Autocatalytic fragmentation of acetoacetate derivatives as acid amplifiers to proliferate acid molecules

Article abstract of DOI:10.1021/ja972353c

A novel concept of acid proliferation to improve the photosensitivity of chemically amplified photoresist materials is described by presenting the autocatalytic fragmentation of acetoacetates having a (sulfonyloxy)methyl residue to liberate the corresponding sulfonic acid. 2-Methyl-2-((methane- or p-toluenesulfonyloxy)methyl)acetoacetates were designed to be subjected to the acid-catalyzed fragmentation through the corresponding acetoacetic acids which are readily decarboxylated and undergo the subsequent β-elimination to give a sulfonic acid which can act as the autocatalyst to lead to the increment of acid concentration in geometric progression. Among the acetoacetates tested, tert-butyl 2-methyl-2-((p-toluenesulfonyloxy)methyl)acetoacetate was the most suitable reagent for the present purpose because of its reasonable thermal stability. A nonlinear generation of the sulfonic acid was confirmed in the acidolytic transformation of the acetoacetate in nonpolar solvents and in a film of poly[4-((tert-butoxycarbonyl)oxy)styrene] (PBOCSt), respectively. It was found that the addition of the acetoacetate to a film of PBOCSt doped with a photoacid generator enhances the photosensitivity characteristics.

Full text of DOI:10.1021/ja972353c

J. Am. Chem. Soc. 1998, 120, 37-45
37
Autocatalytic Fragmentation of Acetoacetate Derivatives as Acid
Amplifiers to Proliferate Acid Molecules
Koji Arimitsu,† Kazuaki Kudo,‡ and Kunihiro Ichimura*,†
Contribution from the Research Laboratory of Resources Utilization, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226, Japan, and Institute of Industrial Science, UniVersity of Tokyo,
7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan
ReceiVed July 14, 1997X
Abstract: A novel concept of acid proliferation to improve the photosensitivity of chemically amplified
photoresist materials is described by presenting the autocatalytic fragmentation of acetoacetates having a
(sulfonyloxy)methyl residue to liberate the corresponding sulfonic acid. 2-Methyl-2-((methane- or p-
toluenesulfonyloxy)methyl)acetoacetates were designed to be subjected to the acid-catalyzed fragmentation
through the corresponding acetoacetic acids which are readily decarboxylated and undergo the subsequent
â-elimination to give a sulfonic acid which can act as the autocatalyst to lead to the increment of acid
concentration in geometric progression. Among the acetoacetates tested, tert-butyl 2-methyl-2-((p-toluene-
sulfonyloxy)methyl)acetoacetate was the most suitable reagent for the present purpose because of its reasonable
thermal stability. A nonlinear generation of the sulfonic acid was confirmed in the acidolytic transformation
of the acetoacetate in nonpolar solvents and in a film of poly[4-((tert-butoxycarbonyl)oxy)styrene] (PBOCSt),
respectively. It was found that the addition of the acetoacetate to a film of PBOCSt doped with a photoacid
generator enhances the photosensitivity characteristics.
A catalyst is defined as a chemical species which initiates
and/or accelerates a thermodynamically favorable reaction
without any change of its concentration during the chemical
transformation. One class of important catalysts is unequivo-
cally acidic substances which enhance rates of versatile organic
reactions including hydrolytic reactions, rearrangements, and
eliminations. If an acid can be produced by an acid-catalyzed
transformation of a precursor substrate to result in the increment
of the amount of acidic species in a manner of geometric
progression, subsequent acidolytic reaction rates should be
enhanced exponentially, being essentially different from the
kinetics of ordinary acidolytic reactions. This idea led us to
carry out the molecular design and the synthesis of an acid
precursor which is reasonably called an acid amplifier since the
compound creates acid molecules more than it reacts with. Acid
amplification processes can be coupled with versatile acidolytic
reactions to develop various types of nonlinear chemical
transformations. Our particular interest is the combination of
a photoacid generator with an acid amplifier since a tiny amount
of a photogenerated strong acid may be bred to result in the
drastic enhancement of a subsequent acidolytic reaction by the
autocatalytic decomposition of the acid amplifier. This leads
to the improvement of photosensitivity of chemical amplification
type photoimaging materials triggered by photogenerated acids.1
Our efforts have been focused on the development of acid
amplifiers which should fulfill the following requirements. First,
an acid amplifier should be readily subjected to an acid-catalyzed
decomposition to liberate a strong acid which is capable of
catalyzing the decomposition of itself. Second, an acid amplifier
should be thermally stable in the absence of an acid at least
under the reaction conditions to advance the autocatalytic
decomposition and a subsequent acid-catalyzed reaction. Third,
a liberated acid should be strong enough to catalyze subsequent
chemical reaction(s) to display a nonlinear chemical transforma-
tion. The major concerns of this paper are to present aceto-
acetate derivatives which meet these requirements and to show
that a novel photoimaging system is realized by the combination
of the acid amplifier with a photoacid generator and an acid-
labile polymer.2 Other acid amplifiers with different chemical
structures have been developed and reported elsewhere.3,4
Results
Molecular Design and Synthesis. An acid formed by the
autocatalytic decomposition of an acid amplifier should be
strong enough to trigger a conjugated acidolytic reaction. We
employed sulfonates as acid precursors, taking the following
into account: sulfonic acids are of sufficiently strong acidity
to catalyze diversified organic reactions and are readily available
by the esterification of alcohols. It has been well established
that acidic molecules are liberated with ease as a result of
â-elimination when the â-carbon is activated by electron-
withdrawing group(s). This suggests reversely that the â-elim-
ination may be suppressed effectively by the absence of
hydrogen atoms at the â-carbon. This had led us to an idea to
* Address correspondence to Prof. Kunihiro Ichimura, Research Labora-
tory of Resources Utilization, Tokyo Institute of Technology, 4259
Nagatsuta, Midori-ku, Yokohama 226, Japan. Tel: +81-45-924-5266.
Fax: +81-45-924-5276. E-mail: kichimur@res.titech.ac.jp.
† Tokyo Institute of Technology.
(2) A preliminary report: Ichimura, K.; Arimitsu, K.; Kudo, K. Chem.
Lett. 1995, 491.
(3) Kudo, K.; Arimitsu, K.; Ichimura, K. Mol. Cryst. Liq. Cryst. 1996,
280, 307.
