Solvent programmable polymers based on restricted rotation

Article abstract of DOI:10.1021/ja904234w

(Chemical Equation Presented) Solvent programmable polymers (SPPs) were developed that can modulate their recognition properties by heating in different solvents. These highly cross-linked polymer gels were able to respond to differences in solvent polari

Full text of DOI:10.1021/ja904234w

Published on Web 08/05/2009
Solvent Programmable Polymers Based on Restricted Rotation
Yagang Zhang, Judith M. Lavin, and Ken D. Shimizu*
Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208
Received May 25, 2009; E-mail: shimizu@mail.chem.sc.edu
Scheme 1. A Solvent Programmable Polymer (SPP) That
Modulates Its Recognition Properties When Heated in Different
Polarity Solventsa
The ability to control and manipulate recognition properties of
polymer gels is important for many applications including chro-
matography, drug delivery, and biosensors.1 An attractive strategy
has been to develop stimuli-responsive gels that can change their
chemical properties in response to heat, solvent, and pH.1d,f,2
A
limitation of this strategy is that the stimuli-induced structural
changes are typically fragile and unstable. Thus, the gels revert
back to their equilibrium states upon removal of the stimuli.
Reported, herein, is a new class of solvent programmable polymers
(SPPs) based on restricted rotation with the ability to respond and
remember their stimuli-induced properties (Scheme 1).3 Like other
stimuli-responsive polymers, the recognition properties can be
modulated by heating in different solvents. At elevated temperatures,
the carboxylic acid recognition groups have free rotation and can
switch their relative orientations in response to the solvent. For
example, heating the SPP in polar solvents increased the number
of solvent accessible carboxylic groups. Conversely, heating in
nonpolar solvents decreases the number of solvent accessible
carboxylic acids. On cooling to rt, these solvent-induced changes
were “saved” due to restricted rotation about the Caryl-Nimide bonds.4
Thus, the orientation of the carboxylic acid groups were maintained
even when the imprinting solvent is removed or exchanged. The
solvent-induced changes are also reversible, and the binding
properties can be modulated by cycling between heating the polymer
in a polar and nonpolar solvent.
a The solvent induced changes are maintained after cooling and removal
of solvent due to the presence of a monomer with restricted rotation.
Scheme 2. Synthesis of the SPP and Depictions of Control
Monomer 3 Which Lacks Restricted Rotation and Atropisomeric
Model Compound 4
The SPP was prepared via ROMP polymerization of monomer
1 with restricted rotation and diimide cross-linker 2 (Scheme 2).5
A high molar percentage of cross-linker (80%) was used to ensure
that 1 was rigidly fixed within the polymer framework. The resulting
polymer gel was not soluble in water or organic solvents. However,
the interiors of the polymers were solvent accessible as they swelled
to 30% v/v and 50% v/v in water and acetonitrile, respectively. A
control polymer (CP) was prepared under the same conditions, using
monomer 3 that has free rotation around its Caryl-Nimide bond. To
verify that the polymerized ring opened products of 1 would also
have restricted rotation, the conformational stability of model
compound 4, which has a similar cis-fused bicyclic framework as
the ring-opened products of 1, was studied.6 The rotamers of 4
were stable at rt. A rotational barrier of 27.7 kcal/mol was measured
by following the kinetics of isomerization. This barrier equates to
a half-life of 0.58 y at 24 °C and 90 min at 83 °C.
The switching and memory properties of the SPP were initially
tested by heating the polymer (83 °C, 26 h) in solvents of varying
polarity from cyclohexane to water (Figure 1). After heating, the
polymers were cooled to rt and the imprinting solvents were
removed in Vacuo. The solvent-induced changes were assessed by
measuring the binding capacities of the polymers for ethyl adenine-
9-acetate (EA9A) in acetonitrile.7 This basic guest is known to form
strong H-bonding interactions with carboxylic acids.4a-c,8 Thus,
an increase or decrease in the number of solvent accessible
carboxylic acids would result in an increase or decrease in the
binding capacity of the polymers for EA9A. The solvent-induced
conformational changes were evident by the wide variation in
binding properties of the polymers (Figure 1). The polymers heated
