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Photopolymerization of methyl methacrylate in carbon tetrachloride solution using 4-isopropyl-4′-N,N- dimethylaminobenzophenone as chromophore
J. Photochem. Photobiol. A: Chem., 67 (1992) 353-361
3.53
Photopolymerization of methyl methacrylate in carbon tetrachloride solution using 4-isopropyl-4’-N, Ndimethylaminobenzophenone as chromophore P. Boschr
and
J. L.
Mateo
Institute de Ciencia y Tecnologia de Polimeror, Juan de la Cierva 3, 28006 Madrid (Spain) (Received
January 23, 1992; accepted March 12, 1992)
Abstract The polymerization of methyl methacrylate (MMA) in solution using 4-isopropyl-4’-N,Ndimethylaminobenzophenone (CUMI) as photoinitiator shows a particular behaviour when Ccl, is used as solvent. The MMA polymerization rate is much higher than that obtained in other solvents (benzene, tetrahydrofuran, cyclohexane and chloroform). In the absence of monomer, quantum yields of CUM1 disappearance higher than unity are found. These results and the analysis of the products obtained after irradiation support the view of a charge-transfer compIex formation between CCll and the triplet excited state of CUMI. Disproportion of the complex leads to Cl a and Ccl,’ radicals, which are responsible for the polymerization initiation step. A suitable mechanism is proposed.
1. Introduction The
photochemistry
and
dark
reactions
of amines
in halomethane
solution
have
been widely studied, even though the process is far from being completely understood. Amines have been shown to form 1:l charge-transfer complexes with halomethanes in the ground state [l-4] through some equilibria, the extent of which depends on the donor and acceptor characteristics as well as on the polarity and solvating capacity of the solvent (Fig. l)_ The charge-transfer process has been shown to be responsible for the marked instability of solutions of amines in CCL, both in the dark and under UV irradiation [5-7]. It is not well established whether the complexes are formed from the first singlet A-I-D
e
[A
. . . . . D]
e
AA=
Acceptor
D=
Donor (amine)
]
-I- D+
(halomethane)
Fig. 1. Charge-transfer
complex equilibria between
+Author to whom correspondence
lOlO-6030/92/$5.00
[A-D+
halomethanes
and amines.
should be addressed.
0 1992 - Elsevier Sequoia.
All rights reserved
354
and/or triplet state of the amine [S, 91, since the excited state involved will also depend on the nature of the solvent [lo]. Light-induced reactions between halomethanes (especially Ccl,) and other functional groups such as ketones [II, 121 and pyridine-SO2 complexes 1131 have also been reported. The subsequent reaction has been proposed to occur by disproportion of the complex, forming either Cl’ and CC13’ radicals [14] or real contact ion pairs [lo]. In the presence of monomers, some examples have been given of “spontaneous” polymerization (in the dark and with T<30 “C) initiated by a CC&.amine combination [15-171 or even by Ccl4 alone [18]. UV irradiation of ketone-amine combinations in the presence of a monomer and a polyhalogenated solvent leads to a notorious increase in the rate of polymerization compared with other non-halogenated solvents [19, 201. The purpose of this paper is the study of methyl methacrylate (MMA) polymerization using Ccl4 as solvent and 4-isopropyl-4’-N,N-dimethylaminobenzophenone (CUMI) as photoinitiator. It has been necessary to gain a better insight into the photochemical behaviour of CUMI-Ccl4 and CUMI-MMA-Ccl4 solutions, which leads us to consider a different process to create radicals from that described previously for this aminobenzophenone 121, 221.