(4) (a) Noguchi, S.; Arimitsu, K; Ichimura, K.; Kudo, K; Ohfuji, T.;
Sasago, M. J. Photopolym. Sci. Technol. 1997, 10, 315. (b) Ohfuji, T.;
Takahashi, M., Sasago, M.; Noguchi, S.; Ichimura, K. J. Photopolym. Sci.
Technol. 1997, 10, 551.
‡ University of Tokyo.
X Abstract published in AdVance ACS Abstracts, December 15, 1997.
(1) Reichmanis, E. In Polymers for Electronic and Photonic Applications;
Wong, C. P., Ed.; Academic Press: San Diego, 1993; p 67.
S0002-7863(97)02353-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/14/1998

38 J. Am. Chem. Soc., Vol. 120, No. 1, 1998
Scheme 1. Autocatalytic Fragmentation of 2a
Arimitsu et al.
Table 1. Acid-Catalyzed Decomposition of the Acetoacetates 2
Scheme 2. Preparation of Acid Amplifiers
concentration
(mmol dm-3
)
run AAa acidb AAa acid
solventc
DE-TL
DE-TL
DE-TL/MeOH Figure 2
2.0 DE-TL/MeOH Figure 2
DE-TL/MeOH Figure 2
remarks
Figure 1
Figure 1
1
2
3
4
5
6
7
8
9
2a none
2a TsOH
2a none
2a TsOH
2a TsOH
2a none
2a TsOH
2a TsOH 290 29
2c none 83
76 23
70
70 25
65
65
65 28
58
70 85
0
0
0
TL
DO
DO
TL
TL
Figure 3
no reaction
no reaction
Figure 4
0
10 2c MsOH
11 2e DCA
12 2e DCA
13 2f none
14 2f DCA
15 2f DCA
Figure 4
80
78
73
73 23
80 63
9.3 DT
9.8 TL
0
no reaction
no reaction
Figure 5
TL
TL
Figure 5
DO
no reaction
negligible reaction
16 2f TsOH 110
8.8 DO
a AA, acetoacetate (2). b TsOH, p-toluenesulfonic acid; MsOH,
methanesulfonic acid; DCA, dichloroacetic acid. c TL, toluene; DE-
TL, a 3:1 (v/v) mixture of diphenyl ether and toluene; MeOH, methanol;
DO, dioxane.
protect â-hydrogen with a suitable acid-labile group like a (tert-
alkylcarbonyloxy) residue. We consequently designed tert-alkyl
2-methyl-2-((sulfonyloxy)methyl)acetoacetates (2a-d) as acid
amplifiers. The sulfonates possesses no hydrogen atom at the
â-carbon so the thermal reaction is expected to be retarded. On
the other hand, the action of acid causes the decomposition of
the ester group, accompanied by the elimination of the corre-
sponding olefin and carbon dioxide, to yield a â-ketosulfonyloxy
compound which readily undergoes the â-elimination of sulfonic
acid owing to the acetyl residue (Scheme 1).
The acetoacetates (2) were prepared according to Scheme 2.
tert-Alkyl acetoacetate was monomethylated, followed by the
hydroxymethylation with formalin under an alkaline condition
to give hydroxylated derivatives (1).5 Four derivatives were
prepared from 2 with the esterification with p-toluenesulfonyl
chloride and methanesulfonyl chloride, respectively. For com-
parison, three carboxylates (2e-g) were prepared by the
esterification of 1 with dichloroacetyl chloride and benzoyl
chloride, respectively. They were purified by column chroma-
tography. Among the compounds thus prepared, the aceto-
acetates substituted with p-toluenesulfonyloxy (TsO) (2b: R
) C6H5, X ) TsO) and methanesulfonyloxy (MsO) (2c: R )
CH3, X ) MsO and 2d: R ) C6H5, X ) MsO) turned black in
color abruptly with the complete consumption of the actoacetates
to form the corresponding acid during the course of storage in
a neat state for few days at room temperature. This sudden
degradation suggests that it proceeds in a nonlinear manner.
They were stored in a refrigerator for relatively longer terms.
Thermal Decomposition in Solutions. The acidolytic
behavior of 2 in a 3:1 mixture of diphenyl ether and toluene-d8
solution at an elevated temperature was followed by NMR
spectroscopy. The formulation and experimental results are
summarized in Table 1. Because of the hard solubility of
p-toluenesulfonic acid (TsOH) in hydrocarbon solvents such as
toluene, the mixture was employed as a solvent. No change in
an NMR spectrum of 2a was observed upon heating of a solution
at 100 °C for 2 h in absence of TsOH (run 1). On the other
hand, the decomposition of 2a was induced by TsOH at the
same temperature (run 2). The acid-catalyzed fragmentation
of 2a was followed by monitoring the decrease of the proton
signals due to tert-butyl (δ ) 1.28) and â-methyl (δ ) 1.28)
groups while the formation of the unsaturated ketone (5) was
checked by the proton signal due to its vinyl group (δ ) 5.50)
with the use of 2-methoxynaphthalene as an internal standard.
Figure 1 shows that both the consumption of 2a and the
formation of 5 take place abruptly to display a sigmoidal time
course, indicating that the fragmentation proceeds autocatalyti-
cally to lead to the proliferation of the acid. As seen in Figure
1, no quantitative formation of 5 was observed. The reduction
of the yield of 5 may be ascribable to a partial loss due to radical
polymerization. The figure indicates also that the rate of
formation of 5 coincides with that of the consumption of 2a.
This reflects probably that the decarboxylation of the acetoacetic
acid 3 and the subsequent â-elimination of â-tosyloxy ketone
4 take place reasonably fast though no identification of these
intermediates was achieved because of complicated NMR
signals of a reaction mixture (Scheme 2). The almost quantita-
tive formation of TsOH in a 98% yield was confirmed separately
by the titration of a reaction mixture of 2a in a THF solution
after 3 h of heating under reflux in the presence of TsOH.
The thermal behavior of 2a displayed considerable solvent
dependence. When methanol was added to a mixed solvent of
diphenyl ether and toluene-d8 to enhance the solubility of TsOH,
no degradation of 2a was caused by heating at 100 °C in the
(5) (a) Hiroi, K.; Abe, J. Chem. Pharm. Bull. 1991, 39, 616. (b) Wilson,
B. D. J. Org. Chem. 1963, 28, 314.

Acid Amplifiers to Proliferate Acid Molecules
J. Am. Chem. Soc., Vol. 120, No. 1, 1998 39
Figure 3. Time course of the consumption of 2a in toluene-d8 at 100
°C.
