Amphiphilic homopolymer as a reaction medium in water: Product selectivity within polymeric nanopockets

Article abstract of DOI:10.1021/ja051107v

A styrene-based water-soluble polymer has been explored for its use as a host for lipophilic substrates in aqueous medium. Unimolecular reactions, namely, photo-Fries rearrangement of naphthyl esters, α-cleavage reaction of 1-phenyl-3-p-tolyl-propan-2-one, and Norrish type I and type II reactions of benzoin alkyl ethers were examined. We find that the hydrophobic domains generated by the polymer not only restrict the mobility of the radicals but also modestly incarcerate the substrate, intermediates, and products during the time scale of the reactions. Comparative studies of the same photoreactions in micelles formed from small molecule surfactants and an amphiphilic diblock copolymer demonstrate that the styrene-based water-soluble polymer aggregates in aqueous medium offer better selectivity.

Full text of DOI:10.1021/ja051107v

Published on Web 09/03/2005
Amphiphilic Homopolymer as a Reaction Medium in Water:
Product Selectivity within Polymeric Nanopockets
Selvanathan Arumugam,† Dharma Rao Vutukuri,‡ S. Thayumanavan,*,‡ and
V. Ramamurthy*,†
Contribution from the Department of Chemistry, UniVersity of Miami,
Coral Gables, Florida 33124, and Department of Chemistry, UniVersity of Massachusetts,
Amherst, Massachusetts 01003
Received February 21, 2005; E-mail: murthy1@miami.edu; thai@chem.umass.edu
Abstract: A styrene-based water-soluble polymer has been explored for its use as a host for lipophilic
substrates in aqueous medium. Unimolecular reactions, namely, photo-Fries rearrangement of naphthyl
esters, R-cleavage reaction of 1-phenyl-3-p-tolyl-propan-2-one, and Norrish type I and type II reactions of
benzoin alkyl ethers were examined. We find that the hydrophobic domains generated by the polymer not
only restrict the mobility of the radicals but also modestly incarcerate the substrate, intermediates, and
products during the time scale of the reactions. Comparative studies of the same photoreactions in micelles
formed from small molecule surfactants and an amphiphilic diblock copolymer demonstrate that the styrene-
based water-soluble polymer aggregates in aqueous medium offer better selectivity.
Introduction
are tucked inside to form nanosized-pockets (average size of
about 50 nm) that are capable of sequestering hydrophobic guest
Although today’s environmental consciousness imposes use
of water as a solvent1 and despite the fact that water is cheap,
organic solvents are still the primary choice. Most of the organic
substrates are not soluble in water, and many reactive molecules
are decomposed or deactivated by water. Surfactants,2 water
soluble dendrimers,3 and organic hosts4 have been used as
reaction media to conduct reactions in an aqueous medium.
Recently a new class of polymer superstructures having a
hydrophilic carboxylic acid moiety and a hydrophobic benzyl
moiety within its monomer unit was reported by one of our
groups (Figure 1).5 In aqueous medium, this amphiphilic
homopolymer (polymer-A) adopts a conformation in which the
hydrophilic units are exposed to water and the hydrophobic units
molecules. This behavior opens up the possibility of utilizing
such a polymer as a reaction medium in aqueous solutions. In
this report, the potential of this polymer’s hydrophobic domains
to provide product selectivity was investigated with four
unimolecular reactions. We show here that this amphiphilic
polymer does indeed provide a unique hydrophobic environment
providing selectivity better than conventional micelles based
on small molecule surfactants and an amphiphilic diblock
copolymer. The photoreactions reported here also serve as
probes to understand the nature of hydrophobic pockets provided
by the polymer.
Results and Discussion
In this study, the styrene-based amphiphilic polymer (Figure
1a; polymer-A) has been utilized as a host for carrying out
† University of Miami.
‡ University of Massachusetts.
