazabicyclo[2,2,2]octane (DABCO) on UV irradiation. They acted effectively as photo-initiated thermal crossJinkers for poly(glycidyl methacrylate) (PGMA). We have also reported that the biftmctional Q A salt having ^^-dimethyldithiocarbamate anions, which was derived from Nfljfjf,tetramethyl hexamethylenediamine, acted as an effective photo-initiated thermal crosslinker for epoxy compounds (11). Counter anions of QA salts affected their photolysis and the efficiency of photo-induced thermal crosslinking of PGMA also depended on the anions. In these papers, QA saltstearingdithiocarbamate anions derivedfromDABCO were mainly examined. In this chapter, we describe effects of #-methylpiperidine or DABCO moieties on photolysis of QA salts and their efficiency as photo-initiated thermal crosslinkers for PGMAfilms.Furthermore, the effects of photo-sensitizers on their photolysis of QA salts were also investigated

EXPERIMENTAL Material QA salts having dithiocarbamate anions were prepared in a similar manner described in the previous paper (11). That is, QA salts having a bromide anion were prepared by the reaction of phenacylbromide and TV-methylpiperidine. The anion exchange of the salts was carried out using sodium NJVdimethyldithiocarbamate. Structures and properties of QA salts are summarized in Table 1. Poly(glycidyl methacrylate) (PGMA) (Mn=104000, Mw/Mn-2.04, Tg-84.0 °C) was prepared by solution polymérisation of glycidyl methacrylate in benzene using 2,2-azobisisobutyronitrile as initiator. Benzophenone (Bp), 4chlorothioxanthone (CTX), phenanthrene (Phn) and anthracene (Ant) were used as sensitizers for photolysis of QA salts after purification. ,

Instruments U V spectra were obtained on a Shimadzu UV-2400PC spectrometer. H NMR spectra were measured on a JEOL GX-270 (270 MHz). Film thickness was determined by two-beam interferometry using a metallurgical microscope (Nikon !


353 ΟΡΉΡΗΟΤΟ XPF-UM), an interference apparatus (Nikon) and a video microscaler (FORA IV-550). Thermal gravimetry was conducted on a Rigaku Denki DSC-8230 at a heating rate of 10 °C /min.

Tablet Structure and Propeties of QA Salts

W

^,,=21700,

\h

£295=11900


0

QAsattU

s CMJ

/W - O - - " - - ^

VÇ-N^

112

i,

«2,2=48300, 6

^,9,00

6296=16200

6259=15800,

H

QAsaltIV

/ - * 0 ^ ©

QAsaltV

Q ^ ^ O

P>

^ "ί i '

1 1 9

no

1 3 1

Π1

PH

v

f"|

8251=25000,

3

108

^

* Measured by TGA

Photo-irradiation method and Photo-initiated thermal insolubilization of PGMA films The polymer thinfilms(ca. 0.3 μηι or 0.5 μιη) with 5 mol % of QA salt were prepared by casting tetrahydrofuran (THF) or chloroform solutions of polymers and salts on a glass plate. U V irradiation was carried out with a low-pressure mercury lamp (Ushio Denki ULO-6DQ, 6W) (light intensity : 1.0 mW/cmT at 254 nm) or a medium-pressure mercury lamp (Ushio Denki UM-102, 100W) in air or under N at room temperature. The light at 366 nm from the medium-pressure mercury lamp was selected by the use of glassfilter(Toshiba glass, UV-36B). The light intensity was determined by the light illuminometer (ORC UV-M02). If necessary, the irradiatedfilmswere followed by baking at a given temperature on a hot plate. After the irradiation or baking, thefilmswere soaked in THF for 10 min at room temperature. The insolublefractionof irradiated polymer films with QA 2


354 salts was calculated from the ratio of the film thickness determined by the interference microscope before and after the soaking.