2. Experimental.
details
2.3. Materials MMA from Fluka was washed three times with portions of 10 vol.% aqueous sodium hydroxide and then three times with distilled water. The monomer was dried over anhydrous magnesium sulphate and distilled under reduced pressure prior to use. Benzene and the other solvents were spectroscopic grade from Aldrich and were used without further purification. Carbon tetrachloride (fluorescence grade from Carlo Erba) was shaken vigorously with a saturated KOH solution and washed first with water, then with sulphuric acid until no colour was found and finally with water again. It was then refluxed over P,05 for a few hours and distilled. 2.2. Polymerizatiun procedure This procedure was performed according to the method described in ref. 22. The rates of all photoinitiated polymerizations were corrected to allow for the changes from run to run of the incident light intensity, assuming in all cases a square root dependence of light intensity_ 2.3. Steady
state
irradiation
procedure
The kinetics of disappearance of the charge-transfer band of CUM1 during irradiation was carried out by taking readings of absorbance at A = 335 nm, at different times of irradiation, such as described in ref. 21. CUM1 concentrations were in the range 10-4-10-5 M. Sa mples were irradiated at h =365 nm both in air or degasified, the latter by bubbling nitrogen until complete evacuation of the oxygen dissolved in solution. UV-visible spectra were recorded using a Shimatzu UV-256 FS spectrophotometer. 2.4. ‘H nuclear magnetic resonance spectra The behaviour of CUMI-CCk, solutions under irradiation was monitored on a Brucker model AM-200 MHz. The solution ([CUMI] = 10m2 M in CC14, with an external
355
dimethyl sulphoxide-&- reference) was placed in a nuclear magnetic resonance (NMR) tube, degasified by bubbling- nitrogen and irradiated at A=365 nm. Several spectra of this sample were recorded at different stages of irradiation. 2.5. Gel permeation chromatography measurements The molar weight distribution was measured as described previously [23]. A 440 Waters absorbance detector was used to detect chromophore end-chain groups. analysis 2.6. Gas chromatography Non-quantitative gas chromatography analyses were done in a Hewlett-Packard M CUM1 in CCL, was 5890 series II, with Tinjection=100 “C. A solution of 1.3 X lo-’ irradiated at A=365 nm until complete disappearance of the CUM1 band in the absorbance spectrum_ The solution was evaporated after irradiation and redissolved in 4 ml of n-hexane. A reference was prepared by dissolving CL&-CC13 in CCL+, evaporating the solution and redissolving it in n-hexane.
3. Synthesis CUM1 was prepared as described in ref. 24. 4-Isopropyl-4’-N,N-dimethylaminobenzophenone hydrochloride was prepared by bubbling pure HCl into a 0.1 M solution of CUMI in diethyl ether. When the solution became cloudy, the solid was filtered off and purified by preparative thin layer chromatography. IR (BrK): 3100-2900 cm-’ (C-H st); 2850-2800 cm-’ (N-CH2 St); 2700-2000 cm-’ (N+-H st); 1650 cm-’ (C=O st).
4. Results
and
4.2. Irradiation
discussion
in the absence
of mummer
UV absorption spectra of CUMI-CC& solutions did not show, over a wide range of concentrations, charge-transfer wmplex formation between CUM1 and CC4 in the ground state. UV spectra in these solutions were similar to those in other solvents (benzene, cyclohexane, tetrahydrofuran and chloroform) not only in the shape of the band but also in the h,, and E values (Table 1). TABLE
1
I-Isopropyl-4’-NJ+dimethylaminobenzophenone Solvent
x max
absorption
characteristics Q
(nm) Cl& Acetonitrile Benzene
335 341 340
19100 20400 21100
Cyclohexane Chloroform Tetrahydrofuran
330 349 337
22800 22500 20900
in several
solvents
356
These solutions were stable in the dark for long periods of time. Degassed solutions were irradiated at h =365 nm and an extremely rapid disappearance of the CUM1 charge-transfer band was observed. Aerated solutions were then irradiated in the same conditions and the same behaviour was obtained (Fig. 2). A rapid decrease in the CUM1 band with Q,d,sap> . 1 was found, and the formation of a new product detected_ This main product of the reaction was relatively stable iii solution but rapidly decomposed during isolation. It was identified by comparison with an authentic sample such as CUM1 hydrochloride 2 (Fig. 3). The formation of amine hydrochlorides in Ccl4 solutions has been reported by other workers [I, 251. The CUM1 disappearance quantum yield in the presence of oxygen increased when the carbon tetrachloride concentration increased (Table 2). All these data indicated a completely different behaviour from that observed by us for these types of compound in other solvents, including a halogenated solvent such as chloroform [Zl-231. The irradiation was also carried out under nitrogen in a 5 mm NMR tube, and the evolution of the reaction was monitored VS. increasing irradiation time (Fig. 4). The modification of the NMR spectra was in accordance with the presence of CUMI hydrochloride since the aromatic protons adjacent to the nitrogen suffered a significant alteration. Other signals appeared, confirming that structure (S=6.6 ppm corresponding to a N+H proton and S =7.2 ppm corresponding to its aromatic ortho protons). Two new singlets appeared at S- 3.2 and S-5.2 ppm, which were assigned to N-CH2-Cl and N-C!H2CC13 protons respectively, corresponding to structures 3 and 4
Abs
0.5
4 0
3&o
2;0
A Fig.
