Reactive & Functional Polymers 80 (2014) 40–47
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Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Cyclic polymers as a building block for cyclic brush polymers and gels
Ke Zhang a,b,⇑, Gregory N. Tew a,⇑
a
b
Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003, USA
State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history:
Available online 12 February 2014
Keywords:
Cyclic polymer
Brush polymer
Polymer gel
Ring-Expansion metathesis polymerization
Click chemistry
a b s t r a c t
Cyclic polymers, as one of the oldest topological polymers, are undergoing resurgence. This is largely
ascribed to the significant achievements in modern polymer chemistry. The novel ring-expansion techniques have conveniently produced varied cyclic polymers with highly topological purity and on large
scales, which should facilitate their use in the near future. Beyond the monocyclic molecular conformations, the combination of controlled polymerization techniques and click chemistry have established a
robust strategy for preparing cyclic polymers with more complex architectures, such as theta, eight,
and tadpole shapes. This diversification in cyclic polymer composition and conformation significantly
broadens interest in the cyclic polymers. However, compared to the synthesis achievements, the exploration of cyclic polymer property and application are lagging behind. Recently, we explored the ringexpansion metathesis polymerization on various functional ring-strained olefin monomers to produce
cyclic functional polymers, which were then used as the building blocks to fabricate cyclic brush polymers and cyclic gel materials and will be discussed here.
Published by Elsevier B.V.
1. Introduction
Cyclic polymers have received significant attention recently with
several important reviews [1–7] and a specific cyclic polymer symposium celebrated at the 245th ACS national meeting. Due to their
unique endless molecular topology, cyclic polymers have markedly
different characteristics from their linear counterparts, including a
smaller hydrodynamic volume and radius of gyration, lower melt
viscosity, higher thermostability, and increased rate of crystallization [1–7].
To date, known synthetic strategies for cyclic polymers can be
generalized into two categories: ring-closure and ring-expansion
methods both have advantages and disadvantages. In the ring-closure method, cyclic polymers are prepared by applying highly efficient coupling chemistry to end-functionalized linear telechelic
polymers. Progress in combining click chemistry with controlled
polymerization has increased the viability of this method for synthesizing cyclic polymers with controlled molecular weight and
low polydispersity, but there are still limitations. To assure high
topological purity, ring-closure reactions typically require dilute
conditions leading to reduced yields. In addition, it is difficult to
obtain cyclic polymers with high molecular weight by this method.
⇑ Corresponding authors at: Department of Polymer Science and Engineering,
University of Massachusetts, Amherst, MA 01003, USA (K. Zhang). Fax: +1 413 545
0082.
E-mail addresses: [email protected] (K. Zhang), [email protected] (G.N.
Tew).
http://dx.doi.org/10.1016/j.reactfunctpolym.2014.01.012
1381-5148/Published by Elsevier B.V.
Alternatively, ring-expansion is based on the insertion of monomer
units into an activated cyclic chain. Because the cyclic polymers remain intact during the whole ring-expansion process, this method
can produce high molecular weight cyclic polymers with high purity, even from concentrated solutions or the bulk. One disadvantage is that it can be difficult to control the molecular weight
and polydispersity of the resultant cyclic polymers. The ring-closure and ring-expansion methods have enabled the preparation
of many types of cyclic polymers, including cyclic polystyrene
[8], polymethacrylate [9], polyacrylate [10] polyacrylamide [11]
polyester [12] polycyclooctene [13] and polynorbornene [14]. In
addition, cyclic polymers with complex architectures have been
developed, including those with theta [15], eight [15], tadpole
[16], block [17], and brush [18–20] shapes. The various synthetic
methods, polymer types, and various cyclic architectures have
been thoroughly described in recent reviews [1–7].
As a complement in this tutorial review, we mainly focus on the
recent progress on cyclic polymers from our research group focused on ring-expansion metathesis polymerization (REMP) to
produce the functional cyclic polyolefins, which were then used
as the universal building blocks to prepare cyclic brush polymers
and cyclic gels.