Figure 1. Time courses of the consumption of 2a (0) and the formation
of methyl isopropenyl ketone ([) in the presence of 25 mmol dm-3
p-toluenesulfonic acid and of the conversion of 2a in the absence of
sulfonic acid (O) in a 3:1 mixture of diphenyl ether and toluene-d8 at
100 °C.
Figure 4. Time courses of the consumption of 2c (0) and the formation
of methyl isopropenyl ketone ([) in the presence of 23 mmol dm-3
methanesulfonic acid and of the conversion of 2c in absence of the
sulfonic acid (O) in toluene-d8 at 100 °C.
Figure 2. Time courses of the consumption of 2a in the absence of
(4) and in the presence of 2.0 mmol dm-3 (O) and 28 mmol dm-3 (0)
p-toluenesulfonic acid and the formation of methyl isopropenyl ketone
([) in the presence of 28 mmol dm-3 of the acid in a 3:1 mixture of
diphenyl ether and toluene-d8 containing 7.5 v/v % of methanol-d4 at
100 °C.
It was found that 2c shows no degradation even at 100 °C
for a while in toluene in absence of methanesulfonic acid
(MsOH) (run 9). A sigmoidal increase in the conversion was
induced when a toluene solution was heated in the presence of
MsOH, as given in Figure 4. Methyl isopropenyl ketone (5)
demonstrated a maximum yield during heating, followed by a
decrease in the yield, probably again because of the polymer-
ization.
The tert-butyl acetoacetate having a dichloroacetoxy residue
(2e) was quite thermally stable even in the presence of
dichloroacetic acid, indicating that this ester cannot serve as an
acid amplifier (runs 11 and 12). It follows that 2-phenyl-2-
propylacetoacetate with a dichloroacetoxy group (2f) was
subjected to the determination of acidolytic reaction in toluene
since the deprotection of the ester proceeds much faster than
the tert-butyl ester (runs 13 and 14).6 As shown in Figure 5,
2f displays an abrupt increase in the conversion in the presence
of dichloroacetic acid, indicating that the disappearance of the
acetoacetate proceeds in a nonlinear manner though the rate
was much slower.
absence and in the presence of a small amount of TsOH (runs
3 and 4 in Table 1). Although a large amount of TsOH results
in the consumption of 2a leading to the formation of 5, no
sigmoidal curve was observed (run 5) (Figure 2). In a sharp
contrast to these results, 2a displayed an abrupt decomposition
with a sigmoidal time conversion curve when the compound
was heated in toluene-d8 as a nonpolar solvent at 100 °C even
in the absence of TsOH (run 6), as shown in Figure 3. The
result suggests that 2a suffers from a pyrolytic decomposition
to give TsOH, which accelerates the fragmentation reaction.
These results all imply that 2a itself is subjected to mono-
molecular fragmentation reaction upon prolonged heating at an
elevated temperature to produce TsOH and that the acidolytic
deprotection of 2a is considerably suppressed by solvents such
as methanol in diphenyl ether which act as weak bases. In fact,
it was confirmed that 2a was quite thermally stable in dioxane-
d8 (runs 7 and 8) even though the solution was contaminated
with TsOH. Dioxane acts as proton acceptor so effectively that
the acid-catalyzed deprotection of tert-butyl ester is severely
retarded.
Thermal Behavior of the Acetoacetates in Polymer Films.
In order to confirm the third requirement for an acid amplifier,
acetoacetate 2a was coupled with a polystyrene substituted with
(tert-butoxycarbonyl)oxy (t-BOC) residues (PBOCSt) (6) as an
(6) Sieber, P.; Iselin, B. HelV. Chim. Acta 1968, 51, 614.

40 J. Am. Chem. Soc., Vol. 120, No. 1, 1998
Arimitsu et al.
Figure 7. Time courses of the consumption of 10 wt % of 2a (0) and
the deprotection of PBOCSt ([) in a film.
Figure 5. Time courses of the consumption of 2f in the absence of
([) and in the presence of (0) 23 mmol dm-3 dichloroacetic acid in
toluene-d8 at 100 °C.
Pyrolytic behavior of the other acetoacetates was determined
in PBOCSt films. 2b having a more acid-labile 2-phenyl-2-
propyl ester group decomposed more rapidly in a film of the
polymer. The CdO bands of both 2b and the polymer
disappeared completely after heating for 3 min at 100 °C,
indicating that this compound is so thermally labile that it is
hard to apply to photoimaging materials, as mentioned below.
On the contrary, the tert-butyl ester having a mesyloxy group
(2c) exhibited different behavior in a polymer film. The heat
treatment of a PBOCSt film containing 100 mol % of 2c at
100 °C resulted in the gradual reduction of the CdO band due
to the ketone of 2c whereas no alteration was observed in the
CdO absorption bands of the polymer. This means evaporation
of the acetoacetate from a polymeric film during heating without
any thermal decomposition. The acetoacetates bearing dichlo-
roacetoxy group (2e and 2f) gave similar results when they were
doped in polymer films. Being judged from these data, it was
concluded that 2a fulfills all requirements for the present purpose
to achieve acid proliferation.
Figure 6. IR spectral change of a film of PBOCSt containing 10 wt
% of 2a (a) before and after heating at 100 °C for (b) 105 and (c) 120
min.
Acid Proliferation in Photosensitive Polymer Films. The
phototriggered acid proliferation as a result of autocatalytic
fragmentation was determined by the use of a film of PBOCSt
doped with 30 wt % of 2a in the presence of 3.6 wt % of
(tosyloxy)methylated benzoin (7) which generates TsOH upon
UV irradiation.8 A thin film of PBOCSt containing 2a and 7
was spin-cast on a silicon wafer. Film thickness was adjusted
to ca. 0.30 µm so that films are put in color due to the
interference of light. In this way, the progress of the acid-
catalyzed polymeric reaction was monitored in real time during
heat treatment after UV irradiation simply by observing the
interference color changes due to the film thickness being
markedly reduced bcause of the elimination of isobutene and
carbon dioxide (Scheme 3). When a film was heated on a hot
plate at 100 °C after UV irradiation through a circular window
of a 3 mm diameter, a rapid color change took place exclusively
at an irradiated area within a few seconds.
Surprisingly, prolonged heating of the film gave birth to the
gradual enlargement of the circle in a concentric manner as a
result of the lateral expansion of the color change to unexposed
areas, as given in Figure 8 when 30 wt % of 2a was added.
