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Mechanisms of radiation induced cationic polymerization in the presence of onium salts
*H CL
Nuclear Instruments and Methods in Physics Research B 105 (1995) 168-174
__
NIIIMI B BeamInteractions
__
with Materials 6 Atoms
I!z
ELSEVIER
Mechanisms of radiation induced cationic polymerization in the presence of onium salts E. Malmstrijm
a, P.E. Sundell b, A. Hult a, SE. Jiinsson ‘, *
’ The Royal Institute of Technology, Depart. of Polymer Tech., Stockholm, Sweden h Swedish Steel Strip Products, Surface Tech., Borliinge, Sweden ’ Fusion W Curing Systems, 7600 Standish Place, Rock&e, MD 20855-2798,
USA
Abstract Cationic polymerization of various monomers in the presence of onium salts were induced by hv, EB and y irradiation. The mechanism for the initiation process involves the photoreduction of onium salts by a direct photolysis or by an indirect redox reaction from organic free radicals or solvated electrons depending on the reduction potentials of the onium salts. For EB and y irradiation only solvated electrons were capable of reducing the onium salts with reduction potentials lower than approximately - 100 kJ/mol. An enhanced production of protons and/or carbenium ions takes place if the reduction potentials of the onium salts are higher than - 60 kJ/mol. This paper will give some indications of useful onium structures that fulfill the needs in EB and y induced cationic polymerization. Typical examples are fragmenting type of dialkylphenacyl and cyclic ringopening phenacylic sulfonium salts. The influence of typical “polymer or monomer backbone” structural groups, such as esters and ethers on the proton formation under high energy irradiation, was studied by UV spectroscopy at 540 nm. The formation of acid was monitored in the presence of various onium salts, and a-naphtylred was used as an indicator. By comparing aromatic versus aliphatic structural group influences on the generation of protons and carbenium ions a good correlation was found between experimental data and theoretical calculations on nucleophilicity, electron charge density distributions and electron scavenging effects by the use of simplified Hiickel calculations @HMO).
1. Introduction The most important breakthrough in the area of radiation-induced acid catalysed reactions was made in the mid 70s by the discovery of various onium salts as latent sources of strong Bronsted acids [ 1,2]. Further advances in the field have shown that the latency of these salts can be broken not only by UV radiation but also thermally [3-51 or by high-energy radiation [6-91. For industrial radiation curing, electron beams are used at high speed production lines. However, since the fundamental radiation effects are essentially the same, y-radiation can be used to study the radiation chemistry of organic compounds. Initially when M, to ionizing radiation, Scheme exposing a “monomer”, 1, monomer radicals, M ., solvated electrons, e,, and protons, H+, are formed. Protons and cation radicals rapidly recombine with solvated electrons, resulting in an increased production of radicals from H-abstraction and
* Corresponding 279 0661.
author.
Tel. +l
301 251 0300, fax +1 301
a-cleavage of a higher excited state “monomer”. Thus, the main products from radiolysis of organic liquids are free radicals which are responsible for most of the following reactions. Route A: Electron transfer reduction of the sulfonium salt by organic free radicals generates a stabilized carbenium ion. Route B: Reduction of the sulfonium salt by solvated electrons liberates the stabilizing counter ion. In the presence of an onium salt, however, solvated electrons can be scavenged which gives two alternative ways of generating initiating species for cationic polymerization. First, by the redox reaction between an electron donating free radical and the onium salt, in which the radical is oxidized to the corresponding cation, stabilized by the counterion (route A). Second, solvated electrons can reduce the onium salt, liberating the counterion and thereby forming a Bronsted acid with the proton (route B). The resulting Bronsted acid, H+X- and the ion pair, M+Xare the true initiators in the system. Any sulfonium salt can be reduced by route B. However, due to the oxidizing ability of the salt, reduction may also proceed by route A, but that requires that the oxidation potential of the radical,
0168-583X/9.5/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)00632-X
160
E. Malmstriim et al. / Nucl. Instr. and Meth. in Phys. Res. B 105 (1995) 168-I 74
Scheme 1. A monomer, M, is exposed to ionizing radiation.
1 Ph cqPh
-
cs
(g)
+
. flObR w
J
AOAR cd;,
Scheme 2. Radical stabilization
of the reduced phenacylsulfonium
E,“” , must be lower than the reduction potential of the sulfonium salt, EOred. This is because the free energy, AC. of the reaction must be negative for the reaction to proceed spontaneously (Eq. (a)). AG=Er’X-E;d
(a)
The mechanism that dominates depends on the reduction potential of the salt and on the radiation yield of oxidizable radicals and solvated electrons. Since the stationary state concentration of radicals is much higher than that of solvated electrons, the radical mechanism will dominate if Eq. (a) is fulfilled. The reduction potential of onium salts depends on their structure. Some onium salts have a relatively low reduction potential, due to the ability to form strongly stabilized [R,S .] radicals (I), Scheme 2. The effect of onium salt structure has previously been reported [7].
Table 1 Structure
+xPh
‘\\ -
‘%h
salt gives a low reduction potential.