2.
curves
UV
absorption
is 5 s.
spectra
of
CUM1
nm in Cl,C
at various
stages
of
irradiation.
ti
between
357
*COeN:cH2c1
3
Fig. 3. Chemical structures of CUM1 (1) and the products identified after irradiation_ TABLE 2 4-isopropyl-4’-N,N-dimethylaminobenzophenone disappearancequantum yield in benzene solution as a function of Cl& concentration ([CUMI] = 10S4 M, 1,=1.114X 10L6 Einsteins s-l 1-l) W4Cl (mol 1-l) 3x10-5 10-4 2x 10-4 10-3 Neat Neat+2.8
0.047 0.145 0.250 0.735 1.350 0.700
M MMA
of Fig. 3. SimuItaneously with the appearance of these signals, a proportional decrease in the integral of the N-Cl& protons was observed. Finally, the irradiation mixture was analysed by gas chromatography where measurable amounts of hexachIoroethane were detected. Considering these experimental resnIts and previous mechanisms described in the literature for similar processes [5, 12, 26, 271 we can propose the following mechanism: CUM1 = R-rjr-CI&
-%
%UMI
=
3CUMI
(1)
CH3 Tuna
R-YCH3
2%
~pzmv41...Cl.qc]
+ c1’ -
CH3 R-y-CHS cw3
R-‘i”tiHz
-
CUMI+Cl’cCi~C’
+ HCI
(2) (3)
C-3 + CC&- -
R-y&H= CHJ
+ HCCI,
(4)
358
0 min
‘irr
%r 30
I
I
0
7
min
I
I
6
I
I
5
4
3
I
2 6
Fig. 4. ‘H NMR
(200
MHz)
R-YcH3 tHc’CH3
R-y&H,
+ CLC -
spectra
I
I
1
0
ppm
of the reaction
mixture
before
and after
30 min irradiation.
R7N
(5)
R-y-CH,Cl
(6)
CH3
CH3
+ Ccl;
CH3
CH3
R-fiT-i3Hz
+ CL’ or CQ’
-
R-y-CH,-Cl CH3
CH3 2C13C’
-
Cl,C-ccl,
where
ISC
indicates
or R-TECH,CC&
(7)
CH3 (8)
intersystem
crossing.
359
The CUM1 triplet and not the singlet is proposed as the reacting species since intersystem crossing is a very efficient reaction for this type of compound. Other workers did not conclude which excited state was reactive, and a reaction from the singlet state does not seem plausible in this particular case. The reaction between the CUM1 triplet and carbon tetrachloride to form the complex (reaction (2)) is faster than the quenching of the triplet by oxygen as the reaction proceeds in air. In the presence of oxygen, disappearance quantum yields higher than unity could be explained by more hydrogen chloride formation through secondary reactions of Ccl,’ with oxygen [5]: ccl;
COCl,
+ 302 -
TABLE
+ Cl’. . . etc.
3
Methyl methacrylate rates of polymerization in several solvents using 4-isopropyl-4’~N,N-dimethylaminobenzophenone as photoinitiator ([CUM11 = 5 x 10e5 M, IO- 5.6 x 10m8 Einsteins s-l 1-l; [MMA] =2.X M) R,x lo6 (M s-l)
Solvent
6.95
Cyclobexane Tetrahydrofuran Chloroform Benzene Cl&
A
8.04 9.81 10.0 40.6
I3
Benzene
Carbon
Tetrachloride
uv
RI III
I
I
II
I 25
30
I
I
II
I 20
V, Fig. 5. Gel permeation chromatography [CUMI] = 10e3 M as photoinitiator [MMA) index detector signal.