Other photoacid generators which genenerate strong acids other
than TsOH resulted in a similar phenomenon. Figure 9 shows
the concentric expansion of color-changed circles as a function
of heating time when diphenyl (4-phenylthiophenyl)sulfonium
acid-labile polymer.7 A thin film of PBOCSt containing 10 wt
% of 2a was spin-coated on a silicon wafer and heated at 100
°C. No essential change in IR spectra of the film was observed
at the early stage of heating. But upon prolonged heating, the
CdO stretching vibration due to the carbonyl residue of 2a at
1719 cm-1 and that due to carbonate of PBOCSt at 1762 cm-l
disappeared quite suddenly, accompanied by the abrupt appear-
ance of a broad absorbance at ca. 3400 cm-1 ascribable to
phenolic OH, supporting the deprotection of the t-BOC residues
(Figure 6). This shows that 2a decomposes thermally in a
polymer matrix just as in the case of a toluene solution upon
prolonged heating and that the acid thus formed initiates acid
amplification as a result of autocatalytic transformation of 2a.
This situation is visualized by plotting peak heights of CdO
vibrations of both of PBOCSt and 2a as a function of heating
period, as given in Figure 7. 2a showed no marked alteration
of an IR absorption spectrum at least for ca. 1 h. The carbonyl
absorption band decreased abruptly after prolonged heating for
about 80 min, accompanied by the decrease in the absorption
band due to the carbonate group of the acid-labile polymer.
These results indicate clearly that the thermal decomposition
of 2a liberates TsOH, which catalyzes both the self-decompsition
of 2a and the deprotection of PBOCSt.
(7) Ito, H.; Willson, C. G.; Fre´chet, J. M. J. Digest of Technical Papers
of 1982 Symposium on VLSI Technology, 1982, 86.
(8) Ro¨schert, H.; Eckes, Ch.; Pawlowski G. SPIE 1993, 1952, 342.

Acid Amplifiers to Proliferate Acid Molecules
J. Am. Chem. Soc., Vol. 120, No. 1, 1998 41
Scheme 3. Working Principle of an Acid Proliferation-Type
Photopolymer System
reversible elimination/addition process.10 The time conversion
curve of this process is sigmoidal, implying an autocatalytic
reaction. As seen in Figure 1, the acid-catalyzed reaction of
the acetoacetate substituted with a tosyloxy residue (2a) proceeds
in a sigmoidal manner, indicating that the consumption of 2a
takes place autocatalytically. The unsaturated ketone 5 as a
product is formed in a nonlinear manner in line with the
disappearance of 2a. This implies that the intermediates such
as the deprotected acetoacetic acid (3) and the â-tosyloxy ketone
(4) are short-lived in the present reaction conditions.
The marked solvent dependence of the autocatalytic process
is interpreted as follows. The deprotection of 2a is initiated by
the protonation of an acidic catalyst at an oxygen atom of the
ester residue so that any solvent molecules having oxygen
atom(s) compete with 2a in the protonation. The level of
suppression of the acidolytic fragmentation of 2a is determined
by the basicity of solvents. As summarized in Table 1, dioxane
quenches completely the acidolytic reaction. Methanol is also
an effective quencher. These results arise from the basicity of
these solvents, pKa’s of conjugated acids of dioxane and
methanol are -2.9211 and -2.2,12 respectively, whereas ester
groups have a pKa of ca. -6.0.13 Besides, these systems involve
a large excess amount of solvent molecules so that even
pyrolytic decomposition of 2a, leading to the formation of
TsOH, has essentially no role in the acidic catalysis. On the
other hand, the autocatalytic fragmentation proceeds in a mixture
solvent of diphenyl ether and toluene-d8 due to the lower bacisity
of the aromatic ether. Figures 3 and 4 demonstrate the sigmoidal
consumption of the corresponding acetoacetates since toluene
molecules are hardly protonated by TsOH and MsOH. In other
words, the behavior of both acetoacetates in toluene reflects
their inherent thermal lability and autocatalytic processes,
respectively. It is safe to say that both tert-butyl esters bearing
tosyloxy and mesyloxy residues (2a and 2c) are not deteriorated
upon heating at 100 °C for 50 min or more. This is important
information concerning their applicability to the acid prolifera-
tion coupled with acidolytic reactions of polymer side chains
to provide a novel photoimaging materials such as chemically
amplified photoresists.
hexafluoroantimonate (8) was employed as a photoacid genera-
tor. In the case of a lower concentration of 10 wt % of the
acetoacetate (2a), the expansion of color-bleached area slowed
down upon prolonged baking, probably because of the gradual
evaporation of the acid amplifier from a film. In order to check
a plasticizing effect of 2a, a system containing 29 wt % of a
related compound, tert-butyl 2-methyl-2-((benzoyloxy)methyl)-
acetoacetate (2g) was examined. This compound liberates
benzoic acid with much weaker acidity and hence cannot act
as an acid amplifier. It was confirmed that no area expansion
is induced at all. These mean that the area expansion of the
polymer decomposition reflects the proliferation of TsOH in a
polymer matrix and is not due to the plasticizing effect.
The effect of addition of 2a on the performances of the so-
called chemically amplified photoresists was examined by using
a typical chemically amplification photoresist consisting of poly-
(t-BOC-styrene) (6)7 and tosyloxylated benzoin (7).8 The results
are shown in Figure 10. Thin films of polymers doped with
the photoacid generator in absence or in the presence of 10 wt
% of 2a were exposed to UV light for various periods, followed
by heat treatment as a postexposure baking. The extent of acid-
catalyzed deprotection of t-BOC side chains was estimated by
measuring film thickness, because the thickness is reduced up
to ca. 60% as a result of the elimination of t-BOC residues.
Although the addition of 2a displays no effect on the photo-
sensitivity characteristics upon heating for 1 min (Figure 10a),
the postexposure baking for 3 min results in an abrupt decrease
in the film thickness at UV exposure period of 200 s or longer,
as seen in Figure 10c. The abrupt changes in IR spectra of
films were also observed to support the deprotection of t-BOC
residues.