2. Ring-expansion metathesis polymerization
REMP was firstly reported by Grubbs and coworkers in 2002
[13]. Following significant contributions from Bielawski, Boydston,
K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47
and Xia in the Grubbs’s research group, REMP has become a powerful method for preparing cyclic polyolefins [13,14,19,21–23].
Fig. 1 shows the key mechanistic steps involved in REMP. Initiation
occurs via the cyclic olefin monomer inserting into the Ru-alkylidene bond to expand the cyclic catalyst ring. Propagation then
proceeds by a repeated insertion of monomer into the growing cyclic polymer. As monomer is consumed, a competing intramolecular
chain transfer reaction decreases the molecular weight of the
growing cyclic polymer, broadens its polydispersity, and generates
cyclic polymers. An additional catalyst release process can produce
the original catalyst and more catalyst-free cyclic polymer.
To date, two series of cyclic Ru-alkylidene catalysts (UC series
and SC series) of five main types (UC-5, 6, 7 and SC-5, 6) (Fig. 2)
have been developed. It was found that both the heterocycle size
and the N-heterocyclic carbene (NHC) ligand electronics significantly impact the catalyst efficiency and the polymerization
behavior as well. The catalyst with larger ring leads to faster polymerization rates and lower catalyst release from cyclic polymer.
The saturated NHC ligand endows the catalyst with much faster
polymerization rate than that of the unsaturated NHC ligand. Furthermore, various cyclic olefin monomers have been successfully
polymerized by REMP. As shown in Fig. 2, they are composed of
not only the commercial available cyclic olefins and their simple
derivatives having a small functional group, but also those having
bulky side groups such as dendrons and even polymers.
As is typical of ring-expansion approaches, REMP can produce
large amounts of pure cyclic polymers at high polymerization concentrations. Furthermore, REMP is unique in that it can obtain cyclic polymers with the highest reported molecular weight. This
makes it a good candidate to prepare cyclic polymer building
blocks for the fabrication of cyclic materials, such as the nano-scale
cyclic brush polymers and the macro-scale cyclic gels.
3. Cyclic brush polymers
Brush polymers are composed of a long polymer backbone with
densely grafted polymer side chains. Due to the steric repulsion between densely grafted bulky side chains, these macromolecules
have an extended, rigid worm-like conformation when the backbone is much longer than the side chains [24,25]. It is this unique
molecular conformation that renders brush polymers as a unique
building block for generating advanced nanomaterials, such as creating well-defined single-molecular hybrid nano-objects and for
fabricating periodic nanomaterials with large domain spacings by
self-assembly [26]. In addition to the linear morphology, the
molecular shapes of brush polymers can be enriched by manipulat-
Fig. 1. Key mechanistic steps involved in REMP [22].
41
Fig. 2. Cyclic Ru-alkylidene catalysts and appropriate cyclic olefin monomers.
ing the backbone topology [27–30]. When cyclic polymers are chosen as backbones, the novel cyclic brush polymers are designed to
bear a donut-like molecular topology.
Although a few reports have been published on synthesizing
cyclic brush polymers [18,31–33], the preparation of brush polymers demonstrating stiff toroidal macromolecular topologies has
remained elusive [13,23,34,35]. REMP is unique here because it
meets many of the necessary requirements for building the backbone of cyclic brush polymers. Inspired by this, REMP has been
successfully utilized to prepare cyclic brush polymers by Grubbs’
and Tew’s research groups independently [14,19,20,36,37]. Applying the three traditional routes to linear brush polymers, [grafting
through (polymerization of macromonomers), grafting onto (coupling of side chains to backbone), and grafting from (polymerization of side chains from a backbone macroinitiator)], cyclic brush
polymers have been prepared by REMP.
4. Grafting through
The first report to synthesize and visualize cyclic dendronized
polymers by the grafting through technique was published by
the Fréchet and Grubbs’ research groups in 2009 [14]. A norbornene-based macromonomer was designed with a second generation
dendron side chain. When using the SC-5 or UC-6 as catalyst, the
cyclic dendronized polymers were obtained by REMP (Fig. 3A).