Analysis of the photo-products of QA salt having N-methylpiperidine moiety by H-NMR spectroscopy J

H-NMR spectra were measured in GDCI3 in a similar manner as described in the previous paper (11). The photolysis of the QA salt was monitored as a decrease at 4.40 ppm due to methylene protons of the QA salt. Mmethylpiperidine and the M#-dimethyldithiocarbamate derivative as photo-products were detected by the appearance of signals at 2.18 ppm and 4.82 ppm, respectively. Downloaded by RUTGERS UNIV on March 2, 2016 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch030

l

Estimation of half-life of QA salts in solvents QA salts were dissolved in various solvents such as water, methanol, acetonitrile, DMF, DMSO and chloroform. U V spectra of the solution (1.0 χ 10" M) were measured after a given time. The half-life of the QA salts was determined by a decrease of absorption band at around 290 nm due to the NJfdimethyldithiocarbamate anions.

4

RESULTS AND DISCUSSION

Photo-chemical behavior of QA salts In previous papers (11,12), we have already reported that QA salts I and II acted as PBGs which generate tertiary amines. The formation of basic compounds in the photolysis of QA salt I was ascertained by a color change of Phenol Red from yellow to red. ^-NMR spectral changes of QA salt I on U V irradiation showed a decrease in a peak at 4.21 ppm due to QA salts and an increase of peaks at 2.81 ppm due to DABCO and at 5.02 ppm due to phenacyl N,Ndimethyldithiocarbamate. These facts show that the photolysis of QA salt I results in the formation of DABCO and phenacyl JV^-dimethyldithiocarbamate as shown in the next equation. QA salts I and Π have an unblocked tertiary amino group. However, the effect of the unblocked amino groups on their photolysis was not investigated in the previous paper. Thus, in this chapter, photolysis of QA salt V which was derived from a mono-fiinctional amine was examined.


355

\—f

Q

\-—/

o f

—

S

Τ

Y-

S

5.02 ppm

/

Τ

4.21 ppm

2.81 ppn

Analysis of photo-products of QA salt V Fig. 1 shows *H-NMR spectral changes of QA salt V on irradiation at 366 nm under N . A decrease of peak at 4.40 ppm due to methylene protons shows the photolysis of QA salt V, and the peaks at 2.18 ppm due to W-methylpiperidine and 4.82 ppm due to A^V-dimethyidithiocarbamate derivatives increased with irradiation The amounts of photolyzed QA salt V and photo-products were calculatedfromthe NMR data. As shown in Table 2, the amounts of the resulting jV-methylpiperidine and MN-dimethyldithiocarbamate derivatives agreed approximately with the amount of decomposed QA salt V. These facts show that QA salt V quantitatively generated JV-methylpiperidine and iV,JVdimethyidithiocarbamate derivatives as photo-products. These results also agreed with those of QA salt I (11), and it can be concluded that the photolysis of these onium salts did not depend on their onium structure.


2

Table 2. Photolysis of QA Salt V in CDC1 and Yields ofPhoto3

Irradiation energy

Conversion of QAsaltV(%)

(mJ/cm ) 1000 40 2000 74 a) NMR analysis b) A : N-methyipiperadine, Β : phenacyi Ν,Ν-dimethyldithiocarbamate 2

Photo-Products (%) b) b> A

36 65

B

40 73

Thermal stability of QA salts and their stability in various solvents QA salts I-V are thermally stable to 110-150 °C depending on their structures as shown in Table 1. However, as they were unstable in some solvents, their stability in solvents was investigated. UV spectral changes of QA salt I kept in methanol at room temperature were very similar as those of its photolysis, though the rate was very slow compared with that of its photolysis. The decomposition of QA salt I in chloroform was followed by H-NMR. Products were the same as photo-products reported previously (11). l



356

Figure 7. Ή-NMR spectral changes ofQA sait Vin CDCI3. Irradiation at 366nm wider FI * [QA sali]- L88x 1CT (mol/l). (Reproduced with permission from reference 34. Copyright 1994 Wiley.) 2


357 Table 3 shows the half-life of QA salts determined from a decrease of the absorption at around 290 nm in various solvents. Although QA salts were stable in polar solvents such as acetonitrile, water, DMF, and DMSO, they were unstable in MeOH and chloroform. In water, they were unstable under pH=7 and the presence of a base such as amines increased their stability. The effect of substituent groups such as phenacyl and benzyl in QA salts on their half-life was also observed. Q A salt V derived ΛΓ-methylpiperidine was unstable compared with QA salt I having a DABCO moiety in solution except for DMSO.