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A. E.; Esfand, R.; Hardy, M. J.; Mitchell, J. Tetrahedron Lett. 1999, 40,
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13200
J. AM. CHEM. SOC. 2005, 127, 13200-13206
10.1021/ja051107v CCC: $30.25 © 2005 American Chemical Society

Amphiphilic Homopolymer as Reaction Medium in H2O
A R T I C L E S
Figure 1. (a) Structure of the polymer used for the study. (b) Idealized view of the cross section of the micellar assembly. (c) Schematic representation of
the micelle with the reaction within the polymer micelle-type nanocontainer.
photochemical reactions in aqueous medium. Polymer-A is
water soluble in its salt form at pH 8.5. In aqueous solution,
polymer-A self-organizes into spherical micellar type aggregates
having a hydrophobic interior and a hydrophilic exterior (Figure
1b). To probe the micropolarity of these hydrophobic domains,
we have employed pyrene as a probe.6 Fluorescence spectra
were recorded for various host/guest ratios, and it was observed
that the fluorescence intensity increases with the addition of a
solution of polymer-A. In aqueous solution, some of the pyrene
molecules exist as aggregates (excimer emission in the region
430-500 nm). But upon addition of the polymer to the aqueous
solution, the emission consisted of only unimolecular fluores-
cence in the region 380-440 nm revealing that the pyrene
molecules are held isolated by the hydrophobic domains of
polymer-A.
The ratio of the intensities of vibrational peaks 1 and 3 (I1/
I3) in the monomer fluorescence spectrum is a measure of the
polarity of the environment around the pyrene moiety.6 It was
Figure 2. Fluorescence spectrum of pyrene (10-5 M) in aqueous NaOH
and in aqueous polymer-A solution (1 × 10-5 M).
observed that pyrene exhibited an I1/I3 ratio of 1.7 in an aqueous
solution of pH 8.5. The ratio decreased from 1.7 to 1.47 upon
addition of polymer-A (Figure 2) which could be attributed to
the presence of a slight hydrophobic environment around pyrene
in the aqueous polymer-A solution. The critical micellar
concentration of the polymer-A was calculated using pyrene as
(6) (a) Kalyanasundaram, K.; Thomos, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
(b) Dong, D. C.; Winnik, M. A. Photochem. Photobiol. 1982, 35, 17.
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J. AM. CHEM. SOC. VOL. 127, NO. 38, 2005 13201

A R T I C L E S
Scheme 1
Arumugam et al.
Table 1. Products Distribution upon Irradiation of 1-Naphthyl
the probe. The change in the intensity I1 of monomer fluores-
cence spectrum of pyrene as a function of the concentration of
the polymer-A was plotted. In this plot, the major change in
the slope is related to the onset of micelle formation and thereby
provides quantitative determination of critical micellar concen-
tration of polymer-A. The critical micellar concentration was
calculated to be 2.4 × 10-6 M from the point of intersection of
extrapolated line segments as shown in Figure 3.7
Benzoate (1)a
product distributionh (%)
benzoic
acid
medium
2
3
4
5
hexane
60
2
>99
87
83
75
50
30
7
60
3
aq. NaOH (pH) 8.5)b
polymer-Ac
CTACd
38
12
14
21
6
1
2
3
SDSe
SDOf
1
PS-b-PSAb,g
23
21
a [1] ) 2.4 × 10-4 M. b Product 4 and benzoic acid resulted from thermal
hydrolysis. c [Polymer-A] ) 1.1 × 10-4 M. d [CTAC] )1 × 10-2 M.
e [SDS] ) 1 × 10-2 M. f [SDO] ) 1 × 10-1 M. g [PS-b-PSA] ) 1.1 ×
10-4 M. h The percentage yield was kept ∼ 20% in all reaction media and
was calculated by gas chromatography using undecane as the internal
standard. The mass balance was estimated to be 90-95%.