Since the most important process of high-energy induced cationic polymerization is the reduction of onium salt, the reactivity of a formulation can be altered by: - the onium salt (Eared) - monomer structure (radiation chemistry, yields of radicals and solvated electrons) - dose rate (yields of radicals and solvated electrons)
2. Experimental 2.1. Materials The synthesis and characterization shown in Table 1, are given elsewhere
of the onium salts [7]. Ethyl benzoate,
q
and reduction
Onium salt
potential
:
of used onium salts
Abbreviation
Ecd
Ph,SSbF,
- 112
PTSSbF,
-63
A .
[kJ/mol]
Ethyl benzyl ether Dlphenyl erha ?-Ethoxy ethyl ether Ethyl benwate Ethyl hexmom’
* ratuated solution
500
IOCQ Dose. Gy
1500
Fig. 1. The results from y-irradiations in presence Ph,SSbF,. The highest yields of protons are obtained hexanoate and 2-ethoxyethyl ether.
11. SURFACE/BULK
of 1 mM from ethyl
MODIFICATIONS
E. Malmstriimet al. / Nucl. Instr. and Meth. in Phys. Rex B 105 (1995) 168-I 74
170
CJ 2-Ethoxyethyl l n
ether
Ethyl benmate Ethyl hexanoate’
0
Fig. 2. The results PTSSbF,.
from
y-irradiations
in presence
of 1 mM
2(x)
4M) 600 Dose, Gy
800
Fig. 3. The yields of protons when 2-ethoxyethyl ated.
ethyl hexanoate and diphenyl ether were supplied by Merck. These Fquids were distilled and stored over molecular sieves (4 A) prior to y-irradiation. 2-ethoxyethyl ether was supplied by Aldrich. It was distilled over P205, bubbled with argon, dried with magnesium sulphate over night and stored over CaCI, prior to irradiation. Ethyl benzyl ether was synthesized from benzylbromide and ethanol using PTC conditions. Benzylbromide was purchased from Merck and distilled before use.
ether is y-irradi-
3.2. Monitoring the proton formation The y-radiation induced proton formation in the organic liquids was measured spectroscopically (8451A Diode Array Spectrophotometer, Hewlett Packard) at 540 nm using cu-naphthyl red as indicator. 3.3. Irradiated samples Irradiated samples were diluted, if necessary, to appropriate concentrations and measured as described above. irradiated liquid without sulfonium salt was used as reference.
3. Procedures
3.1. Irradiations y-rays were obtained from a AECL,,e %oy-cell giving a dose rate of 1080 Gy/h. The irradiations were carried out in polypropylene tubes. The organic liquids with an initiator concentration of 1 X 10m3 M were bubbled with argon for 20 min before irradiation.
4. Results and discussion The results, when using Ph,SSbF,, are illustrated in Fig. 1. The highest yield of protons was obtained from the e.
(1) (2)
PwR
+
FpR
-
~5
(p&R)
-
p&R
(p‘
-
(p
-
+
rd;R
ed’
*
H+
+
+
(3) (4)
H*
R*
Scheme 3. Radiation chemistry
(5)
(6)
of vinyl ethers [7].
E. Malmstriim
et al. /Nucl. Instr. and Meth. in Phys. Res. B 105 (1995) 168-I 74
-
R’ 0 ‘c’ ‘5’
R’
+
171
H+ PII)
Scheme 4. Rearrangement reactions for ether cation-radicals,
aliphatic ester, ethyl hexanoate. One could expect that the other ester, ethyl benzoate, would give the second highest yield but, due to the multiple cc-oxygen hydrogens, 2ethoxyethyl ether gave the second highest proton concentration after irradiation. The lowest proton concentrations were obtained from ethyl benzyl ether and diphenyl ether. The results, for PTSSbF6, are illustrated in Fig. 2. The yields of protons at an equal dose of exposure are higher compared to samples containing Ph,SSbF, which reflects the more favourable reduction potential allowing reduction by both pathways, as shown in Scheme 1. 4.1. Ethers The radiation chemistry of butanedioldivinyl ether, BDDVE, and diethyleneglycoldivinyl ether, DEGDVE, has previously been investigated [8,9], Scheme 3. When an organic substrate is exposed to y-rays, fast electrons are generated in the sample and these dissipate most of their energy causing ionization (1) and excitation (2) of molecules. When a molecule is excited above its ionization potential (lo-12 eV for most organic molecules) it may lose energy by ionization or dissociation (56). The yield of ions in a liquid depends on the distance the ejected electron travels before reaching thermal energy levels (about 0.025 eV at room temperature). This distance depends to a large extent on the electron density of the medium. For liquids with low dielectric constant, E, the G-value for free ion production is very low, indicating that
most electrons rapidly recombine with their geminate positive ions (3). The recombination produces an excited molecule with an energy excess lower than the ionization potential but often high enough to cause radical fragmentation (56). As ionized and excited species undergo further reactions, neutral free radicals are produced by several processes (4,561. The yield of free radicals is much higher than that of free ions. In ethers, having E = 4, the yield of ions is rather high Gi (diethyl ether) = 0.35 [lo], reportedly due to the possibility of electrons becoming solvated
1111. When BDDVE is y-irradiated, one would expect several radicals to be formed. However, we found only one detectable radical in the ESR spectrum of y-irradiated BDDVE, using 2-methyl-2-nitrosopropane as a spin trap. A comparison of the hyperfine splitting constants with those of known radicals shows that the radical formed from y-irradiation of BDDVE is a stable o-ether radical.
