Fine-tuning the transition temperature of a stimuli-responsive polymer by a simple blending procedure

Article abstract of DOI:10.1039/b800266e

Binary mixtures of well-defined, stimuli-responsive elastin-based side-chain polymers show a single transition temperature that depends on blend composition. The Royal Society of Chemistry.

Full text of DOI:10.1039/b800266e

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Fine-tuning the transition temperature of a stimuli-responsive polymer by
a simple blending procedurew
a
b
c
´
Francisco Fernandez-Trillo,z Jan C. M. van Hest, Jens C. Thies,
Thierry Michon,d Ralf Weberskirche and Neil R. Cameron*a
Received (in Cambridge, UK) 9th January 2008, Accepted 8th February 2008
First published as an Advance Article on the web 5th March 2008
DOI: 10.1039/b800266e
Binary mixtures of well-defined, stimuli-responsive elastin-based
side-chain polymers show a single transition temperature that
depends on blend composition.
Recently, we have described the synthesis of elastin-based
side-chain polymers (EBPs),8,9 in which a methacrylate deri-
vative of the pentapeptide VPGVG was polymerised using
controlled radical polymerisation (CRP) techniques to give
well-defined synthetic polymers with similar thermoresponsive
behavior to linear ELPs. These EBPs can be viewed as
chemically accessible variants of ELPs (although it should be
noted that EBPs are pH- as well as thermally-responsive, as
they bear free carboxylic acid groups from the C-terminus of
the pentapeptide side-chains).
Polymers displaying lower critical solution temperature
(LCST) behaviour are potentially useful for a variety of bio-
and nanotechnological applications.1,2 By far the most widely
studied polymer of this type is poly(N-isopropylacrylamide)
(pNiPAm), which displays an LCST in aqueous solution at
around 32 1C.2,3 This value can be altered, normally within a
10 1C range, by changing the end-group functionality, varying
the molecular weight and by the introduction of co-mono-
mers.3–5 An alternative class of thermoresponsive polymers are
elastin-like peptides (ELPs), which are bio-inspired polymers
composed of repeat units of the amino acids
Val–Pro–Gly–Val–Gly (VPGVG). The inherent bio-degrad-
ability and (presumed) biocompatibility of ELPs may provide
advantages over carbon-backbone polymers (pNiPAm) in
certain applications. In addition, the LCST (B35 1C) can be
tuned within a wide temperature range by replacing the fourth
(Val) residue with any other amino acid (VPGXG) except
Pro.6 Such tuning of the thermal response of pNiPAm, ELPs
and other similar polymers is important for applications such
as tumour cell targeting, for which the polymers should be
soluble at physiological temperature (B37 1C) and have a
transition temperature (Tt) below the temperature for local
hyperthermia (B42 1C).3
An investigation of the thermoresponsive behaviour of our
EBPs revealed a strong dependence of Tt on molecular
weight,9 in agreement with the previous findings on ELPs
and recent work on narrow polydispersity pNiPAms prepared
by atom transfer radical polymerisation (ATRP).5 This beha-
vior is consistent with classical theory on the thermodynamics
of polymer solutions, which states that, at the spinodal point,
the interaction parameter between polymer and solvent (w)
depends on the weight-average molecular weight (Mw)
(eqn. (1))10
ꢀ
ꢁ
1
1
1
w ¼
þ
ð1Þ
2 fPMw 1 ꢀ fP
where fP is the volume fraction of the polymer in solution. An
important observation from eqn. (1) is that w does not depend
on polydispersity. Therefore, it should be possible to vary Mw,
and thus w, by mixing together two narrow polydispersity
polymers of different chain length. This should provide a
simple method of fine-tuning Tt, since w p 1/T.
Meyer and Chilkoti demonstrated an inverse dependence
between Tt, and both molecular weight and concentration for
ELPs synthesised by recombinant DNA techniques.7
In order to explore this possibility, well-defined EBPs of
different chain length are required. Our recent work has
employed reversible addition fragmentation chain-transfer
(RAFT) polymerisation to prepare well-defined EBPs.9 RAFT
is arguably the most versatile of the CRP techniques, being
amenable to the polymerisation of virtually all monomers
active in radical polymerisation.11 Thus, VPGVG-methacry-
late derivative 1 was polymerised by RAFT using dithioester 2
and a commercially available azo initiator at 70 1C to give a
series of well-defined, narrow molecular weight distribution
EBPs, varying in number average degree of polymerisation
(DPn) from 29 to 88 (see Scheme 1 and Table 1). The
dithioester end-groups of these EBPs were then removed by
treatment with an excess of azo initiator12 to prevent their
interference in turbidimetry experiments.
a Department of Chemistry and Interdisciplinary Research Centre in
Polymer Science and Technology, Durham University, South Road,
Durham, UK DH1 3LE
b Radboud University Nijmegen, Organic Chemistry, Institute for
Molecules and Materials, Toernooiveld 1, 6525 ED Nijmegen, The
Netherlands
c DSM Research Campus Geleen, Performance Materials—Chemistry
& Technology, Urmonderbaan 22, 6167 RD Geleen, The Netherlands
d INRA, UMR GDPP, IBVM, Interactions Plante Virus, BP 81,
33883 Villenave d’Ornon Cedex, France
e Technische Universitat Munchen, Abteilung Chemie, Lehrstuhl fur
¨
¨
¨
Makromolekulare Stoffe, Lichtenbergstraße 4, 85747 Garching,
Germany
w Electronic supplementary information (ESI) available: General pro-
cedures, calculation of DPn,th and synthetic procedures. See DOI:
10.1039/b800266e