Table 3, Half-Life of QA Salts in Solution

a)

Half-life (hour) DMF DMSO chloroform watermethanol acetonitrile 1.4 1.2 4.2

38.2 21.0 34.0

22.6 14.5 _J0

38.7 60.8 _t>)

2.2 7.2 7.2

22.0 2.1 7.4 2.2 OA salt V a)ISah] = 1.0Xi0" (moyi) b) no data.

35.0

__b)

_>

11.0

19.3

16.9

49.4

4.1

QA salt I QA salt I Q A salt HI

39.0 4.1 12.0

QA salt Ν

b

4

Photo-initiated thermal insolubilization ofPGMA films Photo-initiated thermal insolubilization of PGMA films with 5 mol% of QA salts I-IV was investigated (Table 4). The films with QA salts I and Π turned insoluble with only irradiation at 254 nm, but the photo-insolubilization of the films with other QA salts were not observed even at 60 mJ/cm . The degree of insoluble fraction of the irradiatedfilmswas increased by post-baking at 80 °C as shown in Table 4. The photolysis rate of QA salt I was faster than that of other QA salts, and the efficiency of photo-initiated thermal insolubilization of PGMA films decreased in the order of QA salt I > QA salt II > QA salt ΠΙ > QA salt IV. The photolysis of QA salt I was fastest among them. However, not much difference in their photolysis rate except QA salt I was observed. Thus, further studies on the quantitative yields of products in their photolysis are required for an understanding of the crosslinking rate. The effect of ammonio groups of QA salts on photo-initiated thermal insolubilization of PGMA films was investigated. Fig.2 shows photo-initiated insolubilization of PGMA films with QA salts I or V. The efficiency of photo- and post-thermal insolubilization of PGMA films with QA salt I was higher than that with QA salt V. It is apparent that QA salt I worked more effectively than QA salt V in the insolubilization. As there is no difference in their photolysis rate, the 2



Ο

50

100

150

200

Irradiation energy (mJ/cno?)

Irradiation energy (mJ/cn?) Figure 2. Effect of ammonio groups on (a) photo-insolubilizaiion and (b) post-thermal insolubilization o/PGMA films. (•,•) QA salt Ι, ( Ο , · ) QA salt V.


359 above result is thought to be due to the difference in basicity and nucleophilicity of resulting amines. The pKa of iV-methylpiperidine is 10.1 and that of DABCO 8.7 and 3.0, and this result was not explained by the difference in their basicity. Imidazole is known as a good curing reagent irrespective of its low basicity (pKa=6.92). Thus, bifunctionality of DABCO and two lone pairs on nitrogen without steric hindrance are thought to be more effectiveforthe crosslinking. Table 4. Photo-Initiated Thermal Insohibilization of PGMA Funis with QA Salts


QAsalt

% Insohibilization Irradiation Energy Post-Baking at (mJ/cm ) at 254 nm Only Irradiation 80 "C for 5 min

I II m IV

2

30 60 30 60 30 60 30 60

60 60 30 35 0 0 0 0

95 100 80 80 65 80 60 70

[Salt]-5(mol%).

Effects of sensitizers on the photolysis of QA salts From the viewpoint of effective use of energy, the efficient use of U V light at 366 nm emitted from the medium-pressure mercury lamp is very attractive. Fig.3 shows the relationship between the photolysis of QA salt I in CDG1 under N . The presence of CTX, BP, and Phn promoted the photolysis of QA salt I, though the effect of Ant was not observed. The activity of photosensitizers in the photolysis decreased in the order of C T X > BP >Phn. The yield of DABCO depended on the degree of the photolysis of QA salt I. On the basis of their excited triplet energy levels: EKBP)=69.0, ET=42.0 (kJ/mol), and the order of their molecular absorption coefficient at 366nm:CTX>BP>Phn, it is thought that the sensetization was induced by those QA salts which have triplet energy higher than about 60 kJ/mol and that the apparent order of their sensitization depended on the order of their absorption coefficient. Furthermore, the photolysis of QA salt I was depressed in air. These results suggest that the photosensitization occurs by triplet-triplet energy transfer. Fig.4 shows photo-initiated thermal ^solubilization of PGMA films with Q A salts and photo-sensitizers at 366 nm irradiation. In this experiment, post-baking 3