11111111111111 35
2s
30
Vr traces of poly(methy1 methacrylaie) obtained using = 2.8 M: UV, ultraviolet detector signal; RI. refraction
360 4.2. Methyl methacrylate photopolymerization in Ccl, solution The reaction pathway between CUM1 and CC1, was apparently not modified when it was conducted in the presence of high concentrations of MMA (up to 2.8 M). The observed effect was only a decrease in the value found for mdisap(Table 2). Kinetic measurements for MMA polymerization photoinitiated by CUM1 in Ccl_+ solution were carried out by dilatometry as described in the experimental part. Compared with values obtained in other solvents the rates of polymerization were found to be four to ten times higher in CCL, (Table 3). The photoinitiated polymerization proceeded without an inhibition period and the polymers obtained did not show a UV signal attributable to chromophore (CUMI) end-chain groups, which is always present in the polymers obtained in other solvents (Fig. 5). The elemental analysis of the polymers also showed the presence of chlorine. In previous papers [21-23, 281 we described a mechanism for the radical polymerization photoinitiated by CUM1 and other aminobenzophenones in different solvents. In non-halogenated solvents and in chloroform, initiating radicals are alkylamino or solvent radicals, which are generated respectively through excimer formation or by direct hydrogen atom abstraction in a well known process depending on the type of solvent and on the photoinitiator concentration (Scheme 1).
R’ Pi
_
hu
‘?I, Kd
Scheme
CL,ST
-
3Pl,
PI + Pl
Kd 1
I
PI
PI
-----Y KCZf
h-i
+
-N-b-$
1.
In CCb the mechanism is clearly different as can be inferred from the experimental results. Two active radicals are produced by an absorbed photon which are ‘Cl and C13C’. These radicals have been described as capable of initiating the polymerization process of acrylic monomers 1293, and in this case this occurs in a very efficient way, since higher R, values are obtained and no chromophore end-chain groups are detected. This behaviour appears to be very general as our preliminary results indicate for other non-conjugated aminobenzophenones examined as potential chromophores.
5. Conclusions Irradiation of CUMI in CCL, solutions exhibits a different behaviour in other solvents. All the results indicate that a charge-transfer complex between CCI, CC&* radicals.
and the excited
triplet
state
of CUMI,
which
decomposes
from that is formed
into
Cl’
and
The radicals formed are able to abstract a hydrogen atom from the N-methyl group of the amine, forming hydrogen chloride and chloroform. Hydrogen chloride reacts with CUMI to form the hydrochloride.
361
both radicals initiate a free-radical In the presence of an acrylic monomer, polymerization very efficiently. No chromophore is detected as an end-chain group in any case. Elemental analysis of the polymers showed the presence of chlorine. The behaviour is similar when other aminoketones are irradiated in CCb solution. This behaviour does not occur when chloroform is used as solvent.
Acknowledgment Thanks are due to the Comisidn Interministerial 0488) for their financial support.
de Ciencia y Tecnologia
(MAT91-
References 1 2 3 4
5 6
D. P. Stevenson and G. M. Coppinger, .T. Am. Chem. Sot., 84 (3962) 149. R. Foster and T. J. Thomson, Trans. Famday Sot., 58 (1962) 861. K. M. C. Davis and M. F. Farmer, X Chem. Sec. & (1967) 28. K. B. Wbetsel and J. H. Lady, 3. Phys. #rem., 68 (1964) 1010. W. J. Lautenberger, E. N. Jones and J. G. Miller, 1. Am. Chem. Sot., 90 (1968) 1110. W. C. Meyer, 3. Whys. Clzem., 74 (1970) 2118, 2122, 2127. K. L. Kurtz, R. Hutton and F. Westheimer, J. Am. Chem. Sot., 83 (1961) 584. A. Aviram, Macromolecules, 11 (1978) 1275.