in the most polar solvent (water) had a 50% higher binding capacity
than the polymers heated in nonpolar solvent (cyclohexane). A
sigmoidal correlation was observed between the polarities (ETN) of
the imprinting solvent and the binding capacities of the solvent-
imprinted polymers.9 These solvent studies demonstrate the ability
to attenuate and tailor the binding capacities of the SPP simply by
heating in a solvent of appropriate polarity. The solvent trends were
also consistent with the carboxylic acids favoring orientations that
maximized their contact with solvents of matching polarity.
To establish that the changes in the binding capacity of the SPP
were coupled to restricted rotation, the rates of the solvent
imprinting processes were measured (Figure 2) and compared with
the rotational barriers of atropisomer model system 4. A high-
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12062 J. AM. CHEM. SOC. 2009, 131, 12062–12063
10.1021/ja904234w CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. Binding capacities of SPP for EA9A in acetonitrile after heating
(83 °C, 26 h) in solvents of varying polarity (From left to right: cyclohexane,
dioxane, ethyl acetate, tert-butanol, acetonitrile, 2-propanol, ethanol, ethanol/
water (1:1), water).
Figure 3. Measured EA9A binding capacities of SPP ([) and CP (9)
after repeatedly heating (83 °C, 26 h) in acetonitrile and then in water. The
SPP was also repeatedly shaken in acetonitrile and then water without
heating (2).
In summary, the programmable polymers based on restricted
rotation were developed that can be reversibly shaped at elevated
temperatures and can maintain these solvent-induced conformational
changes on cooling to room temperature.
Acknowledgment. This work was supported by the NSF (CHE
0616442).
Supporting Information Available: Experimental details, NMR
spectra, and crystallographic data. These material is available free of
charge via Internet at http://pubs.acs.org.
Figure 2. Change in binding capacity over 26 h for a high-affinity SPP
heated (83 °C) in acetonitrile (9) and a low-affinity SPP heated (83 °C) in
water ([).
affinity SPP (prepared by heating in water) was heated in a less
polar solvent (acetonitrile) at 83 °C. The systematic transformation
into a low-affinity SPP was followed over 26 h (Figure 2, 9). The
complementary experiment was also performed in which a low-
affinity SPP (prepared by heating in CH3CN) was heated in water.
The systematic transformation into a high-affinity polymer appeared
to occur at the same rate. This was confirmed by kinetic analyses
of the rate curves. Both rate curves could be fit to first-order kinetic
equations, yielding rates of 3.1 and 3.6 × 10-4 s-1. The rate order
and activation energies (27.1 and 27.0 kcal/mol) of the solvent-
imprinting processes were of similar magnitudes to the rotational
barrier of 4 and other structurally similar N-arylimides.4,10 The high
activation barriers were also consistent with the ability of the SPPs
to maintain their solvent-induced changes for a week without a
measurable change in binding capacity.11
We have previously demonstrated that the conformational
changes of atropisomeric systems are reversible, allowing the
properties to be repeatedly “written” and “erased”.4a-c To test
whether similar levels of reversibility could be observed in the SPP,
a sample was repeatedly heated (83 °C, 26 h) in acetonitrile and
water (Figure 3, [). The solvent-induced switching was shown to
be reversible and proceeded with high fidelity. The binding
capacities of the high- and low-affinity SPP states remained constant
over 5 cycles. Control studies were also carried out to verify that
the solvent-induced changes were due to rotation about the
Caryl-Nimide bonds and were not due to differences in hydration or
swelling of the polymers after heating in the different solvents. First,
the SPP was subjected to the same cycle of immersion in CH3CN
and water but without heating (Figure 3, 2). Second, a control
polymer (CP) made with monomer 3 was tested that lacked
restricted rotation. Both cases did not display the solvent-memory
effects, as the binding capacities remained relatively constant
(Figure 3, 9). The control studies showed that (1) the combination
of solvent and heat were necessary to change the binding properties,
(2) the presence of monomer 1 with restricted rotation was essential,
and (3) the switching process cannot be explained by the presence
of residual solvent in the polymers gels.
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