However in an aqueous solution in the absence of polymer-A
at pH 8.5, 1 thermally underwent hydrolysis into 1-naphthol
and benzoic acid. But in an aqueous polymer-A solution (1 ×
10-4 M), hydrolysis was completely suppressed, and when
irradiated, it yielded 2-benzoyl 1-naphthol (2) as the only
photoproduct. The selectivity obtained within this polymer-A
assembly is better than that obtained within micelles from
classical surfactant small molecules such as cetyl trimethylam-
monium chloride [CTAC], sodium dodecyl sulfate [SDS], and
sodium dodecanoate [SDO] where a significant amount of para-
rearrangement product 3 (12-21%) was obtained. For com-
parison, photolysis was also conducted in an aqueous solution
of the diblock copolymer, polystyrene-b-poly (sodium acrylate)
[PS-b-PSA].10 In a 10-4 M aqueous solution of [PS-b-PSA] at
pH 9.5, 1 underwent thermal hydrolysis to yield R-naphthol and
benzoic acid, and when irradiated, ortho- and para-rearrange-
ment products resulted in the ratio 1:8. The product distributions
upon irradiation in various media are presented in Table 1.
Absence of hydrolysis product suggests that the naphthyl ester
is protected from the basic aqueous medium by homopolymer-
A. Note that all photochemical reactions were kept at about
20% conversion, which is a standard operating procedure in
mechanistic photochemistry.11 This avoids complications in
analysis due to secondary photoreactions from the products of
the reaction under investigation.
Figure 3. A Plot of pyrene (1 × 10-5 M) fluorescence intensity I1 as
function of homopolymer concentration.
To examine the usefulness of polymer-A as a reaction
medium, four unimolecular photochemical reactions, namely,
photo-Fries rearrangement of 1-naphthyl benzoate (1), photo-
Fries rearrangement of 1-naphthyl phenyl acyl ester (6), Norrish
type I reactions of 1-phenyl-3-p-tolyl-propan-2-one (14), and
benzoin alkyl ethers (16a-c) were investigated. In all the above
reactions, upon irradiation, a molecule from either its excited
singlet or triplet state fragments to form a radical pair.
Depending upon the mobility and confinement provided by the
medium, the primary intermediates yield various products as
discussed below.
The first reaction carried out was the photo-Fries rearrange-
ment of 1-naphthyl benzoate (1). This molecule upon irradiation
in hexane afforded four products, as shown Scheme 1.8,9
(7) At first sight, this result seems inconsistent with our previous report,5 where
we mention that the integrity of the micelles is intact even at 10-9
M
concentration based on TEM. Note however that the CMC determination
using pyrene is a measure of the partition coefficient of the hydrophobic
probe in the micellar interior vs the bulk solvent (see: Zhao, J.; Allen, C.;
Eisenberg, A. Macromolecules 1997, 30, 7143). Therefore, it is possible
that the integrity of the micelle is intact at nanomolar concentration, but
the partition coefficient of a molecule with polarity similar to that of pyrene
into that micelle is low at this concentration. Moreover, note that the
concentration (10-4 M) that we use for the photochemical reactions in this
report is well above the CMC measured using pyrene as the hydrophobic
probe. Therefore, the comparison of this polymer above its CMC with other
micelle forming molecules above their respective CMCs is consistent and
valid.
(10) It has been previously reported that a 10-4 M aqueous solution of PS-b-
PSA provides a micellar environment; see: (a) Astafieva, I.; Zhong, X.
F.; Eisenberg, A. Macromolecules 1993, 26, 7339. (b) Zhang, L.; Eisenberg,
A. Macromolecules 1996, 29, 8805. To confirm this with the polymers
purchased from commercial sources, we performed the pyrene-based CMC
determination experiments with this polymer. We obtained a CMC value
of 1 × 10-7 M, which is consistent with the values reported in the literature
(data provided in the Supporting Information). Since we perform our
experiments at 10-4 M concentration, we are indeed working well above
the CMC of this polymer.
(8) Miranda, M. A. In CRC Handbook of Photochemistry and Photobiology;
Horspool, W. M., Song, P. S., Eds.; CRC Press: Boca Raton, FL, 1995; p
570.