7 8 9 C. A. Chesta, J. J. Cosa and C. M. Previtali, J. Phutochem. Phutobiol. A: Chem., 45 (1988) 9. 10 H. Shimamori and H. Uegaito, J. Phys. Chem., 95 (1991) 6218. 11 J. Chilton, L. Giering and C. Steel, .7. Am. Chem. Sot., 98 (1976) 1865. 12 J. 0. Pavlik, P. I. PIooard, A. C. Somersall and J. E. Guillet, Can. J. Chem., 51 (1973) 1435. 13 M. Matsuda and Y. Ishiroshi, J. Po&m. Sci. A, 8 (1970) 387. 14 M. C. Biondic and R. Erra-B&ells, J. Photochem. Photobioi., 51 (1990) 341. 15 S. Hussain, S. D. Baruah and N. N. Dass, .X PO&m. Sci., PO&~. Left. Edn., 16 (1978) 167. 16 N. N. Dass and A. K. Gogoi, X Mucromol. Sci. Chem. A, 20 (1983) 207. 17 K. Tsuda, S. Kondo. K. Yamasita and K Ito, Makzumol. Chem., 18.5 (1984) 81. 18 J. Lingnau and G. Meyerhoff, J. MacromuZ. ScL Chem. A, 18 (1982) 503. 19 P. Ghosh and A. R. Bandyopadhyay, J. PoZym. Mater., 2 (1985) 1. 20 P. Ghosh and R. Ghosh, Eur. PoZym. 1, 17 (1981) 545. 21 J. L. Mateo, J. A. Manzarbeitia and R. Sastre, J. Photochem. Photobiol. A: Chem., 40 (1987) 169. 22 J. L. Mateo, P. Bosch, E. Vsizquez and R. Sastre, MakromoL Chem,’ 189 (1988) 1219. 23 J. L. Mateo, P. Bosch, F. Catalina and R. Sastre, .T. PO&~. Sci., A: Chem., 28 (1990) 1445. 24 J. L. Mateo, J. A. Manzarbeitia and R. Sastre, Angew_ MakronwL Chem., 142 (1986) 123. 25 R. Foster, Chem. Ind. (London), (1960) 1354. 26 L. J. Johnston and K. U. IngoId, J. Am. Chem. Sot., 108 (1986) 2343. 27 K. I. Goldberg and R. G. Bergman, J. Am. Chem. Sot., 121 (1989) 1285. 28 J. L. Mateo, P. Bosch, R. Sastre and F. Catalina, J_ PO&m_ Sci A, 29 (1991) 1955. 29 G. Bemer. R. Kirchmayr and G. Rist, J. Oil Corour C&m. Assoc., 62 (1978) 105.
3.53
Photopolymerization of methyl methacrylate in carbon tetrachloride solution using 4-isopropyl-4’-N, Ndimethylaminobenzophenone as chromophore P. Boschr
and
J. L.
Mateo
Institute de Ciencia y Tecnologia de Polimeror, Juan de la Cierva 3, 28006 Madrid (Spain) (Received
January 23, 1992; accepted March 12, 1992)
Abstract The polymerization of methyl methacrylate (MMA) in solution using 4-isopropyl-4’-N,Ndimethylaminobenzophenone (CUMI) as photoinitiator shows a particular behaviour when Ccl, is used as solvent. The MMA polymerization rate is much higher than that obtained in other solvents (benzene, tetrahydrofuran, cyclohexane and chloroform). In the absence of monomer, quantum yields of CUM1 disappearance higher than unity are found. These results and the analysis of the products obtained after irradiation support the view of a charge-transfer compIex formation between CCll and the triplet excited state of CUMI. Disproportion of the complex leads to Cl a and Ccl,’ radicals, which are responsible for the polymerization initiation step. A suitable mechanism is proposed.
1. Introduction The
photochemistry
and
dark
reactions
of amines
in halomethane
solution
have
been widely studied, even though the process is far from being completely understood. Amines have been shown to form 1:l charge-transfer complexes with halomethanes in the ground state [l-4] through some equilibria, the extent of which depends on the donor and acceptor characteristics as well as on the polarity and solvating capacity of the solvent (Fig. l)_ The charge-transfer process has been shown to be responsible for the marked instability of solutions of amines in CCL, both in the dark and under UV irradiation [5-7]. It is not well established whether the complexes are formed from the first singlet A-I-D
e
[A
. . . . . D]
e
AA=
Acceptor
D=
Donor (amine)
]
-I- D+
(halomethane)
Fig. 1. Charge-transfer
complex equilibria between
+Author to whom correspondence
lOlO-6030/92/$5.00
[A-D+
halomethanes
and amines.
should be addressed.
0 1992 - Elsevier Sequoia.