The results shown in Figure 7 demonstrate unequivocally that
the autocatalytic fragmentation of 2a is brought about even in
polymeric films and that the acid-generated TsOH catalyzes the
deprotection of tert-butyl carbonate residues. The unique
features of this system are brought into relief by the results
shown in Figures 8 and 9. The area, where tert-butyl carbonate
side chains are deprotected by TsOH, increases upon heat
treatment in a manner of geometrical progression. Therefore,
it is reasonable to refer the autocatalytic acid generation as the
acid proliferation reaction. 2c does not meet the requirements
for the acid proliferation in polymeric films because of the
volatility leading to the escape during heat treament. However,
as Figure 4 shows, this mesyloxylated acetoacetate seems to
be a good candidate for acid proliferation systems if the
suppression of the volatility is enhanced, taking into account
the faster fragmentation, when compared with the tosyloxylated
counterpart shown in Figure 3. The suppression of the volatility
should be overcome by suitable ways, including the increase
in the molecular weight of a mesyloxylated acetoacetate.
Discussion
There have been only few reports on the autocatalytic
generation of a strong acid. It was mentioned by Winstein et
al. in a series of their systematic studies on solvolytic reactions
without any experimental support that the reaction of a
p-methoxyneophyl toluenesulfonate proceeds autocatalytically
in benzene.9 On the other hand, Nenitzescu et al. suggested
that a cycloalkyl benzenesulfonate demonstrates autocatalytic
elimination reaction to produce a cycloalkene and benzene-
sulfonic acid which adds to the cycloalkene again to result in a
(10) Nenitzescu, C. D.; Ioan, V.; Teodorescu, L. Chem. Ber. 1957, 90,
585.
(11) Arnett, M.; Wu, C. Y. J. Am. Chem. Soc. 1960, 82, 4999.
(12) Gold, V.; Laali, K.; Morris, K. P.; Zdunek, L. Z. J. Chem. Soc.,
Parkin Trans. 2 1985, 865.
(9) Smith, S. G.; Fainberg, A. H.; Winstein, S. J. Am. Chem. Soc. 1961,
83, 618.
(13) Bagno, A.; Scorrano, G.; Lucchini, V. Gazz. Chim. Ital. 1987, 117,
475.

42 J. Am. Chem. Soc., Vol. 120, No. 1, 1998
Arimitsu et al.
Figure 8. Photographs of a silicon wafer spin-cast with a 0.3 µm thick film of PBOCSt containing 3.6 wt % of the photoacid generator (7) and
30 wt % of 2a upon heating at 100 °C for (a) 4, (b) 10, (c) 15, and (d) 20 min after UV irradiation through a circular window of 3 mm diameter.
influenced by the size of acidic species and comes to less than
100 nm for TsOH upon heating for 60 min.15 It was also
mentioned that a remaining solvent in a chemically amplified
photoresist enlarges the diffusion range of an acidic species; a
diffusion range is ca. 200 nm when 1 wt % of a solvent
remains.16 These values are too small when compared with the
extent of the lateral expansion shown in Figure 9. This implied
that the area expansion of acidolytic deprotection occurs through
a phenomenon quite different from the diffusion in polymer
solids.
It was found that the lateral expansion is practically prohibited
by covering a photosensitive polymer film with a spin-cast
Novolak film or simply with a glass slide. These facts support
that the theory that the marked lateral expansion of the present
system does not arise from the diffusion of TsOH within a
polymer matrix but from the evaporation from and landing of
acid molecules on a film surface, leading to the “air infection”.
Although the evaporation of a photogenerated acid from a
polymeric layer has been detected and discussed in connection
with the improvement of performances of photoresists,18,19 it is
evident that the evaporation mechanism of the present system
is quite different from that of chemically amplified photoresists.
As seen in Figure 9, no expansion of the area was observed on
a millimeter scale when no acid amplifier was added to give a
conventional chemically amplified photoresist.
Although the effect on the photosensitivity characteristics of
a chemically amplified photoresist was observed as seen in
Figure 10, the level of amplification by autocatalytic acid
generation is not so remarkable. Three possilibities should be
considered. The first one is that the acid-calalyzed deprotection
of t-BOC side chain takes place much faster than the acid-
catalyzed fragmentation of 2a so that no marked enhancement
in photosensitivity is realized. This is not the case as revealed
by IR spectral changes of a film doped with 2a (Figure 6). Both
t-BOC and 2a disappear suddenly upon heating, indicating that
there is no marked difference in the deprotection rate between
t-BOC residues and 2a. The second possibility is that 2a acts
Figure 9. Expansion of the diameter of circular deprotected area of a
film of PBOCSt containing 5.5 wt % of diphenyl (4-(phenylthio)-
phenyl)sulfonium hexafluoroantimonate as a photoacid generator in the
absence of (1) and in the presence of 10 wt % (O) and 30 wt % (b)
of 2a upon heataing at 100 °C.
From a practical viewpoint, the drastic expansion of acidolytic
areas upon heating as shown in Figures 8 and 9 is a fatal
drawback when this system is applied to microphotopatterning
owing to the considerable decrease in the resolution power. On
one hand, it was reported that photogenerated acid molecules
migrate within a sphere representing a diffusion range in
polymer matrices.14-17 On the basis of a chain length of the
acidolytic deprotection of t-BOCSt polymer, the radius of this
sphere was estimated to be approximately 5 nm when resist
films are heated for 1 min.14 On the other hand, it was described
that the diffusion range of an acid photogenerated in a Novolak
resin film into another fresh acid-labile polymer film is markedly
(14) (a) McKean, D. R.; Schaedeli, U. P.; MacDonald, S. A. J. Polym.
Sci., Part A, Polym. Chem. 1989, 27, 3927. (b) McKean, D. R.; Alem, R.
D.; Kasai, P. H.; Schaedeli, U. P.; MacDonald, S. A. SPlE 1992, 1672, 94.
(15) Schlege. L.; Ueno, T.; Hayashi, N.; Iwayanagi, T. Jpn. J. Appl. Phys.
1991, 30, 3132.
(16) Asakawa, K.; Ushirogouchi, T.; Nakase, M. J. Vac. Sci. Technol.
1995, B13, 833.
(18) Ro¨schert, H.; Echkes, Ch.; Endo, H.; Kinoshita, Y.; Kudo, T.;
Masuda, S.; Okazaki, H.; Padmanaban, M.; Przybilla, K.-L.; Spiess, W.;
Suehiro, N.; Wengenroth, H.; Pawlowski, G. SPIE 1993, 1925, 14.
(19) Kihira, N.; Saito, S.; Ushirogouchi, T.; Nakase, M. J. Photopolym.
Sci. Technol. 1995, 8, 561.
