Polymer catenanes via a supramolecularly templated ATRP initiator

Article abstract of DOI:10.1039/c1cc13162a

Synthesis of polymer catenanes via a living radical polymerization and supramolecular template approach are demonstrated. The ring closure was performed via atom transfer radical cross coupling (ATRC) to obtain polymer catenanes from the linear polymer metal complex precursor.

Full text of DOI:10.1039/c1cc13162a

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Polymer catenanes via a supramolecularly templated ATRP initiatorw
Ajaykumar Bunha, Maria Celeste Tria and Rigoberto Advincula*
Received 29th May 2011, Accepted 24th June 2011
DOI: 10.1039/c1cc13162a
Synthesis of polymer catenanes via a living radical polymer-
ization and supramolecular template approach are demon-
strated. The ring closure was performed via atom transfer
radical cross coupling (ATRC) to obtain polymer catenanes
from the linear polymer metal complex precursor.
catenanes via a templating method. Herein, we report a synthetic
strategy (Scheme 1) to obtain polymer catenanes by: (1)
designing supramolecularly templated ATRP initiators
followed by polymerization and (2) ring-closure of the supra-
molecular polymer via an intramolecular end-to-end cyclization
method.
Synthesis of molecules with complex topologies such as
catenanes, rotaxanes, and molecular knots requires the use
of chemical templates. Various types of supramolecularly
assembled templates have been reported based on metal ion
templates, hydrogen-bonded templates, and p-donor, p-acceptor
templates.5 A metal ion template such as a phenanthroline
copper (I) complex as demonstrated by Sauvage and
co-workers4,22 could be the best choice for designing supra-
molecularly templated ATRP initiators because of the ease
of end-group modifications in the 2,9-bis(p-hydroxyphenyl)-
1,10-phenanthroline ligand and the stability of the template
under ATRP polymerization conditions.22 The synthesis of the
ATRP initiator (1) was reported in 68.8% yield using bromo
ethyl, 2-bromopropionic acid ester and the previously reported
2,9-bis(p-hydroxyphenyl)-1,10-phenanthroline ligand.
Polymers with different types of topologies play an important
role in tuning macromolecular properties.1 Various polymers
with special architectures, such as star, hyperbranched,
dendronized, cylindrical, and macrocyclic, have been reported.2,3
Although numerous examples of catenanes, rotaxanes, and
trefoil knots with considerably low molecular weight have
been reported in the literature,4–7 obtaining such orderly
entangled topology in the case of high molecular weight
polymers as well as block copolymers remains challenging
due to their synthetic obstacles. Nevertheless, the interest on
entanglements is an integral part of modern polymer physics,
notably in the fields of rheology,8 adhesion,9 crystallinity,10
surface and interfaces,11 block copolymers,12 and viscoelasticity.13
Many aspects of chemical topology from DNA to stereochemical
reactions have been studied.14 These topologies can be
explained by the realm of orderly molecular entanglements
of well-reported interlocked molecular and supramolecular
architectures as demonstrated by Dietrich-Bucheker and
Sauvage,7 Amabilino and Stoddart,4 Wasserman,15 Stephenson
and Busch,16 and Walba.17 A number of reports have demon-
strated intricate sequences of steps (threadings, cross-overs,
ring closings, and other linkages) in order to form complicated
orderly knot entanglements.18
The supramolecular assembly of ATRP initiators (1) was
formed by adding a degassed solution of Cu(CH3CN)4ÁBF4 in
CH2Cl2 to the degassed solution of the ATRP initiator (2 eq)
in CH2Cl2. The resulting reddish solid product of the initiator
complex was characterized by UV-Vis spectroscopy as shown
in Fig. 1a. The formation of the initiator complex (2) was
The development of controlled radical polymerization
methods, especially atom transfer radical polymerization
(ATRP) and nitroxide mediated polymerization (NMP), has
opened the door to experimentally more lenient syntheses of
well-defined telechelic linear precursors.19 Since a catenane is
an analogue of a cyclic ‘‘end-less’’ structure, there is a possibility
of using existing methods of forming cyclic polymers, such
as coupling reactions of telechelics in dilute solution,20
or macrocycles via step-growth polymerization21 to form
Department of Chemistry and Department of Chemical and
Biomolecular Engineering, University of Houston, TX 77204-5003,
USA. E-mail: radvincula@uh.edu; Fax: +1 713-743-1755;
Tel: +1 713-743-1760
w Electronic supplementary information (ESI) available: Detailed
procedures for the chemical synthesis and polymerization, character-
ization by 1H-NMR, GPC and AFM image analysis. See DOI:
10.1039/c1cc13162a
Scheme 1 Schematic route for the preparation of polymer catenanes
from the templated ATRP initiator (1).