All rights reserved
354
and/or triplet state of the amine [S, 91, since the excited state involved will also depend on the nature of the solvent [lo]. Light-induced reactions between halomethanes (especially Ccl,) and other functional groups such as ketones [II, 121 and pyridine-SO2 complexes 1131 have also been reported. The subsequent reaction has been proposed to occur by disproportion of the complex, forming either Cl’ and CC13’ radicals [14] or real contact ion pairs [lo]. In the presence of monomers, some examples have been given of “spontaneous” polymerization (in the dark and with T<30 “C) initiated by a CC&.amine combination [15-171 or even by Ccl4 alone [18]. UV irradiation of ketone-amine combinations in the presence of a monomer and a polyhalogenated solvent leads to a notorious increase in the rate of polymerization compared with other non-halogenated solvents [19, 201. The purpose of this paper is the study of methyl methacrylate (MMA) polymerization using Ccl4 as solvent and 4-isopropyl-4’-N,N-dimethylaminobenzophenone (CUMI) as photoinitiator. It has been necessary to gain a better insight into the photochemical behaviour of CUMI-Ccl4 and CUMI-MMA-Ccl4 solutions, which leads us to consider a different process to create radicals from that described previously for this aminobenzophenone 121, 221.
2. Experimental.
details
2.3. Materials MMA from Fluka was washed three times with portions of 10 vol.% aqueous sodium hydroxide and then three times with distilled water. The monomer was dried over anhydrous magnesium sulphate and distilled under reduced pressure prior to use. Benzene and the other solvents were spectroscopic grade from Aldrich and were used without further purification. Carbon tetrachloride (fluorescence grade from Carlo Erba) was shaken vigorously with a saturated KOH solution and washed first with water, then with sulphuric acid until no colour was found and finally with water again. It was then refluxed over P,05 for a few hours and distilled. 2.2. Polymerizatiun procedure This procedure was performed according to the method described in ref. 22. The rates of all photoinitiated polymerizations were corrected to allow for the changes from run to run of the incident light intensity, assuming in all cases a square root dependence of light intensity_ 2.3. Steady
state
irradiation
procedure
The kinetics of disappearance of the charge-transfer band of CUM1 during irradiation was carried out by taking readings of absorbance at A = 335 nm, at different times of irradiation, such as described in ref. 21. CUM1 concentrations were in the range 10-4-10-5 M. Sa mples were irradiated at h =365 nm both in air or degasified, the latter by bubbling nitrogen until complete evacuation of the oxygen dissolved in solution. UV-visible spectra were recorded using a Shimatzu UV-256 FS spectrophotometer. 2.4. ‘H nuclear magnetic resonance spectra The behaviour of CUMI-CCk, solutions under irradiation was monitored on a Brucker model AM-200 MHz. The solution ([CUMI] = 10m2 M in CC14, with an external
355
dimethyl sulphoxide-&- reference) was placed in a nuclear magnetic resonance (NMR) tube, degasified by bubbling- nitrogen and irradiated at A=365 nm. Several spectra of this sample were recorded at different stages of irradiation. 2.5. Gel permeation chromatography measurements The molar weight distribution was measured as described previously [23]. A 440 Waters absorbance detector was used to detect chromophore end-chain groups. analysis 2.6. Gas chromatography Non-quantitative gas chromatography analyses were done in a Hewlett-Packard M CUM1 in CCL, was 5890 series II, with Tinjection=100 “C. A solution of 1.3 X lo-’ irradiated at A=365 nm until complete disappearance of the CUM1 band in the absorbance spectrum_ The solution was evaporated after irradiation and redissolved in 4 ml of n-hexane. A reference was prepared by dissolving CL&-CC13 in CCL+, evaporating the solution and redissolving it in n-hexane.
3. Synthesis CUM1 was prepared as described in ref. 24. 4-Isopropyl-4’-N,N-dimethylaminobenzophenone hydrochloride was prepared by bubbling pure HCl into a 0.1 M solution of CUMI in diethyl ether. When the solution became cloudy, the solid was filtered off and purified by preparative thin layer chromatography. IR (BrK): 3100-2900 cm-’ (C-H st); 2850-2800 cm-’ (N-CH2 St); 2700-2000 cm-’ (N+-H st); 1650 cm-’ (C=O st).