(9) (a) Cui, C.; Weiss, R. G. J. Am. Chem. Soc. 1993, 115, 9820. (b) Pitchumani,
K.; Warrier, M.; Cui, C.; Weiss, R. G.; Ramamurthy, V. Tetrahedron Lett.
1966, 37, 6251. (c) Pitchumani, K.; Warrier, M.; Ramamurthy, V. J. Am.
Chem. Soc. 1966, 118, 9428. (d) Pitchumani, K.; Warrier, M.; Ramamurthy,
V. Res. Chem. Intermed. 1999, 25, 623.
(11) For examples, see: (a) Mori, T.; Inoue, Y.; Weiss, R. G. Org. Lett. 2003,
5, 4661. (b) Hirano, T.; Li, W.; Abrams, L.; Krusic, P. J.; Ottaviani, M. F.;
Turro, N. J. J. Org. Chem. 2000, 65, 1319. (c) ref 9a.
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Amphiphilic Homopolymer as Reaction Medium in H2O
A R T I C L E S
Scheme 2
Table 2. Products Distribution upon Irradiation of 1-Naphthyl
Phenacyl Acetate (6)a
solution. However, we have noticed significant differences in
the formation of the photoproducts. The absence of para-
rearrangement product (12) upon irradiation of 6 within
polymer-A reveals the more confined nature of its hydrophobic
domains compared to that of micelles formed by surfactants
and block copolymer.
To address the question, whether the hydrophobic pockets
of polymer-A can restrict the mobility of guest molecules even
on a longer time scale, Norrish type I reactions of 1-phenyl-3-
p-tolyl-propan-2-one (14)13,14 and benzoin alkyl ethers (16a-
c)15,16 were investigated. These two systems cleave from their
triplet excited states to yield triplet radical pairs. Under such
conditions, coupling of radicals can take place only if the radical
pair can intersystem cross to a singlet state or escape from the
cage to form free radicals. Results discussed below on these
two systems reveal that the hydrophobic reaction cavities of
polymer-A confine the guest molecules even on a microsecond
time scale.
Upon irradiation, 1-phenyl-3-p-tolyl-propan-2-one (14) un-
dergoes R-cleavage to form the radical pairs C and C′, followed
by the decarbonylation to yield the radicals A and B (Scheme
3). When the radicals A and B are held in a cage, the only
expected product is the recombination product AB.17 The cage
effect of the medium is given by the formula [AB - (AA +
BB)]/[AA + AB + BB]. It was also observed that in reaction
media such as a dimerized cavitand, or water soluble dendrimers,
the radical pair C and C′ intersystem cross to a singlet state to
yield the rearrangement products such as 15a (Scheme 3).18
However, no rearrangement products were observed in the
polymer-A medium. The cage effect in various reaction media
are presented in Table 3.
About 80% cage effect was observed when the reaction was
carried out in aqueous polymer-A solution (1.1 × 10-4 M). To
understand the observed cage effect, the roles of the concentra-
tion of the polymer-A, the concentration of the substrate, and
dilution were examined. The cage effect increased with an
increase in the polymer concentration, as shown in Figure 5.
medium
product distribution (%)g
7
8
9
10
11
12
13
hexane
18
2
11
2.5
9
39
19
24
1.5
aq NaOH (pH ) 8.5)
polymer-Ab
CTACc
74
>99
86
83
81
2
2
2
12
15
17
12
SDSd
SDOe
PS-b-PSAf
15
74
a [6] ) 2.0 × 10-4 M. b [Polymer-A] ) 1.1 × 10-4 M. c [CTAC] ) 1
× 10-2 M. d [SDS] ) 1 × 10-2 M. e [SDO] ) 1 × 10-1 M. f [PS-b-PSA]
) 1.1 × 10-4 M; Product 7 was resulted from thermal hydrolysis. g The
percentage yield was kept ∼20% in all reaction media and was calculated
by gas chromatography using undecane as the internal standard. The mass
balance was estimated to be 90-95%.