´
z Current address: Departamento de Quımica Organica, Facultade de
Quımica, Universidade de Santiago de Compostela, Avda. das Cien-
´
cias S/N, 15782, Santiago de Compostela, Espana.
In a first experiment, polymers A and D (Table 1) were
mixed in a 1 : 1 volume ratio in aqueous solution. The
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Scheme 1 Structure of elastin-based side-chain polymers (EBPs),
elastin-based monomer 1 and RAFT agent 2.
Fig. 1 Turbidimetry profiles of polymers A ( ), D ( ) and a 1 : 1
volume ratio of A : D (solid line) measured by UV-Vis spectrometry at
pH 3.2 in an aqueous phosphate-buffered solution (0.65 mg mLꢀ1).
Absorbance values are normalised to 1.
polymers were chosen carefully to give a sufficient difference in
their Tt values to be able to determine the effect of mixing on
Tt (see Table 1).
In agreement with eqn. (1), only one transition was observed
in the turbidimetry profile of the mixture of polymers A and D
(Fig. 1). Furthermore, the Tt value of the mixture (38.2 1C)
agrees well with the average (40.2 1C) of the individual Tt
values of polymers A and D. The Tt values of ELPs7 and
narrow polydispersity pNiPAms5 have both been shown to be
strongly dependent on molecular weight. However, surpris-
ingly, the behaviour of mixtures of such polymers of different
chain lengths has, as far as we can determine, not been
reported. This is despite the fact that the thermally-induced
phase transition has been described as a cooperative process,13
and that intermolecular aggregation has been described for
pNiPAm.14
polymers. This is obviously much more practical than synthe-
sising a range of polymers of different chain lengths.
We believe that these results could have important practical
implications, for example regarding the immobilisation of
enzymes onto surfaces employing reversible (‘smart’) co-as-
sembly. In previous work on elastin-mediated surface assem-
bly, an ELP immobilised on a gold surface was employed to
immobilise the same polymer (as part of a fusion protein) in
solution.15 The ELP in solution underwent self-assembly and
simultaneously co-assembly with the polymer attached to the
surface. Co-assembly between the molecules (and their aggre-
gates) in solution and hydrophobic ELPs on the surface occurs
due to hydrophobic interactions,16 which could lead to pro-
blems with non-specific adsorption onto unfunctionalised
surfaces. Such problems could be overcome by attaching a
relatively high molecular weight (low Tt) EBP to a surface and
using a lower DPn (high Tt) EBP in solution. This should bring
about co-assembly between the EBP in solution and that on
the surface, instead of self-assembly of the low DP polymer in
solution, thus preventing non-specific adsorption.
To analyse further the possibility of fine-tuning the transi-
tion temperature, two more polymers (B and C, Table 1), with
a difference in their Tt values of around 10 1C, were prepared
and blended together at a range of compositions. As shown in
Fig. 2(a), only one transition could be observed for all
mixtures of the two polymers. An important consequence of
this cooperative behaviour, as can be seen in Fig. 2(b), is that
there is a linear dependence between the fraction of the lower
molecular weight polymer in the mixture and Tt. This allows
Tt to be fine-tuned between the values corresponding to the
individual polymers, simply by varying the ratio of the two
Table 1 Molecular weight data and thermoresponsive behavior of
EBPs
Mnc/kg molꢀ1
PDId
Tte/1C
a
b
Polymer
DPn,th
DPn
A
B
C
D
a
29
34
76
88
43
45
105
113
23.5
24.3
57.0
61.6
1.05
1.05
1.20
1.25
47.9
45.7
36.2
32.4
Theoretical number-average degree of polymerisation (see ESIw for
Fig. 2 (a) Turbidimetry profiles for mixtures of polymers B and C of
different volume fractions (x) of the lower molecular weight polymer
(polymer B; K xB = 0.00; xB = 0.17; xB = 0.33; xB = 0.50;
xB = 0.67; xB = 0.83; xB = 1.00). Experiments were carried out
in an aqueous phosphate-buffered solution (pH adjusted to 3.2;
[polymer] = 0.65 mg mLꢀ1). Absorbance values are normalized to
1. (b) Relation between the transition temperature (Tt) and the volume
fraction of polymer B (DPn = 34). Values quoted are the inflection
points of the heating curves.
b
calculation formula). Number-average degree of polymerisation,
c
determined by aqueous size exclusion chromatography. Number
average molecular weight, determined by aqueous size exclusion
d
chromatography. Polydispersity index (Mw/Mn), determined by aqu-
e
eous size exclusion chromatography. Transition temperature, calcu-
lated from turbidimetry experiments in phosphate buffer (pH adjusted
to 3.2; [polymer] = 0.65 mg mLꢀ1). Values quoted are the inflection
points of the heating curves.
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In conclusion, we have shown that mixtures of elastin-based
side-chain polymers (EBPs) of different molecular weights
undergo a simultaneous thermal transition, the temperature
of which is linearly dependent on the composition of the
mixture. This could lead to novel applications of EBPs that
exploit their co-assembly, such as smart surface functionalisa-
tion.
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This work was performed within the European Research
Training Network SMASHYBIO (Smart Assembly of Hybrid
Biopolymers) under contract (HPRN-CT-2001-00188). We
would also like to thank the rest of the SmashyBio Network
for discussion and Dr John Sanderson (Durham University)
for his help with peptide synthesis.
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