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backbone of the polymer was confirmed by the UV-Vis
analysis as shown in Fig. 1b, which still shows the ligand-
centered bands (260 nm and 336 nm) and the MLCT and
LMCT transition peaks (438 nm and 580 nm, respectively),
consistent with the observed results of the ATRP initiator
complex (2). The UV-visible result was also supported by
performing a control study (see ESIw) of demetallation reac-
tion of the PS complex (3) and characterization by GPC.
Even though intramolecular click chemistry24 and intra-
molecular ring closing metathesis25 have proven to be highly
efficient in producing macrocycles, the synthetic modification
of the polymer chains prior to the cyclization reaction yields
an additional step(s) to the overall synthetic route and could
lead to multiple side products. We have utilized a simple
method for the synthesis of the PS catenanes (4) based on
atom transfer radical coupling (ATRC) chemistry, which only
involves one step towards ring closure.26
The formation of the PS catenanes (4) over step polymers
can be favored by intramolecular reaction of the PS complex
(3) under pseudo-high dilution conditions. Thus, a THF
solution of the PS complex (3) was added dropwise at a fairly
low rate (B0.75 mL hÀ1), into a refluxing THF solution of
CuBr, Me6TREN, and Cu(0). The resulting PS catenanes (4)
were characterized by AFM and GPC analysis. The GPC
analysis of the closed polymer catenane (4) (Mn: 4188 g molÀ1
,
Fig. 1 UV-visible analysis of (a) ATRP initiator (1) and ATRP
initiator complex (2) (inset: visible region of the ATRP initiator
complex (2) spectrum), (b) PS complex (3) and PS control (inset:
visible region of the PS complex (3) spectrum).
Mw: 6533 g molÀ1) showed a very slight shift to a higher
retention volume (Rv) (Rv = 24.27 mL) as compared to the
open PS complex (3) (Rv = 23.84 mL) (Fig. 2). This shift in the
retention volume is due to the formation of the reduced
confirmed by the presence of the intense ligand-centered
transitions (p–p*) at 283 nm and 321 nm and the weaker
absorption peaks at the visible regions, 437 nm and 585 nm
corresponding to metal-to-ligand (MLCT) and the ligand-to-
metal (LMCT) transitions, respectively.23
hydrodynamic radius (Rh) of the PS catenane (4) (Rh
=
4.6 nm) after the ring-closure of the precursor PS complex (3)
(Rh = 4.9 nm). As observed, the shift in Rv is not too evident
as compared to other GPC traces reported in the literature
that compares linear and cyclic polymer counterparts. This
observation could be explained by the fact that our open PS
complex system (3) is not completely linear and is much closer
to a semicyclic structure due to the geometric constraints
coming from the complexed phenanthroline ligand in between
the PS chains. The formation of a broader GPC peak for
polymer (4) (PDI = 1.56) as compared to polymer (3) (PDI =
1.22) signifies the presence of the mixture of PS catenanes (4)
and the unreacted acyclic PS complex (3). Nonetheless, the
observed ATRC products were predominantly PS catenanes
(4) as evidenced by a relatively high oG> value (i.e. the ratio
of the peak average molecular weight (Mp) of the coupled
product relative to that of the acyclic precursor) of 0.79, which
is comparable to other literature values of cyclic polymers
produced by ATRC coupling26 and anionic methods.27
Using the ATRP initiator complex (2), styrene was
polymerized by the Cu(I)Br catalyst with the hexamethyl
triethylene tetraamine (HMTETA) ligand in anisole at 110 1C
after a series of freeze–pump–thaw cycles to remove the
oxygen from the reaction environment. The resulting poly-
styrene (PS) complex (3) was purified by passing through a
neutral alumina plug to remove the copper salts. The unreacted
monomer and solvent were removed by precipitation of the
polymer in cold methanol. The gel permeation chromatography
(GPC) traces of the resulting PS complex (3) (Fig. 2) show
narrow polydispersity with good control over molecular
weight (Mn: 6405 g molÀ1, Mw: 7814 g molÀ1, PDI: 1.22).