4.1.1. Z-Ethoxyethyl ether (Fig. 3) a-ether radicals have oxidation potentials around -80 kJ/mol which makes a redox reaction with PTSSbF, feasible (AC = - 17 kJ/mol according to Eq. (a)) but not with Ph,SSbF, (AC = 32 kJ/mol). Thus, PTSSbF, can be reduced by both radicals and solvated electrons and consequently the highest yields of protons are obtained in those samples. The primarily formed cation-radical (II), is easily rearranged to give a radical and .I proton (III). Scheme 4.
Scheme 5.
II. SURFACE/BULK MODIFICATIONS
E. Malmstriim et al. /Nucl.
172
Instr. and Meth. in Phys. Res. B 105 (1995) 168-I 74 0.60
PtqSSbF6
,I
0.00 j//y,, .-. 0
S(K)
loo0
Dose. Gy
The high yield of protons can be explained by the multiplicity of a-oxygen hydrogens in the ether molecule. Ethyl benzyl
ether
(Fig. 4)
Only data for the Ph,SSbF,-salt is available since PTSSbF, is not soluble in ethyl benzyl ether. The primary formed cation-radical, (IV), gives an a-ether radical and a proton (V). Scheme 5. The content of a-oxygen hydrogens is much lower in ethyl benzyl ether than in 2-ethoxyethyl ether. However, other mechanisms might be activated as well (VI). It is possible that energy enough has been brought into the molecule to cleave the C-O bond. This bond is more unlikely to be broken in 2-ethoxyethyl ether, as the resulting radical does not have the possibility to stabilize by resonance. This mechanism does not give any protons. 4.1.3.
Diphenyl
ether
2lw
Dose (Gy)
Fig. 4. The yield of protons when ethyl benzyl ether is exposed to y-rays in presence of 1 mM Ph,SSbF,.
4.1.2.
15mJ
(Fig. 5)
PTSSbF, is not soluble in diphenyl ether. In an initial phase diphenyl ether gives no protons (VII, VIII) which is
Fig. 5. The yield of protons when diphenyl Ph,SSbF,
ether and 1 mM
are exposed to y-rays.
expected since there are no a-ether hydrogens in the molecule. Scheme 6. The energy contribution at low doses is not enough to break the C-O bond. Another mechanism, “in cage” rearrangement, seems to be activated above about 600 Gy (IX, X), as the yield of protons suddenly increases [ 121. 4.2. Esters By -y-irradiation of esters, electrons are ejected from the carbonyl oxygen (XI). Scheme 7. The formed cationradical is stabilized by resonance, Scheme 7, between (XI) and (XII). The cation-radical may then be rearranged to give protons and an a-ether type radical at the ester group (XII). This resulting radical will be split into an aldehyde and an acyl radical (XIII). The stability of these products is influencing the yield of protons. The aliphatic ester might undergo a McLafferty rearrangement, Scheme 8. The mechanism involves a P-H
@a
-@=p-
l
(v,II)
rearomatization 1 @#-O Scheme 6.
+
H+
(X)
E. Malmstrijm
et al. /Nucl.
173
Phys. Res. B 105 (1995) 168-174
ii
0
R K O-R,
+
e,
(XT)
.
ii RK 0-R.
-
.I
I R
Instr. and Meth. in
*O+AR,
-
Fig. 6. The yields of protons in ethyl benzoate in the presence of mM sulfonium salt.
-
R
O-R’
+
H+
(XII)
ethyl benzoate scavenges yield of protons.
electrons
1
and hence reduces the
4.3. SMHO calculations + of the primarily
formed
abstraction from the alkyl chain to the carbonyl centered cation radical.
oxygen
Scheme
7. The
resonance
(XIII)
structures
cation-radical.
4.2.1. Aromatic versus aliphatic esters. Ethyl hexanoateethyl benzoate (Fig. 6) The soiubility of the two onium salts in ethyl hexanoate is very limited. It was not possible to make 1 mM solutions, therefore the concentrations were GZ 1 mM for both onium salts in this case. However, the yield of protons is lower from ethyl benzoate than from ethyl hexanoate, even though the concentrations of the onium salts in ethyl bensoate were all 1 mM. This can be explained by the McLafferty rearrangement that produces protons. Another additional explanation will be that the aromatic part of
In Table 2 are shown as an example typical values of electron charge distributions for dialkylethers versus diarylethers and ethyl propionate versus ethyl benzoate. By comparing the charge distributions in the neutral molecule to the corresponding charge distributions in the cation-radical intermediate, formed after the electron ejection under EB or y exposure, the following observations and interpretations can be made.