4. Results
and
4.2. Irradiation
discussion
in the absence
of mummer
UV absorption spectra of CUMI-CC& solutions did not show, over a wide range of concentrations, charge-transfer wmplex formation between CUM1 and CC4 in the ground state. UV spectra in these solutions were similar to those in other solvents (benzene, cyclohexane, tetrahydrofuran and chloroform) not only in the shape of the band but also in the h,, and E values (Table 1). TABLE
1
I-Isopropyl-4’-NJ+dimethylaminobenzophenone Solvent
x max
absorption
characteristics Q
(nm) Cl& Acetonitrile Benzene
335 341 340
19100 20400 21100
Cyclohexane Chloroform Tetrahydrofuran
330 349 337
22800 22500 20900
in several
solvents
356
These solutions were stable in the dark for long periods of time. Degassed solutions were irradiated at h =365 nm and an extremely rapid disappearance of the CUM1 charge-transfer band was observed. Aerated solutions were then irradiated in the same conditions and the same behaviour was obtained (Fig. 2). A rapid decrease in the CUM1 band with Q,d,sap> . 1 was found, and the formation of a new product detected_ This main product of the reaction was relatively stable iii solution but rapidly decomposed during isolation. It was identified by comparison with an authentic sample such as CUM1 hydrochloride 2 (Fig. 3). The formation of amine hydrochlorides in Ccl4 solutions has been reported by other workers [I, 251. The CUM1 disappearance quantum yield in the presence of oxygen increased when the carbon tetrachloride concentration increased (Table 2). All these data indicated a completely different behaviour from that observed by us for these types of compound in other solvents, including a halogenated solvent such as chloroform [Zl-231. The irradiation was also carried out under nitrogen in a 5 mm NMR tube, and the evolution of the reaction was monitored VS. increasing irradiation time (Fig. 4). The modification of the NMR spectra was in accordance with the presence of CUMI hydrochloride since the aromatic protons adjacent to the nitrogen suffered a significant alteration. Other signals appeared, confirming that structure (S=6.6 ppm corresponding to a N+H proton and S =7.2 ppm corresponding to its aromatic ortho protons). Two new singlets appeared at S- 3.2 and S-5.2 ppm, which were assigned to N-CH2-Cl and N-C!H2CC13 protons respectively, corresponding to structures 3 and 4
Abs
0.5
4 0
3&o
2;0
A Fig.
2.
curves
UV
absorption
is 5 s.
spectra
of
CUM1
nm in Cl,C
at various
stages
of
irradiation.
ti
between
357
*COeN:cH2c1
3
Fig. 3. Chemical structures of CUM1 (1) and the products identified after irradiation_ TABLE 2 4-isopropyl-4’-N,N-dimethylaminobenzophenone disappearancequantum yield in benzene solution as a function of Cl& concentration ([CUMI] = 10S4 M, 1,=1.114X 10L6 Einsteins s-l 1-l) W4Cl (mol 1-l) 3x10-5 10-4 2x 10-4 10-3 Neat Neat+2.8
0.047 0.145 0.250 0.735 1.350 0.700
M MMA
of Fig. 3. SimuItaneously with the appearance of these signals, a proportional decrease in the integral of the N-Cl& protons was observed. Finally, the irradiation mixture was analysed by gas chromatography where measurable amounts of hexachIoroethane were detected. Considering these experimental resnIts and previous mechanisms described in the literature for similar processes [5, 12, 26, 271 we can propose the following mechanism: CUM1 = R-rjr-CI&
-%
%UMI
=
3CUMI
(1)
CH3 Tuna
R-YCH3
2%
~pzmv41...Cl.qc]
+ c1’ -
CH3 R-y-CHS cw3
R-‘i”tiHz
-
CUMI+Cl’cCi~C’
+ HCI
(2) (3)
C-3 + CC&- -
R-y&H= CHJ
+ HCCI,
(4)
358
0 min
‘irr
%r 30
I
I
0
7
min
I
I
6
I
I
5
4
3
I
2 6
Fig. 4. ‘H NMR
(200
MHz)
R-YcH3 tHc’CH3
R-y&H,
+ CLC -
spectra
I
I
1
0
ppm
of the reaction
mixture
before
and after
30 min irradiation.
R7N
(5)
R-y-CH,Cl
(6)
CH3
CH3
+ Ccl;
CH3
CH3
R-fiT-i3Hz
+ CL’ or CQ’
-
R-y-CH,-Cl CH3
CH3 2C13C’
-
Cl,C-ccl,
where
ISC
indicates
or R-TECH,CC&
(7)
CH3 (8)
intersystem
crossing.