(17) Itani, T.; Yoshino, H.; Hashimoto, S.; Yamana, M.; Samoto, N.;
Kasama, K. Jpn. J. Appl. Phys. 1996, 35, 6501.

Acid Amplifiers to Proliferate Acid Molecules
J. Am. Chem. Soc., Vol. 120, No. 1, 1998 43
is based on the entrapment of a generated acid species by some
kind of proton acceptor(s) in polymer films. In fact, as given
in Table 1, solvent molecules having proton acceptor site(s) such
as dioxane and methanol suppress critically the autocatalytic
fragmentation. In the present polymeric systems, there seem
to be many trapping sites capable of protonation by TsOH
including t-BOC side chains, phenolic OH residues as a
deprotected functional residue, and some degradation products
derived from 2a. Although detailed investigation is required,
it is very likely that the acid proliferation is suppressed by this
mechanism. In other words, an appropriate choice of acid-labile
polymers with much lower basicity may exhibit the marked
enhancement of photosensitivity in the presence of an acid
amplifier. Endeavor to boost the photosensitivity of chemically
amplified photoresist systems has been concentrated so far
predominantly on the improvement of quantum efficiency of
photoacid generation and the optimization of chemical structures
of acid-labile polymers. We anticipate that the concept of acid
prolifieration coupled with acidolytic reactions of polymer
systems provides a novel way to improve the performances of
photopolymers.20 It is worthy to mention that an acid amplifier
is practically applicable to 193 nm microphotolithography to
enhance performances including superfine resolution of a
chemically amplified photoresist.4b
Conclusion
Some of acetoacetate derivatives having (tosyloxy)methyl as
well as (mesyloxy)methyl residues are subjected to the auto-
catalytic fragmentation to produce TsOH or MsOH which
subsequently catalyzes the deprotection of tert-butyl carbonate
side chains attached to polystyrene. This type of reaction is
reasonably called the acid proliferation on account of the
generation of catalytic acids in a manner of geometrical
progression. Among the compounds tested, 2a meets the re-
quirements for the application to chemically amplified photo-
imaging materials because of its reasonable thermal stability
and the ability to generate TsOH which possesses enough acidity
to catalyze subsequent acidolytic reactions such as deprotection
of polymer side chains. The addition of 2a to a conventional
chemically amplified photoresist consisting of t-BOC-styrene
and a photoacid generator results in the enhancement of
photosensitivity characteristics.
Experimental Section
Materials. tert-Butyl 3-ketobutanoate was commercially available
while 2-phenyl-2-propyl ketobutanoate was prepared according to a
literature.21 Diphenyl (4-phenylthiophenyl)sulfonium hexafluoro-
antimonate was a gift from Midori Chemicals Co., Ltd. Poly(4-((tert-
butoxycarbonyl)oxy)styrene) (PBOCSt)7 and (tosyloxy)methylated ben-
zoin8 were prepared according to literatures.
tert-Butyl 2-Methyl-2-(hydroxymethyl)-3-ketobutanoate (1a). To
a suspension of sodium hydride (20 g; 0.51 mol) in 140 mL of THF
was added tert-butyl acetoacetate (70 mL; 0.42 mol) dropwise under
ice cooling, followed by stirring for 1 h. A solution of methyl iodide
(31 mL; 0.5 mol) in 140 mL of THF was added to the reaction mixture
under ice cooling, and the mixture was stirred for 2 days at room
temperature. After the conventional workup, a colorless oil of bp7 60
°C was obtained and weighed 54 g. Gas chromatography revealed that
the product was a 2:1 mixture of monomethylated and dimethylated
esters. The esters (24 g) were treated with 7.2 mL of formalin and
0.14 g of potassium hydroxide in 12 mL of ethanol. After 6 days of
stirring at room temperature, the mixture was neutralized with dilute
Figure 10. Photosensivitity curves of films of PBOCSt containing 3.6
wt % of the photoacid generator (7) in the absence of (0) and in the
presence of 10 wt % of 2a ([). The films were exposed to UV light,
followed by heating at 100 °C for (a) 1, (b) 2, and (c) 3 min,
respectively.
as a quencher of excited state(s) of a photoacid generator. It
seems to be likely that triplet energy transfer occurs in solid
films. This is also not feasible, however, because both 2a and
a photoacid generator (7) bear a common aliphatic CdO
chromophore so that the specific quenching of triplet excited
state of 7 by 2a takes place only scarcely. Another possibility
(20) Arimitsu, K.; Kudo, K.; Hayashi, Y.; Ichimura, K. J. Photopolym.
Sci. Technol. 1996, 9, 29.
(21) Lawesson, S.-O.; Gronwall, S.; Sandberg, R. Organic Syntheses;
Wiley: New York, 1973; Collect. Vol. 5, pp 155-157.

44 J. Am. Chem. Soc., Vol. 120, No. 1, 1998
Arimitsu et al.
hydrochloric acid and extracted with benzene. An organic layer was
washed with water, dried over magnesium sulfate, and subjected to
column chromatography on silica gel to yield a yellowish oil weighing
7.6 g (20% yield). 1H-NMR (CDCl3): δ (ppm) 1.37 (s, 3H,
C(CH3)CO), 1.50 (s, 9H, C(CH3)3), 2.24 (s, 3H, CH3CO), 2.89 (br t, J
) 6 Hz, 1H, OH), 3.82 (br d, J ) 6 Hz, 2H, CH2O). IR (neat, cm-1):
3502 (OH), 1725 (>CdO of ester), 1711 (>CdO). Anal. Found: C,
58.92; H, 9.23. Calcd for C10H18O4: C, 59.39; H, 8.97.
a saturated sodium bicarbonate solution and a sodium chloride solution.
Drying over magnesium sulfate and evaporation of the solvent gave
an oily residue which was purified by column chromatography on silica
gel with benzene as an eluent. A viscous yellowish oily product
weighed 1.2 g (97% yield). 1H-NMR (CDCl3): δ (ppm) 1.50 (s, 12H,
C(CH3)3, C(CH3)CO), 2.22 (s, 3H, CH3CO), 3.05 (s, 3H, OSO2CH3),
4.50 (s, 2H, CH2OSO2). 13C-NMR (CDCl3): δ (ppm) 17.2 (CH3), 27.2
(CH3), 27.5 (CH3), 36.9 (CH3), 59.9 (>C<), 71.1 (>C<), 83.1 (CH3),
168.3 (CdO), 202.7 (CdO). IR (neat, cm-1): 2981, 1738 (>CdO of
ester), 1714 (>CdO). Anal. Found: C, 47.33; H, 7.45; S, 10.3%.