Table 1 SMHO calculations: Electron charge ROR+ * and ArOAr, ArOAr+ *
for
x W*ROE
z CWRCE
z CWiilGE
distributions
r
0+
Lg.,,,e.,,
-o-
---
+0.0415 _______..__._._..__.__~...-
._
. _ _. _ _ _ _ &r = -0.1176 __ _ _ . _ . _
I I ._ ._____._..........__...---I I
-
I+O.4156
/-0.5672
I
I
+0.3502
ROW
, ArOAr*’
E&Et . .._._____----.-
/
Scheme 8. McLafferty duce protons.
OH
rearrangement
+
of B-hydrogen
0
esters pro-
&I = +O.OlO _. _. _ _. _
br =
I
I +0.4920
Et
%., = +0.133!
._.__..._
._
I
Et%o,Et . .._____.__.._.-
H’
..
+0.1247
-0.5922
+0.6969
co.2767
R
+0.1347 _
._. .
I I 1I ______._._._..
+
‘]iR ’
Et
._
+0.1174
+
CWAIGE
Ar I
ROR / ArOAr
H 2J’
RYyH
ROR.
._..._.._.-
I
+0.3575 _.
+0.0744
I I +0.3502
4 .0.1247
-0.4672
11. SURFACE/BULK
+0.7234 _ _ _. _ _. _
&r = +1.0065
-
MODlFlCATlONS
174
E. Malmstrtim et al. /Nucl. Ins&. and Meth. in Phys. Res. B 105 (199s) 168-l 74
4.3. I. Ethers For the diakylether cation-radical there is a great increase in charge on the ether oxygen from +0.0415 to + 0.6969, meaning that = 70% of the positive charge is located at the ether oxygen. For the dialylether, the much smaller increase from + 0.1174 to + 0.2767, indicates that only =: 28% of the positive charge in the aromatic structure is located at the ether oxygen. 72% of the charge is distributed in the aryl rings. The strong delocalization of charge into the aromatic parts of the diarylether and the absence of cr-hydrogens further support the mechanism outlined in Scheme 4 and for the intermediates (VII-Xl. 4.3.2. Esters The positive charge distibution at the ester carbon in the aliphatic ester has increased moderately from + 0.4156 in the neutral molecule to f0.4920, whereas the ester carbon in the aromatic ester does not change. This is due to the “node point activity” of the carbonyl carbon in the aromatic ester structure. However, more surprising is the great difference in charge localization at the carbonyl oxygen between the two structures. For the aliphatic ester an increase from - 0.5672 to + 0.0744 is noticeable. The corresponding increase at the carbonyl oxygen in the aromatic ester is much smaller, -0.5922 to -0.4672, which is still a high localization of the electron charge. At the ether carbon center in the ester group a moderate increase is valid for the aliphatic ester, while the aromatic do not change. The main difference is the total delocalization of the positive charge into the aryl ring for the aromatic ester (+ 1.0085) and the complete localization of the positive charge into the ester group for the aliphatic ester. These fundamental differences between the two esters also reflect their reactivities, The strong localization of charge in the aliphatic esters promote the rearrangement mechanisms outlined in Schemes 7 and 8 (MacLafferty type) and for the intermediates (XII and XIII>. Due to the more delocalized positive charge distribution in the aryl ring, this carbocation radical will have a longer life time and therefore the rearrangement reactions metioned above will be less favoured. Added to this also
secondary reactions such as termination by solvated eiectrons elsewhere in the system and other strong nucleophiles will be more pronounced.
5. Conclusion The influence of a prepolymer or “monomer” backbone structure on the y-ray induced decomposition of sulfonium salts has been investigated. Generally Ph,SSbF, gave lower yields of protons than PTSSbF,, reflecting the more favourable reduction potential of the latter. Aromatic structures in the backbone act as electron scavengers, hence lowering the yield of oxidizable radicals, solvated electrons and ultimately the acid production. Thus, from a technical point of view, when formulating a cationic EBcurable composition, it is important to consider the radiation chemistry of the organic structural parts in the “monomer” and prepolymer backbone.
References
111J.V. Crivello, J.H.W. Lam, Macromolecules
10 (1977) 1307. J.V. Crivello, J.H.W. Lam, J. Pol. Sci., Polym. Chem. 17 (1979) 977. 131 A. Ledwith, Polymer 19 (1978) 1217. t41 S.P. Pappas, L.W. Hill, J. Coatings Techn. 53 (1981) 43. 151 P.E. Sundell, E.S. Jiinsson, A. Hult, J. Pol. Sci., Polym. Chem. 29 (1991) 1535. [61 S. Mah, Y. Yamamoto, K. Hayashi, Macromolecules 10 (1983) 681. 171 S.C. Lapin, RadTech 86 Conf. Proc. (Assoc. for Finish. Proc., Baltimore, 1986). Bl P.E. Sundell, Ph.D. Thesis RIT, Dept. of Polymer Technology, Stockholm, Sweden (1990). [9] P.E. Sundell, E.S. Jiinsson, A. Hult, J. Pol. Sci., Polym. Chem. 29 (1991) 1525. [lo] W.F. Schmidt, A.O. Allen, J. Phys. Chem. 72 (1968) 3730. [ll] S. Arai, M.C. Sauer Jr., J. Chem. Phys. 44 (1966) 2297. [12] J.L. Dektar, N.P. Hacker, J. Am. Chem. Sot. 112 (1990) 6004.