2


Ο

10

20

Irradiationtime(min) Figure 5. Photolysis of QA mit I in the presence ofsensitizers under (a) Decomposedfraction ofQA sait I with photo-sensitizers. φ) Yield of amine detectedfromirradiated solution ofQA salt/. (X) with CTX, (Δ) with Bp, (•) with Ant, (O) withPhn, (O) without sensitizer. [QA saltl]~[sensitizer]=3xl0 (mol/l). 2


361


was not carried out. CTX was effective for insolubilization of PGMAfilms.On the other hand, the films with BP and Phn were not insolubilized at this irradiation energy, because their absorption coefficient at 366 nm was very small or zero, respectively.

0

100

200

300

Irradiation time (sec) Figure 4. Photo-insolubilization of PGMAfilmswith 5 mol% of QA salt I at 366 nm. (•) without sensitizer, ( O ) with CTX, ( Δ ) with Phn, (·) BP.

CONCLUSIONS We have investigated the photolysis of QA salts bearing onium moieties derived N-methylpiperidine or DABCO and an Ν,Ν-dimethylditWocarbamate anion. They were thermally stable under 100 °C. They were also stable in acetonitrile, DMF, DMSO and H 0 , though their stability depended on their structure. QA salt I having DABCO moiety was more stable than QA salt V having N-methylpiperidine moiety in the solvents except DMSO. All QA salts became stable at pH>7 in aq. alkaline solution. Photolysis of QA salt I was sensitized by CTX, BP and Phn. The presence of CTX made it possible for PGMA films with QA salt I to become insoluble at 366 nm irradiation. The QA salts were proved to be useful PBGs in photo-initiated thermal insolubilization of epoxides. 2


362 Acknowledgement : This work was supported by a Grant-in-Aid for Scientific Research (c) (10650870)fromthe Japan Society of the Promotion of Science.

REFERENCES 1. 2.


3.

4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15.

Fouassier, J. P. Photoinitiation, Photopolymerization and Photocuring: Fundamentals and Applications; Hansen Munich, 1995; pp 102-245. Fouassier, J. P.; Rabek, J. F. Radiation Curing in Polymer Science and Technology; Elsevier Applied Science: London, 1993; Vol. 2, pp 435-554. Crivello, J. V.; Dietliker, K. Photoinitiators for Free Radical, Cationic & Anionic Photopolymerization; Bradley, G., Ed.; Wiley: New York, 1999; pp 329-566. Frechet, J. M. J. Pure Appl. Chem. 1992, 64, 1239. Shirai, M.; Tsunooka, M. Prog. Polym. Sci. 1996, 21, 1. Shirai, M.; Tsunooka, M. Bull. Chem. Soc. Jpn. 1998, 71, 2483. Shirai, M.; Suyama, K.; Tsunooka, M. Trends Photochem. Photobiol. 1999, 5, 169. Hanson, J. E.; Jensen, K. H.; Gargiulo, N.; Motta, D.; Pingor, D. A.; November, A. E.; Mixon, D. A.; Kometani. J. M.; Knurek, C. Polym. Mater. Sci. 1995, 72, 201. Kaneko, Y.; Sarker, A. M.; Neckers, D. C. Chem. Mater. 1999,11,170. Sarker, A. M.; Kaneko, Y.; Neckers, D. C. J. Photochem. Photobiol. A Chem. 1999,121,83. Tachi, H.; Yamamoto, T.; Shirai, M.; Tsunooka, M. J. Polym. Sci. Part A Polym. Chem. 2001, 39, 1329. Tsunooka, M.; Tachi, H.; Yamamoto, T.; Akitomo, K.; Shirai, M. J. Photopolym. Sci. Technol. 2001, 14, 153. Tachi, H.; Tsunooka, M. J. Photopolym. Sci. Technol. 2000, 13, 153. Tsunooka, M.; Tachi, H.; Shirai, M. Proc. RadTech 2000 North America, Baltimore, 2000;p358. Tachi, H.; Tsunooka, M. J. Photopolym. Sci. Technol. 1999,12,313.


Quaternary Ammonium N,N-Dimethyldithiocarbamates as Photobase

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