359
The CUM1 triplet and not the singlet is proposed as the reacting species since intersystem crossing is a very efficient reaction for this type of compound. Other workers did not conclude which excited state was reactive, and a reaction from the singlet state does not seem plausible in this particular case. The reaction between the CUM1 triplet and carbon tetrachloride to form the complex (reaction (2)) is faster than the quenching of the triplet by oxygen as the reaction proceeds in air. In the presence of oxygen, disappearance quantum yields higher than unity could be explained by more hydrogen chloride formation through secondary reactions of Ccl,’ with oxygen [5]: ccl;
COCl,
+ 302 -
TABLE
+ Cl’. . . etc.
3
Methyl methacrylate rates of polymerization in several solvents using 4-isopropyl-4’~N,N-dimethylaminobenzophenone as photoinitiator ([CUM11 = 5 x 10e5 M, IO- 5.6 x 10m8 Einsteins s-l 1-l; [MMA] =2.X M) R,x lo6 (M s-l)
Solvent
6.95
Cyclobexane Tetrahydrofuran Chloroform Benzene Cl&
A
8.04 9.81 10.0 40.6
I3
Benzene
Carbon
Tetrachloride
uv
RI III
I
I
II
I 25
30
I
I
II
I 20
V, Fig. 5. Gel permeation chromatography [CUMI] = 10e3 M as photoinitiator [MMA) index detector signal.
11111111111111 35
2s
30
Vr traces of poly(methy1 methacrylaie) obtained using = 2.8 M: UV, ultraviolet detector signal; RI. refraction
360 4.2. Methyl methacrylate photopolymerization in Ccl, solution The reaction pathway between CUM1 and CC1, was apparently not modified when it was conducted in the presence of high concentrations of MMA (up to 2.8 M). The observed effect was only a decrease in the value found for mdisap(Table 2). Kinetic measurements for MMA polymerization photoinitiated by CUM1 in Ccl_+ solution were carried out by dilatometry as described in the experimental part. Compared with values obtained in other solvents the rates of polymerization were found to be four to ten times higher in CCL, (Table 3). The photoinitiated polymerization proceeded without an inhibition period and the polymers obtained did not show a UV signal attributable to chromophore (CUMI) end-chain groups, which is always present in the polymers obtained in other solvents (Fig. 5). The elemental analysis of the polymers also showed the presence of chlorine. In previous papers [21-23, 281 we described a mechanism for the radical polymerization photoinitiated by CUM1 and other aminobenzophenones in different solvents. In non-halogenated solvents and in chloroform, initiating radicals are alkylamino or solvent radicals, which are generated respectively through excimer formation or by direct hydrogen atom abstraction in a well known process depending on the type of solvent and on the photoinitiator concentration (Scheme 1).
R’ Pi
_
hu
‘?I, Kd
Scheme
CL,ST
-
3Pl,
PI + Pl
Kd 1
I
PI
PI
-----Y KCZf
h-i
+
-N-b-$
1.
In CCb the mechanism is clearly different as can be inferred from the experimental results. Two active radicals are produced by an absorbed photon which are ‘Cl and C13C’. These radicals have been described as capable of initiating the polymerization process of acrylic monomers 1293, and in this case this occurs in a very efficient way, since higher R, values are obtained and no chromophore end-chain groups are detected. This behaviour appears to be very general as our preliminary results indicate for other non-conjugated aminobenzophenones examined as potential chromophores.
5. Conclusions Irradiation of CUMI in CCL, solutions exhibits a different behaviour in other solvents. All the results indicate that a charge-transfer complex between CCI, CC&* radicals.
and the excited
triplet
state
of CUMI,
which
decomposes
from that is formed
into
Cl’
and
The radicals formed are able to abstract a hydrogen atom from the N-methyl group of the amine, forming hydrogen chloride and chloroform. Hydrogen chloride reacts with CUMI to form the hydrochloride.
361
both radicals initiate a free-radical In the presence of an acrylic monomer, polymerization very efficiently. No chromophore is detected as an end-chain group in any case. Elemental analysis of the polymers showed the presence of chlorine. The behaviour is similar when other aminoketones are irradiated in CCb solution. This behaviour does not occur when chloroform is used as solvent.
Acknowledgment Thanks are due to the Comisidn Interministerial 0488) for their financial support.
de Ciencia y Tecnologia
(MAT91-
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5 6
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