M
Nuclear Instruments and Methods in Physics Research B 105 (1995) 168-174
__
NIIIMI B BeamInteractions
__
with Materials 6 Atoms
I!z
ELSEVIER
Mechanisms of radiation induced cationic polymerization in the presence of onium salts E. Malmstrijm
a, P.E. Sundell b, A. Hult a, SE. Jiinsson ‘, *
’ The Royal Institute of Technology, Depart. of Polymer Tech., Stockholm, Sweden h Swedish Steel Strip Products, Surface Tech., Borliinge, Sweden ’ Fusion W Curing Systems, 7600 Standish Place, Rock&e, MD 20855-2798,
USA
Abstract Cationic polymerization of various monomers in the presence of onium salts were induced by hv, EB and y irradiation. The mechanism for the initiation process involves the photoreduction of onium salts by a direct photolysis or by an indirect redox reaction from organic free radicals or solvated electrons depending on the reduction potentials of the onium salts. For EB and y irradiation only solvated electrons were capable of reducing the onium salts with reduction potentials lower than approximately - 100 kJ/mol. An enhanced production of protons and/or carbenium ions takes place if the reduction potentials of the onium salts are higher than - 60 kJ/mol. This paper will give some indications of useful onium structures that fulfill the needs in EB and y induced cationic polymerization. Typical examples are fragmenting type of dialkylphenacyl and cyclic ringopening phenacylic sulfonium salts. The influence of typical “polymer or monomer backbone” structural groups, such as esters and ethers on the proton formation under high energy irradiation, was studied by UV spectroscopy at 540 nm. The formation of acid was monitored in the presence of various onium salts, and a-naphtylred was used as an indicator. By comparing aromatic versus aliphatic structural group influences on the generation of protons and carbenium ions a good correlation was found between experimental data and theoretical calculations on nucleophilicity, electron charge density distributions and electron scavenging effects by the use of simplified Hiickel calculations @HMO).
1. Introduction The most important breakthrough in the area of radiation-induced acid catalysed reactions was made in the mid 70s by the discovery of various onium salts as latent sources of strong Bronsted acids [ 1,2]. Further advances in the field have shown that the latency of these salts can be broken not only by UV radiation but also thermally [3-51 or by high-energy radiation [6-91. For industrial radiation curing, electron beams are used at high speed production lines. However, since the fundamental radiation effects are essentially the same, y-radiation can be used to study the radiation chemistry of organic compounds. Initially when M, to ionizing radiation, Scheme exposing a “monomer”, 1, monomer radicals, M ., solvated electrons, e,, and protons, H+, are formed. Protons and cation radicals rapidly recombine with solvated electrons, resulting in an increased production of radicals from H-abstraction and
* Corresponding 279 0661.
author.
Tel. +l
301 251 0300, fax +1 301
a-cleavage of a higher excited state “monomer”. Thus, the main products from radiolysis of organic liquids are free radicals which are responsible for most of the following reactions. Route A: Electron transfer reduction of the sulfonium salt by organic free radicals generates a stabilized carbenium ion. Route B: Reduction of the sulfonium salt by solvated electrons liberates the stabilizing counter ion. In the presence of an onium salt, however, solvated electrons can be scavenged which gives two alternative ways of generating initiating species for cationic polymerization. First, by the redox reaction between an electron donating free radical and the onium salt, in which the radical is oxidized to the corresponding cation, stabilized by the counterion (route A). Second, solvated electrons can reduce the onium salt, liberating the counterion and thereby forming a Bronsted acid with the proton (route B). The resulting Bronsted acid, H+X- and the ion pair, M+Xare the true initiators in the system. Any sulfonium salt can be reduced by route B. However, due to the oxidizing ability of the salt, reduction may also proceed by route A, but that requires that the oxidation potential of the radical,
0168-583X/9.5/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)00632-X
160
E. Malmstriim et al. / Nucl. Instr. and Meth. in Phys. Res. B 105 (1995) 168-I 74
Scheme 1. A monomer, M, is exposed to ionizing radiation.
1 Ph cqPh
-
cs
(g)
+
. flObR w
J
AOAR cd;,
Scheme 2. Radical stabilization
of the reduced phenacylsulfonium
E,“” , must be lower than the reduction potential of the sulfonium salt, EOred. This is because the free energy, AC. of the reaction must be negative for the reaction to proceed spontaneously (Eq. (a)). AG=Er’X-E;d
(a)
The mechanism that dominates depends on the reduction potential of the salt and on the radiation yield of oxidizable radicals and solvated electrons. Since the stationary state concentration of radicals is much higher than that of solvated electrons, the radical mechanism will dominate if Eq. (a) is fulfilled. The reduction potential of onium salts depends on their structure. Some onium salts have a relatively low reduction potential, due to the ability to form strongly stabilized [R,S .] radicals (I), Scheme 2. The effect of onium salt structure has previously been reported [7].
Table 1 Structure
+xPh
‘\\ -
‘%h
salt gives a low reduction potential.