The second reaction investigated was the photo-Fries rear-
rangement of 1-naphthyl phenyl acyl ester (6).12 Upon irradiation
in hexane, this compound afforded seven products 7-13
(Scheme 2). Unlike naphthyl benzoate, when taken in an
aqueous solution at pH 8.5, it remained stable against thermal
hydrolysis and upon irradiation, 1-naphthol (7), 2-phenylacyl-
1-naphthol (11), and 4-phenylacyl-1-naphthol (12) were ob-
tained. The product distributions in various media are presented
in Table 2. But when irradiated in an aqueous polymer-A
solution of 1 × 10-4 M, 2-phenacyl-1-naphthol was the only
photoproduct observed. The selectivity increased with an
increase in concentration of the polymer (Figure 4). The
selectivity observed during photolysis of 1-naphthyl benzoate
and 1-naphthyl phenyl acyl ester could be attributed to the
restricted mobility of the radicals that are confined within the
hydrophobic pockets. The photo-Fries rearrangement of 6 was
also carried out in aqueous micellar solutions of CTAC, SDS,
SDO, and PS-b-PSA. Reaction of 6 within surfactant micellar
solution resulted in both ortho- and para-rearrangement
products (11 and 12) in the ratio ∼6:1 along with ∼2% of
1-naphthol. Photolysis of 6 in an aqueous micellar solution of
PS-b-PSA yielded ortho- and para-rearrangement products in
the ratio 8:1 along with appreciable amount of R-naphthol
(∼15%) which resulted from thermal hydrolysis (pH of PS-b-
PSA ) 9.5). The difference in the formation of the hydrolysis
product 7 could be attributed to the difference in the pH of the
(13) Robbins, W. K.; Eastman, R. H. J. Am. Chem. Soc. 1970, 92, 6076. (b)
Engel, P. S. J. Am. Chem. Soc. 1970, 92, 6074.
(14) For characterization of 14, see: Rabjohn, N., Ed. Organic Synthesis;
Wiley: New York, 1963; Vol. IV, p 176.
(15) Lewis, F. D.; Lauterbach, R. T.; Heine, H. G.; Hartmann, W.; Rudolph, H.
J. Am. Chem. Soc. 1975, 97, 1519.
(16) For the synthesis of benzoin ethers, see: Fischer, E. Chem. Ber. 1983, 26,
2412.
(12) (a) Gu, W.; Warrier, M.; Ramamurthy, V.; Weiss, R. G. J. Am. Chem.
Soc. 1999, 121, 9467. (b) Gu, W.; Hill, A. J.; Wang, X.; Cui, C.; Weiss,
R. G. Macrmolecules 2000, 33, 7801. (c) Gu, W.; Weiss, R. G. Tetrahedron
2000, 56, 6913. (d) Gu, W.; Weiss, R. G. J. Photochem. Photobiol. C 2001,
2, 117. (e) Gu, W.; Weiss, R. G. J. Org. Chem. 2001, 66, 1775. (f) Gu,
W.; Bi, S.; Weiss, R. G. Photochem. Photobiol. Sci. 2002, 1, 52. (g) Warrier,
M.; Kaanumalle, L. S.; Ramamurthy, V. Can. J. Chem. 2003, 81, 620.
(17) Fredrick, B.; Johnson, L. J.; De Mayo, P.; Wong, S. K. Can. J. Chem.
1984, 62, 403.
(18) (a) Kaanumalle, L. S.; Gibb, C. L. D.; Gibb, B. C.; Ramamurthy, V. J.
Am. Chem. Soc. 2004, 126, 14366. (b) Kaanumalle, L. S.; Nithyanandhan,
J.; Pattabiraman, M.; Jayaraman, N.; Ramamurthy, V. J. Am. Chem. Soc.
2004, 126, 8999.
9
J. AM. CHEM. SOC. VOL. 127, NO. 38, 2005 13203

A R T I C L E S
Arumugam et al.
Figure 4. Concentration effect of polymer-A on the products distribution upon photolysis of 1-naphthyl phenyl acyl ester (6).