Calcd for C11H20O6S: C, 47.13; H, 7.19; S, 11.44.
2-Phenyl-2-propyl 2-Methyl-2-(hydroxymethyl)-3-ketobutanoate
(lb). To a suspension of sodium hydride (4.72 g; 0.118 mol) in 40
mL of THF was added 2-phenyl-2-propyl 3-ketobutanoate (22 g; 0.099
mol) in 20 mL of THF dropwise under ice cooling, followed by stirring
for 1 h. Subsequently, a solution of methyl iodide (6.1 mL; 0.098
mol) in 20 mL of THF was added to the mixture under ice cooling.
The mixture was stirred for 22 h at room temperature, followed by
dilution with ether and neutralization with dilute hydrochloric acid,
followed by washing with a saturated aqueous solution of sodium
hydogen sulfite, a sodium bicarbonate solution, and a sodium chloride
solution. The organic layer was dried over anhydrous sodium sulfate
and evaporated to dryness to give 23 g of a crude methylated ester.
The product was dissolved in 10 mL of ethanol and mixed with 7.8
mL of formalin and 0.1 g of potassium hydroxide. After stirring for
41 h at room temperature, benzene was added to the reaction mixture
to be washed with dilute hydrochloric acid, a sodium bicarbonate
solution, and a saturated sodium chloride solution. After the mixture
was dried over anhydrous sodium sulfate, the solvent was evaporated
off and an oily residue was purified by column chromatography on
silica gel with a 10:1 mixture of hexane and ethyl acetate as an eluent
to give 9.0 g (23% yield) of a yellowish oil. 1H-NMR (CDCl3): δ
(ppm) 1.32 (s, 3H, C(CH3)CO), 1.76 (s, 3H, OC(CH3)2), 1.80 (s, 3H,
(CH3)2), 2.20 (s, 3H, CH3CO), 2.70 (t, J ) 7.0 Hz, 1H, OH), 3.80 (d
of AB q, J ) 7.0, 13 Hz, 2H, CH2O), 7.30 (s, 5H, Ar-H). IR (neat,
cm-1): 3500 (OH), 1731 (>CdO of ester), 1711 (>CdO). Anal.
Found: C, 66.32; H, 7.49. Calcd for C14O20O4: C, 66.65, H, 7.99.
tert-Butyl 2-Methyl-2-((p-toluenesulfonyloxy)methyl)-3-keto-
butanoate (2a). To 20 mL of dichloromethane was added 4.5 g (24
mmol) of p-toluenesulfonyl chloride, 2.4 g (24 mmol) of triethylamine,
and 1.2 g (10 mmol) of 4-(dimethylamino)pyridine, followed by stirring
for 15 min at room temperature. A solution of 4.0 g of tert-butyl
2-methyl-2-(hydroxymethyl)-3-ketobutanoate (1a) in 15 mL of dichlo-
romethane was added dropwise to the reaction mixture and heated for
8.5 h under reflux. The mixture was diluted with ether, washed with
a saturated aqueous solution of copper sulfate and sodium chloride
solution, and dried over magnesium sulfate to evaporate the solvent.
An oily residue was purified by column chromatography on silica gel
with a 10:1 mixture of hexane and ethyl acetate to afford 3.6 g (51%
yield) of colorless crystals melting at 52-53 °C. 1H-NMR (CDCl3):
δ (ppm) 1.38 (s, 3H, C(CH3)CO), 1.40 (s, 9H, OC(CH3)3), 2.15 (s,
3H, CH3CO), 2.47 (s, 3H, Ar-CH3), 4.28 (AB q, J ) 10 Hz, 2H,
CH2OSO2), 7.38 (d, J ) 7.7 Hz, 2H, Ar-H), 7.77 (d, J ) 7.7 Hz, 2H,
Ar-H). IR (KBr, cm-1): 1738 (>CdO of ester), 1719 (>CdO). Anal.
Found: C, 57.18; H, 6.90; S, 8.84. Calcd for C17H24O6S: C, 57.29;
H, 6.79; S, 9.00.
2-Phenyl-2-propyl 2-Methyl-2-((methanesulfonyloxy)methyl)-3-
ketobutanoate (2d). This was prepared in a similar way in a 71%
yield as a yellowish oil by the reaction of 2-phenyl-2-propyl 2-methyl-
2-(hydroxymethyl)-3-ketobutanoate (1b) with methanesulfonyl chloride.
1H-NMR (CDCl3): δ (ppm) 1.49 (s, 3H, C(CH3)CO), 1.75 (s, 3H,
OC(CH3)2), 1.80 (s, 3H, O(CH3)2), 2.20 (s, 3H, CH3CO), 2.90 (s, 3H,
OSO2CH3), 4.48 (dd, J ) 13 Hz, 2H, CH2OSO2), 7.30 (s, 5H, Ar-H).
IR (neat, cm-1): 2985, 1738 (>CdO of ester), 1714 (>CdO).
tert-Butyl 2-Methyl-2-((dichloroacetoxy)methyl)-3-ketobutanoate
(2e). A solution of 0.73 g (5.0 mmol) of dichloroacetyl chloride in 5
mL of THF was added dropwise to a solution of 1.0 g (4.9 mmol) of
tert-butyl 2-methyl-2-(hydroxymethyl)-3-ketobutanoate (1a) and 0.53
mL (5.0 mmol) of 2,6-lutidine in 5 mL of THF under ice cooling and
stirred for 3 h at room temperature. The reaction mixture was diluted
with ether, washed with dilute hydrochloric acid, saturated sodium
bicarbonate solution, and saturated sodium chloride solution, and dried
over sodium sulfate. Evaporation of the solvent gave an oil which
was subjected to column chromatography on silica gel with hexane as
an eluent to give 0.28 g (18% yield) of the ester as a yellowish oil.
1H-NMR (CDCl3): δ (ppm) 1.45 (s, 12H, C(CH3)3, C(CH3)CO), 2.20
(s, 3H, CH3CO), 4.56 (s, 2H, CH2OCO), 5.92 (s, 1H, CHCl2). IR (neat,
cm-1): 2981, 1755 (>CdO of ester), 1713 (>CdO). Anal. Found:
C, 46.09; H, 5.97; Cl, 22.84. Calcd for C12H18O5Cl2: C, 46.02; H,
5.79; Cl, 22.64.