Since the most important process of high-energy induced cationic polymerization is the reduction of onium salt, the reactivity of a formulation can be altered by: - the onium salt (Eared) - monomer structure (radiation chemistry, yields of radicals and solvated electrons) - dose rate (yields of radicals and solvated electrons)
2. Experimental 2.1. Materials The synthesis and characterization shown in Table 1, are given elsewhere
of the onium salts [7]. Ethyl benzoate,
q
and reduction
Onium salt
potential
:
of used onium salts
Abbreviation
Ecd
Ph,SSbF,
- 112
PTSSbF,
-63
A .
[kJ/mol]
Ethyl benzyl ether Dlphenyl erha ?-Ethoxy ethyl ether Ethyl benwate Ethyl hexmom’
* ratuated solution
500
IOCQ Dose. Gy
1500
Fig. 1. The results from y-irradiations in presence Ph,SSbF,. The highest yields of protons are obtained hexanoate and 2-ethoxyethyl ether.
11. SURFACE/BULK
of 1 mM from ethyl
MODIFICATIONS
E. Malmstriimet al. / Nucl. Instr. and Meth. in Phys. Rex B 105 (1995) 168-I 74
170
CJ 2-Ethoxyethyl l n
ether
Ethyl benmate Ethyl hexanoate’
0
Fig. 2. The results PTSSbF,.
from
y-irradiations
in presence
of 1 mM
2(x)
4M) 600 Dose, Gy
800
Fig. 3. The yields of protons when 2-ethoxyethyl ated.
ethyl hexanoate and diphenyl ether were supplied by Merck. These Fquids were distilled and stored over molecular sieves (4 A) prior to y-irradiation. 2-ethoxyethyl ether was supplied by Aldrich. It was distilled over P205, bubbled with argon, dried with magnesium sulphate over night and stored over CaCI, prior to irradiation. Ethyl benzyl ether was synthesized from benzylbromide and ethanol using PTC conditions. Benzylbromide was purchased from Merck and distilled before use.
ether is y-irradi-
3.2. Monitoring the proton formation The y-radiation induced proton formation in the organic liquids was measured spectroscopically (8451A Diode Array Spectrophotometer, Hewlett Packard) at 540 nm using cu-naphthyl red as indicator. 3.3. Irradiated samples Irradiated samples were diluted, if necessary, to appropriate concentrations and measured as described above. irradiated liquid without sulfonium salt was used as reference.
3. Procedures
3.1. Irradiations y-rays were obtained from a AECL,,e %oy-cell giving a dose rate of 1080 Gy/h. The irradiations were carried out in polypropylene tubes. The organic liquids with an initiator concentration of 1 X 10m3 M were bubbled with argon for 20 min before irradiation.
4. Results and discussion The results, when using Ph,SSbF,, are illustrated in Fig. 1. The highest yield of protons was obtained from the e.
(1) (2)
PwR
+
FpR
-
~5
(p&R)
-
p&R
(p‘
-
(p
-
+
rd;R
ed’
*
H+
+
+
(3) (4)
H*
R*
Scheme 3. Radiation chemistry
(5)
(6)
of vinyl ethers [7].
E. Malmstriim
et al. /Nucl. Instr. and Meth. in Phys. Res. B 105 (1995) 168-I 74
-
R’ 0 ‘c’ ‘5’
R’
+
171
H+ PII)
Scheme 4. Rearrangement reactions for ether cation-radicals,
aliphatic ester, ethyl hexanoate. One could expect that the other ester, ethyl benzoate, would give the second highest yield but, due to the multiple cc-oxygen hydrogens, 2ethoxyethyl ether gave the second highest proton concentration after irradiation. The lowest proton concentrations were obtained from ethyl benzyl ether and diphenyl ether. The results, for PTSSbF6, are illustrated in Fig. 2. The yields of protons at an equal dose of exposure are higher compared to samples containing Ph,SSbF, which reflects the more favourable reduction potential allowing reduction by both pathways, as shown in Scheme 1. 4.1. Ethers The radiation chemistry of butanedioldivinyl ether, BDDVE, and diethyleneglycoldivinyl ether, DEGDVE, has previously been investigated [8,9], Scheme 3. When an organic substrate is exposed to y-rays, fast electrons are generated in the sample and these dissipate most of their energy causing ionization (1) and excitation (2) of molecules. When a molecule is excited above its ionization potential (lo-12 eV for most organic molecules) it may lose energy by ionization or dissociation (56). The yield of ions in a liquid depends on the distance the ejected electron travels before reaching thermal energy levels (about 0.025 eV at room temperature). This distance depends to a large extent on the electron density of the medium. For liquids with low dielectric constant, E, the G-value for free ion production is very low, indicating that
most electrons rapidly recombine with their geminate positive ions (3). The recombination produces an excited molecule with an energy excess lower than the ionization potential but often high enough to cause radical fragmentation (56). As ionized and excited species undergo further reactions, neutral free radicals are produced by several processes (4,561. The yield of free radicals is much higher than that of free ions. In ethers, having E = 4, the yield of ions is rather high Gi (diethyl ether) = 0.35 [lo], reportedly due to the possibility of electrons becoming solvated
1111. When BDDVE is y-irradiated, one would expect several radicals to be formed. However, we found only one detectable radical in the ESR spectrum of y-irradiated BDDVE, using 2-methyl-2-nitrosopropane as a spin trap. A comparison of the hyperfine splitting constants with those of known radicals shows that the radical formed from y-irradiation of BDDVE is a stable o-ether radical.