Scheme 3
Table 3. Products Distribution upon Photolysis of
the aqueous exterior. Also the number of hydrophobic units will
increase with an increase in polymer concentration, which
enhances the solvophobic interaction of the hydrophobic
domains with the aqueous phase. In other words, the hydro-
phobic pockets are more protected from an aqueous phase at a
higher polymer concentration, and hence leaking of the radicals
into bulk environment is minimized. The cage effect was
constant up to 4.5 × 10-4 M concentration of the reactant
ketone, and then it decreased as shown in Figure 6. Either
multiple occupancy of guest molecules in a single cage or
leaking to aqueous phase from the hydrophobic interior due to
the saturation of the substrate should be causing the decrease
in the cage effect with increased reactant concentration.
As a part of a control experiment, a 10 mL sample with the
substrate concentration 2.2 × 10-4 M and the polymer
concentration 1.1 × 10-4 M was prepared. The sample was then
diluted to 25 mL, 50 mL, and 100 mL. Under these conditions
the ratio of the polymer to reactant molecules remained constant.
If there is strong binding between the reactant and the polymer,
the cage effect should remain constant. However the cage effect
decreased with dilution almost linearly as shown in Figure 7.
1-Phenyl-3-p-tolyl-propan-2-one (14)a
product distributiong (%)
cage effect
Fc
)
AB
AA
−
+
[AA
+
+
BB]/
medium
AA
AB
BB
AB
BB
hexane
22
22
4
16
16
18
15
53
52
90
61
64
61
66
25
26
6
18
20
21
19
0.05
0.04
0.80
0.27
0.28
0.22
0.32
aq NaOH (pH ) 8.5)
polymer-Ab
CTACc
SDSd
SDOe
PS-b-PSAf
a [14] ) 2.2 × 10-4 M. b [Polymer-A] ) 1.1 × 10-4 M. c [CTAC] ) 1
× 10-2 M; Rearrangement product 15a was observed in small amounts
(∼5%) upon photolysis of 14 within CTAC micelles. d [SDS] )1 × 10-2
M. e [SDO] ) 1 × 10-1 M. f [PS-b-PSA] ) 1.1 × 10-4 M. g The percentage
yield was kept ∼ 20% in all reaction media and was calculated by gas
chromatography using undecane as the internal standard. The mass balance
was estimated to be 90-95%.
Figure 5. Cage effect as a function of polymer-A concentration in Norrish
type I reaction of 1-phenyl-3-p-tolyl-propan-2-one (14).
This behavior could be attributed to two factors. The number
of hydrophobic domains in the medium increases with an
increase in polymer concentration, which would enhance the
reactant uptake by the polymer matrix preventing reaction from
Figure 6. Cage effect dependence on the substrate concentration in Norrish
type I reaction of 1-phenyl-3-p-tolyl-propan-2-one (14).
9
13204 J. AM. CHEM. SOC. VOL. 127, NO. 38, 2005

Amphiphilic Homopolymer as Reaction Medium in H2O
A R T I C L E S
Scheme 4
Table 4. Products Distribution upon Photolysis of Benzoin Alkyl
Ethers 16a, 16b, and 16c
We attribute this effect to both the solubility of the ketone in
water and to a decrease in the concentration of the polymer
aggregates. The decrease in cage effect upon dilution suggests
that the complex between the substrate and the polymer
aggregates is responsible for the cage effect rather than the
complex between the reactant and a single polymer chain. The
cage effect obtained within the aggregates of polymer-A was
also compared with that of surfactant micelles and micelles
obtained from PS-b-PSA, and the results are tabulated in Table
3. Moderate cage effect (22-32%) obtained within conventional
micelles and micelles formed by PS-b-PSA suggests that the
aggregates of polymer-A provide a better confining environment
for the reactive intermediates.
The final unimolecular reaction examined was the Norrish
type I and type II reactions of benzoin alkyl ethers (16a-c).