The integrity of the ligand copper (I) complex within the
To give a visual representation of the polymers, AFM
imaging was also performed. The samples of PS catenanes
(4) for AFM analysis were prepared by spin-coating a very
dilute solution of the polymer in dry THF onto freshly cleaved
mica. As shown in Fig. 3, interlocked cyclic structures of
polymers were statistically observed, exhibiting different or-
ientations on the surface (see Fig. S2, ESIw). Line profile
analysis of the structure revealed the presence of the cavities
inside the catenanes where the overlapping region (middle of
the catenanes) is double the height of the individual ring of a
catenane. Statistically, other structures could represent partial
Fig. 2 GPC traces of PS complex (3) and PS catenanes (4) in THF
eluent.
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technical support from Dr Jason Sanchez of Malvern (formerly
Viscotek) Inc. and Agilent Technologies (formerly Molecular
Imaging).
Notes and references
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R. B. Seymour and G. S. Kirshenbaum, Elsevier, New York, 1986,
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Fig. 3 (a) AFM image of the PS catenanes on a 9 Â 9 mm scale on
mica, (b) magnified images of the representative PS catenanes from (a),
and (c) line profile of one of the PS catenanes from (b).
2 For reviews, see: (a) K. Matyjaszewski and J. Xia, Chem. Rev., 2001,
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open arms or intermolecular coupling of the 4 arms of the PS
complex (3), or simply different conformations of catenanes
(Fig. S2, ESIw). But the formation of the catenanes is clear.
In order to obtain metal-free polymer catenanes, copper (I)
from the template was removed by stirring the polymer
catenanes (4) with saturated solution of KCN in THF: methanol
(3 : 1). The GPC traces of the demetallation step (see Fig. S3,
ESIw) of the PS catenanes (4) show that the molecular weight
3 (a) Y. Tezuka and K. Fujiyama, J. Am. Chem. Soc., 2008, 128,
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of the PS catenanes (5) (Mn: 7374 g molÀ1, Mw: 7922 g molÀ1
)
barely changed even after removing the copper, indicating an
interlocked structure of cyclic polymers. The peak broadening
of the GPC trace of the PS catenanes (4) (Fig. 2) due to the
unreacted acyclic precursor was also supported by the GPC
analysis of the demetallation step in Fig. S3 (ESIw). The
appearance of the broad shoulder at a higher retention volume
(Mn: 2769 g molÀ1, Mw: 2982 g molÀ1) denotes the formation
of decomplexed acyclic PS with approximately half the molecular
weight of the PS complex (3). On the basis of the relative areas
of the GPC traces of the PS catenanes (5) (Fig. S3, ESIw), the
amount of cyclic polymer catenanes formed in the reaction
was estimated to be 60% of the product mixture.
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In conclusion, a new, simple, and versatile method for the
synthesis of polymer catenanes with complex and higher order
of cyclic topology was demonstrated. The supramolecularly
assembled ATRP initiator (1) was able to polymerize styrene
with a good control over molecular weight. The intramolecular
cyclization of the PS complex (3) via ATRC was proven to be
an effective method to obtain polymer catenanes based on the
UV-vis spectroscopy, AFM imaging and GPC results. The
presented method is promising in extending the synthesis to
higher order knotted polymeric structures like trefoil knots
(non-trivial knots) through designed templates. This would
open doors for the investigation of important physico-
mechanical properties and possible applications of these
knotted structures. Moreover, it is possible to extend this
towards other ring-expansion living polymerization methods
as well as block copolymer topologies.
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The authors acknowledge funding from NSF CHE-10-41300,
DMR-10-06776, CBET-0854979, Texas NHARP 01846, and
Robert A. Welch Foundation, E-1551. Assistance on the
synthesis of initiators by Joey Mangadlao and Shemaiah
Pascua is greatly acknowledged. The authors also acknowledge
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9173–9175 9175