4.1.1. Z-Ethoxyethyl ether (Fig. 3) a-ether radicals have oxidation potentials around -80 kJ/mol which makes a redox reaction with PTSSbF, feasible (AC = - 17 kJ/mol according to Eq. (a)) but not with Ph,SSbF, (AC = 32 kJ/mol). Thus, PTSSbF, can be reduced by both radicals and solvated electrons and consequently the highest yields of protons are obtained in those samples. The primarily formed cation-radical (II), is easily rearranged to give a radical and .I proton (III). Scheme 4.
Scheme 5.
II. SURFACE/BULK MODIFICATIONS
E. Malmstriim et al. /Nucl.
172
Instr. and Meth. in Phys. Res. B 105 (1995) 168-I 74 0.60
PtqSSbF6
,I
0.00 j//y,, .-. 0
S(K)
loo0
Dose. Gy
The high yield of protons can be explained by the multiplicity of a-oxygen hydrogens in the ether molecule. Ethyl benzyl
ether
(Fig. 4)
Only data for the Ph,SSbF,-salt is available since PTSSbF, is not soluble in ethyl benzyl ether. The primary formed cation-radical, (IV), gives an a-ether radical and a proton (V). Scheme 5. The content of a-oxygen hydrogens is much lower in ethyl benzyl ether than in 2-ethoxyethyl ether. However, other mechanisms might be activated as well (VI). It is possible that energy enough has been brought into the molecule to cleave the C-O bond. This bond is more unlikely to be broken in 2-ethoxyethyl ether, as the resulting radical does not have the possibility to stabilize by resonance. This mechanism does not give any protons. 4.1.3.
Diphenyl
ether
2lw
Dose (Gy)
Fig. 4. The yield of protons when ethyl benzyl ether is exposed to y-rays in presence of 1 mM Ph,SSbF,.
4.1.2.
15mJ
(Fig. 5)
PTSSbF, is not soluble in diphenyl ether. In an initial phase diphenyl ether gives no protons (VII, VIII) which is
Fig. 5. The yield of protons when diphenyl Ph,SSbF,
ether and 1 mM
are exposed to y-rays.
expected since there are no a-ether hydrogens in the molecule. Scheme 6. The energy contribution at low doses is not enough to break the C-O bond. Another mechanism, “in cage” rearrangement, seems to be activated above about 600 Gy (IX, X), as the yield of protons suddenly increases [ 121. 4.2. Esters By -y-irradiation of esters, electrons are ejected from the carbonyl oxygen (XI). Scheme 7. The formed cationradical is stabilized by resonance, Scheme 7, between (XI) and (XII). The cation-radical may then be rearranged to give protons and an a-ether type radical at the ester group (XII). This resulting radical will be split into an aldehyde and an acyl radical (XIII). The stability of these products is influencing the yield of protons. The aliphatic ester might undergo a McLafferty rearrangement, Scheme 8. The mechanism involves a P-H
@a
-@=p-
l
(v,II)
rearomatization 1 @#-O Scheme 6.
+
H+
(X)
E. Malmstrijm
et al. /Nucl.
173
Phys. Res. B 105 (1995) 168-174
ii
0
R K O-R,
+
e,
(XT)
.
ii RK 0-R.
-
.I
I R
Instr. and Meth. in
*O+AR,
-
Fig. 6. The yields of protons in ethyl benzoate in the presence of mM sulfonium salt.
-
R
O-R’
+
H+
(XII)
ethyl benzoate scavenges yield of protons.
electrons
1
and hence reduces the
4.3. SMHO calculations + of the primarily
formed
abstraction from the alkyl chain to the carbonyl centered cation radical.
oxygen
Scheme
7. The
resonance
(XIII)
structures
cation-radical.
4.2.1. Aromatic versus aliphatic esters. Ethyl hexanoateethyl benzoate (Fig. 6) The soiubility of the two onium salts in ethyl hexanoate is very limited. It was not possible to make 1 mM solutions, therefore the concentrations were GZ 1 mM for both onium salts in this case. However, the yield of protons is lower from ethyl benzoate than from ethyl hexanoate, even though the concentrations of the onium salts in ethyl bensoate were all 1 mM. This can be explained by the McLafferty rearrangement that produces protons. Another additional explanation will be that the aromatic part of
In Table 2 are shown as an example typical values of electron charge distributions for dialkylethers versus diarylethers and ethyl propionate versus ethyl benzoate. By comparing the charge distributions in the neutral molecule to the corresponding charge distributions in the cation-radical intermediate, formed after the electron ejection under EB or y exposure, the following observations and interpretations can be made.