The photochemical behavior of benzoin methyl ether, benzoin
ethyl ether, and benzoin isopropyl ether in aqueous polymer-A
solution was investigated. Irradiation of 16a, 16b and 16c in
benzene yields the corresponding radical pair D, which escapes
to form benzil (17) and the respective pinacol ether (18) as
shown in Scheme 4.19 However, benzaldehyde was observed
only in trace amounts when the reaction was carried out in
aqueous solution without the polymer (pH ) 8.5). Only minor
amounts of oxetenol (20) and deoxybenzoin (21) (Norrish type
II products) via γ-hydrogen abstraction were obtained (Table
type I productsi (%)
17 18 22
type II productsi (%)
type I/
type II
medium
20
21
Benzoin Methyl Ether (16a)a
benzene
33
64
94
44
48
38
59
44
3
6
32
15
aq NaOH (pH ) 8.5) trace
polymer-Ab
CTACc
trace
34
18
56
18
44
15
56
0.8
4.5
0.9
3.1
0.8
SDSd
SDOe
26
PS-b-PSAf
Benzoin Ethyl Ether (16b)g
benzene
32
63
80
14
54
35
47
46
5
4
32
4
aq NaOH (pH ) 8.5) trace
16
59
19
9
24
35
polymer-Ab
CTACc
trace
trace
19
8
8
13
4
14
23
37
21
11
0.37
1.4
1.17
1.22
1.17
SDSd
SDOe
PS-b-PSAf
Benzoin Isopropyl Ether (16c)h
benzene
33
64
84
10
35
27
51
40
3
2
32
48
aq NaOH (pH ) 8.5) 12
polymer-Ab
CTACc
trace
23
67
36
54
24
50
0.5
1.8
0.9
3.2
1.0
29
19
25
10
SDSd
SDOe
PS-b-PSAf
a [16a] ) 2.3 × 10-4 M. b [Polymer-A] ) 1.1 × 10-4 M. c [CTAC] )
1 × 10-2 M. d [SDS] ) 1 × 10-2 M. e [SDO] ) 1 × 10-1 M. f [PS-b-
PSA] ) 1 × 10-4 M. g [16b] ) 2.3 × 10-4 M. h [16c] ) 2.2 × 10-4 M.
i The percentage yield was kept at ∼ 20% in all reaction media and was
calculated by gas chromatography using benzophenone as the internal
standard. The mass balance was estimated to be 90-95%.
4). When taken in aqueous polymer-A solution, the photochemi-
cal behavior of these ethers was significantly different. It was
observed that the hydrophobicity of the alkyl part of the ether
also had an influence on the product distribution upon irradiation
in an aqueous polymer-A solution. Irradiation of 16a included
Figure 7. Cage effect as a function of volume of the reaction sample that
contains 23 mg of the polymer and 0.5 mg of 14 in Norrish type I reaction
of 1-phenyl-3-p-tolyl- propan-2-one (14).
(19) (a) Turro, N. J.; Gould, I. R.; Baretz, H. B. J. Phys. Chem. 1983, 87, 531.
(b) Lunazzi, L.; Ingold, K. U.; Scaiano, J. C. J. Phys. Chem. 1983, 87,
529.
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A R T I C L E S
Arumugam et al.
within the polymer matrix yielded pinacol ether 18 (type I) and
deoxybenzoin 21 (type II) in the ratio 44:56, while irradiation
in aqueous solution without polymer-A (pH 8.5) yielded pinacol
ether and deoxybenzoin in the ratio 94:06. In the case of 16b,
irradiation within aggregates of polymer-A resulted in 73% of
Norrish type II products 20 and 21 along with 13% of para-
rearrangement product (22b) due to Norrish type I reaction,
while irradiation in aqueous solution without polymer-A yielded
∼80% of type I products as given in Table 4.20 Similarly,
irradiation of 16c within polymer-A yielded 67% of deoxyben-
zoin along with 23% of para-rearrangement product (22c).