2-Phenyl-2-propyl 2-Methyl-2-((dichloroacetoxy)methyl)-3-keto-
butanoate (2f). This was synthesized in a similar way in a 77% yield
as colorless crystals melting at 37-39 °C. 1H-NMR (CDCl3): δ (ppm)
1.47 (s, 3H, COC(CH3)CO), 1.80 (s, 6H, OC(CH3)2), 2.20 (s, 3H,
CH3CO), 4.62 (s, 2H, CH2OCO), 5.90 (s, 1H, CHCl2), 7.30 (s, 5H,
Ar-H). IR (KBr, cm-1): 2985, 1770 (>CdO of ester), 1753 (>CdO
of ester), 1714 (>CdO). Anal. Found: C, 53.88; H, 5.29; Cl, 19.79.
Calcd for C17H20O5Cl2: C, 54.41; H, 5.37; Cl, 18.90.
tert-Butyl 2-Methyl-2-((benzoyloxy)methyl)-3-ketobutanoate (2g).
In a similar manner, benzoyl chloride instead of dichloroacetyl chloride
was treated with tert-butyl 2-methyl-2-(hydroxymethyl)-3-ketobutanoate
(1a) to give the ester in 52% yield as colorless crystals of mp 42-43
°C. 1H-NMR (CDCl3): δ (ppm) 1.40 (s, 9H, C(CH3)3), 1.48 (s, 3H,
C(CH3)CO), 2.25 (s, 3H, CH3CO), 4.65 (s, 2H, CH2OCO), 7.30-7.60
(m, 3H, Ar-H), 7.90-8.07 (m, 2H, Ar-H). IR (KBr, cm-1): 2981,
1726 (>CdO of ketone, >CdO of ester). Anal. Found: C, 66.31;
H, 7.33. Calcd for C17H22O5: C, 66.65; H, 7.24.
2-Phenyl-2-propyl 2-Methyl-2-((p-toluenesulfonyloxy)methyl)-3-
ketobutanoate (2b). This was prepared in a similar manner in a 69%
yield as a yellowish oil by the reaction of 2-phenyl-2-propyl 2-methyl-
2-(hydroxymethyl)-3-ketobutanoate (1b) with p-toluenesulfonyl chlo-
ride. lH-NMR (CDCl3): δ (ppm) 1.39 (s, 3H, C(CH3)CO), 1.75 (s,
3H, OC(CH3)2), 1.79 (s, 3H, OC(CH3)2), 2.12 (s, 3H, CH3CO), 2.44
(s, 3H, Ar-CH3), 4.30 (AB q, J ) 14 Hz, 2H, CH2OSO2), 7.30 (s, 5H,
Ar-H), 7.35 (d, J ) 8.7 Hz, 2H, Ar-H), 7.74 (d, J ) 8.7 Hz, 2H, Ar-
H). IR (neat, cm-1): 2983, 1738 (>CdO of ester), 1716 (>CdO).
Anal. Found: C, 62.98; H, 6.48; S, 6.76. Calcd for C22H26O6S: C,
63.14; H, 6.26; S, 7.66.
Thermal Decomposition in Solutions. A 3-ketobutanoate (2) and
2-methoxynaphthalene as an internal standard were dissolved in a
solvent containing a small amount of tetramethylsilane. The solution
placed in an NMR tube was heated at 100 °C in an oven to be subjected
to intermittent NMR measurement using a JEOL FX-90 NMR
spectrometer to monitor the thermal stability of 2. When no alteration
in the NMR spectra was observed upon prolonged heating, p-
toluenesulfonic acid monohydrate was added to a fresh solution to
follow the acidolysis of the ester by means of NMR spectra. The
compositions for the evaluation of thermal stability are summarized in
Table 1. Since p-toluenesulfonic acid monohydrate was hardly soluble
in toluene-d8 and a 3:1 mixed solvent of diphenyl ether and toluene-
d8, 7.5 v/v % of methanol-d4 was added to the 3:1 mixed solvent to
enhance the solubility. The consumption of the tert-butyl esters and
2-phenyl-2-propyl esters was followed by monitoring the decrease of
peak height of proton signals due to CO(CH3)CO and tert-butyl and
that due to COOC(CH3)2Ph with the use of the methyl protons of
2-methoxynaphthalene as an internal standard while the formation of
tert-Butyl
2-Methyl-2-((methanesulfonyloxy)methyl)-3-keto-
butanoate (2c). To a solution of 0.90 g (4.5 mmol) of tert-butyl
2-methyl-2-(hydroxymethyl)-3-ketobutanoate (1a) and 0.52 g (5.1
mmol) of triethylamine in 2 mL of dichloromethane was added 0.51 g
(4.5 mmol) of methanesulfonyl chloride in 2 mL of dichloromethane
dropwise under ice cooling. After 1.5 h of stirring at room temperature,
the reaction mixture was diluted with dichloromethane and washed with

Acid Amplifiers to Proliferate Acid Molecules
J. Am. Chem. Soc., Vol. 120, No. 1, 1998 45
a fragmentation product, methyl isopropenyl ketone, was traced by the
increment of peak height of vinyl protons.
Observation of Expansion of Areas Displaying the Reduction of
Film Thickness. A cyclohexanone solution containing PBOCSt, a
photoacid generator, and an acid amplifier was spin-coated on a silicon
wafer to give a thin film of ca. 0.3 µm thickness. After being prebaked
for 1 min at 100 °C, the film showing violet interference color was
irradiated with a 500 W super-high-pressure mercury arc through a
circular window of a 3 mm diameter and subsequently heated at 100
°C on a hot stage. The color of the irradiated area was bleached
immediately, followed by the gradual expansion of the bleached circular
area which was monitored with a video camera.
Evaluation of Thermal Stability in Polymeric Films. A 4 wt %
solution of PBOCSt in chloroform containing 100 mol % of an
acetoacetate ester was spin-coated on a silicon wafer to give a thin
film which was dried in vacuo for 20 min. The wafer was placed on
a hot stage to heat at 100 °C and was subjected to IR absorption
measurement using a JEOL JIR-3505 spectrometer at intervals.
Deprotection of PBOCSt by 2a. A solution of 4 wt % PBOCSt in
chloroform containing the (tosyloxy)methyl ester (10 wt % relative to
the polymer) (2a) was spin-coated on a silicon wafer and dried in Vacuo
at room temperature. A thin film was heated at 100 °C on a hot stage
to be subjected to IR absorption spectral measurement.
JA972353C