Table 1 SMHO calculations: Electron charge ROR+ * and ArOAr, ArOAr+ *
for
x W*ROE
z CWRCE
z CWiilGE
distributions
r
0+
Lg.,,,e.,,
-o-
---
+0.0415 _______..__._._..__.__~...-
._
. _ _. _ _ _ _ &r = -0.1176 __ _ _ . _ . _
I I ._ ._____._..........__...---I I
-
I+O.4156
/-0.5672
I
I
+0.3502
ROW
, ArOAr*’
E&Et . .._._____----.-
/
Scheme 8. McLafferty duce protons.
OH
rearrangement
+
of B-hydrogen
0
esters pro-
&I = +O.OlO _. _. _ _. _
br =
I
I +0.4920
Et
%., = +0.133!
._.__..._
._
I
Et%o,Et . .._____.__.._.-
H’
..
+0.1247
-0.5922
+0.6969
co.2767
R
+0.1347 _
._. .
I I 1I ______._._._..
+
‘]iR ’
Et
._
+0.1174
+
CWAIGE
Ar I
ROR / ArOAr
H 2J’
RYyH
ROR.
._..._.._.-
I
+0.3575 _.
+0.0744
I I +0.3502
4 .0.1247
-0.4672
11. SURFACE/BULK
+0.7234 _ _ _. _ _. _
&r = +1.0065
-
MODlFlCATlONS
174
E. Malmstrtim et al. /Nucl. Ins&. and Meth. in Phys. Res. B 105 (199s) 168-l 74
4.3. I. Ethers For the diakylether cation-radical there is a great increase in charge on the ether oxygen from +0.0415 to + 0.6969, meaning that = 70% of the positive charge is located at the ether oxygen. For the dialylether, the much smaller increase from + 0.1174 to + 0.2767, indicates that only =: 28% of the positive charge in the aromatic structure is located at the ether oxygen. 72% of the charge is distributed in the aryl rings. The strong delocalization of charge into the aromatic parts of the diarylether and the absence of cr-hydrogens further support the mechanism outlined in Scheme 4 and for the intermediates (VII-Xl. 4.3.2. Esters The positive charge distibution at the ester carbon in the aliphatic ester has increased moderately from + 0.4156 in the neutral molecule to f0.4920, whereas the ester carbon in the aromatic ester does not change. This is due to the “node point activity” of the carbonyl carbon in the aromatic ester structure. However, more surprising is the great difference in charge localization at the carbonyl oxygen between the two structures. For the aliphatic ester an increase from - 0.5672 to + 0.0744 is noticeable. The corresponding increase at the carbonyl oxygen in the aromatic ester is much smaller, -0.5922 to -0.4672, which is still a high localization of the electron charge. At the ether carbon center in the ester group a moderate increase is valid for the aliphatic ester, while the aromatic do not change. The main difference is the total delocalization of the positive charge into the aryl ring for the aromatic ester (+ 1.0085) and the complete localization of the positive charge into the ester group for the aliphatic ester. These fundamental differences between the two esters also reflect their reactivities, The strong localization of charge in the aliphatic esters promote the rearrangement mechanisms outlined in Schemes 7 and 8 (MacLafferty type) and for the intermediates (XII and XIII>. Due to the more delocalized positive charge distribution in the aryl ring, this carbocation radical will have a longer life time and therefore the rearrangement reactions metioned above will be less favoured. Added to this also
secondary reactions such as termination by solvated eiectrons elsewhere in the system and other strong nucleophiles will be more pronounced.
5. Conclusion The influence of a prepolymer or “monomer” backbone structure on the y-ray induced decomposition of sulfonium salts has been investigated. Generally Ph,SSbF, gave lower yields of protons than PTSSbF,, reflecting the more favourable reduction potential of the latter. Aromatic structures in the backbone act as electron scavengers, hence lowering the yield of oxidizable radicals, solvated electrons and ultimately the acid production. Thus, from a technical point of view, when formulating a cationic EBcurable composition, it is important to consider the radiation chemistry of the organic structural parts in the “monomer” and prepolymer backbone.
References
111J.V. Crivello, J.H.W. Lam, Macromolecules
10 (1977) 1307. J.V. Crivello, J.H.W. Lam, J. Pol. Sci., Polym. Chem. 17 (1979) 977. 131 A. Ledwith, Polymer 19 (1978) 1217. t41 S.P. Pappas, L.W. Hill, J. Coatings Techn. 53 (1981) 43. 151 P.E. Sundell, E.S. Jiinsson, A. Hult, J. Pol. Sci., Polym. Chem. 29 (1991) 1535. [61 S. Mah, Y. Yamamoto, K. Hayashi, Macromolecules 10 (1983) 681. 171 S.C. Lapin, RadTech 86 Conf. Proc. (Assoc. for Finish. Proc., Baltimore, 1986). Bl P.E. Sundell, Ph.D. Thesis RIT, Dept. of Polymer Technology, Stockholm, Sweden (1990). [9] P.E. Sundell, E.S. Jiinsson, A. Hult, J. Pol. Sci., Polym. Chem. 29 (1991) 1525. [lo] W.F. Schmidt, A.O. Allen, J. Phys. Chem. 72 (1968) 3730. [ll] S. Arai, M.C. Sauer Jr., J. Chem. Phys. 44 (1966) 2297. [12] J.L. Dektar, N.P. Hacker, J. Am. Chem. Sot. 112 (1990) 6004.
M