The above results suggest that the excited-state behavior of
16a-c is altered by homopolymer-A. Two observations are
important to note: (a) The type II reaction which is almost
negligible becomes predominant in the presence of polymer-
A. (b) The rearrangement products 22b and 22c that are not
formed in solution are formed in minor amounts in the presence
of polymer-A.
Both of these observations suggest that the hydrophobic
pockets provided by polymer-A control the diffusion of the
caged radical pair D (Scheme 4). In the absence of cage escape
even if the type I cleavage occurs, the radical pair would be
expected to recombine to yield the reactant under these
conditions. The efficiency of type I products formation would
be expected to be low. The type II process whose rate may be
slow would eventually lead to the products in greater amount.
This model fits the observed results. The yield of rearrangement
products 22 increases with the hydrophobicity of the alkyl part
of the ether. For example, while the most hydrophobic 16c gives
the highest amount of 22c, the least hydrophobic 16a gives no
rearrangement product. This suggests that the escape of the
primary radical D from the hydrophobic cage provided by the
polymer depends on the alkyl chain. The photochemical
behaviors of 16a-c were also studied within surfactant micelles
(CTAC, SDS, and SDO) and micelles obtained from PS-b-PSA.
Photolysis of 16a-c within conventional surfactant micelles
yielded more type I products compared to photolysis within
homopolymer aggregates. The rearrangement product 22 which
was observed from the photoreaction in aqueous homopolymer
solution was not obtained from the reaction within surfactant
micelles. The photobehavior of 16a, less hydrophobic among
the other ethers, within PS-b-PSA micelles was very similar to
that within homopolymer aggregates. However the photochemi-
cal behaviors of 16b and 16c were significantly different within
PS-b-PSA. Photoreaction of 16b and 16c within PS-b-PSA
yielded more type I product relative to reaction within ho-
mopolymer. Also the rearrangement product 22 was absent from
photolysis of 16b and 16c within PS-b-PSA micelles. All the
above observations suggest that the homopolymer aggregates
effectively incarcerate the relatively more hydrophobic ethers
16b and 16c compared to PS-b-PSA micelles, whereas the less
hydrophobic ether 16a resides at the interfacial region in both
cases. Evidently homopolymer aggregates in aqueous medium
are capable of providing a better confining hydrophobic reaction
cavity for benzoin alkyl ethers compared to micelles obtained
from conventional small molecule surfactants and PS-b-PSA.
Conclusion
In summary, we have demonstrated that the self-assembly
of the styrene-based amphiphilic homopolymer can be used as
a supramolecular reaction cavity to perform photochemical
reactions in aqueous solution, giving rise to an environmentally
friendly reaction system. Based on our investigation of four
unimolecular reactions in polymer micelle solution, we have
shown the following: (i) the amphiphilic polymer is capable
of acting as an effective nanoscale container for carrying out
reaction of lipophilic compounds in water. (ii) The polymer
nanoparticle is capable of providing confined environments to
provide products in a selective fashion despite the fact that these
particles are about 50 nm in size. (iii) The environment provided
by the amphiphilic homopolymer is more rigid and confined
compared to that formed by conventional small molecule
surfactant micelles (CTAC, SDS, and SDO) and micelles
obtained from the diblock amphiphilic copolymer PS-b-PSA.
With these comparisons, it is also clear that the charge on the
surfactant has very little influence on the selectivities. Also,
the amphiphilic diblock copolymer behavior seems to be more
similar to the small molecule surfactants than our amphiphilic
homopolymer. The significant differences between micelles
obtained from homopolymer and block copolymer (PS-b-PSA)
in terms of selectivities warrant further investigation on the
polymer assembly itself.
Acknowledgment. V.R. thanks the National Science Founda-
tion for financial support (CHE-0212041), and S.T. acknowl-
edges the National Science Foundation for a CAREER award
(CHE-0353039).
Supporting Information Available: Experimental Section.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(20) For thermal synthesis and characterization of the photoproducts of this
reaction, see: Ramamurthy, V.; Corbin, D. R.; Eaton, D. F. J. Org. Chem.
1990, 55, 5269.
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