The donut-like molecular topology was visualized by AFM with a
diameter of 30–40 nm (Fig. 3B). Following the same concept, they
further investigated the direct REMP of norbornene-based
macromonomers with a bulky polymer side chain (Fig. 3C) [19].
A series of cyclic brush polymers were then produced with ultrahigh molecular weight and various polymer side chains, including
polystyrene, polylactide, and poly(n-butyl acrylate). In this work,
the macromonomers were efficiently prepared by controlled polymerization techniques and click chemistry. For a moderate to high
polymerization conversion of these bulky macromonomers, the
more active SC-series catalysts were required. The donut-like
molecular topology was visualized by AFM and most cyclic brush
polymers have diameters in the range of 100–180 nm (Fig. 3D),
which is much larger than that from the cyclic dendronized polymers. The increased molecular size is presumably caused by the
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K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47
Fig. 3. (A) Synthesis scheme of cyclic dendronized polymers from the combination of REMP and grafting from technique, (B) the corresponding AFM characterization
(reproduced and adapted from Ref. [14]), (C) synthesis scheme of cyclic brush polymers from the combination of REMP and grafting from technique, and (D) the
corresponding AFM characterization (reproduced and adapted from Ref. [19]. Copyright 2011 Wiley).
longer and significantly extended backbone conformation. In addition to the cyclic topology, linear contaminants were occasionally
observed by AFM in this method (Fig. 3D), however, it was demonstrated that they were caused by mechanical scission of the cyclic
backbone during AFM characterization.
5. Grafting onto
The combination of REMP and the grafting onto technique for
the formation of cyclic brush polymers was firstly developed in
our group in 2011 [20]. In this approach (Fig. 4), a functional norbornene-based monomer (M1) was designed to have an activated
pentafluorophenol ester side group. When applying REMP on M1
(Fig. 4(i)), high molecular weight functional cyclic polymers (cyclic
Poly-1) were produced bearing activated ester side groups on each
repeat unit. The highly efficient nucleophilic substitution of activated esters by amines enabled cyclic Poly-1 to be used as a template for preparing cyclic brush polymers when coupling with
amine end-functionalized polymer side chains. As an example to
demonstrate this concept, monofunctional poly(ethylene glycol)
(PEG) with an amine end group and with an average of 13 repeat
units was chosen as the coupling polymer side chains (Fig. 4(ii)).
A high grafting density of >95% was confirmed in this case. The
conformation of the resultant cyclic brush polymers was characterized by AFM and a cyclic morphology with an outer diameter of ca.
30 nm was observed.
Considering the difficulty of preparing amine end-functionalized polymers, we derivatized the cyclic Poly-2 template with a
clickable alkyne side group on each repeat unit. Given that azide
end-functionalized polymers are conveniently obtained by atomtransfer radical polymerization (ATRP), cyclic Poly-2 was easily
functionalized with various polymer side chains when employing
the Cu(I)-catalyzed azide-alkyne click chemistry. A cyclic polymer
brush containing polystyrene (PS) side chains was prepared to
illustrate the concept (Fig. 4(iii)). PS-N3 was obtained by substituting the terminal bromine of PS-Br from ATRP by an azide moiety.
The highly efficient Cu(I)-catalyzed azide-alkyne click chemistry
resulted in quantitative grafting density.
Fig. 4. Synthesis of cyclic brush polymers from the combination of REMP and grafting onto technique and the corresponding AFM characterization [20].
K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47
Most recently, we explored the use of supramolecular chemistry techniques to prepare cyclic brush polymers by combining
REMP and the grafting onto technique [37]. In order to include organic/inorganic hybrid concept as well, the metal–ligand supramolecular interaction was chosen as the building unit for the
preparation of cyclic brush polymers. Due to the outstanding chelating ability to various transition-metal ions, the terpyridine ligand was introduced as the side group on each repeat unit of the
cyclic Poly-3 (Fig. 5). The classic two-step terpyridine-Ru(III)/Ru(II)
supramolecular chemistry was specifically chosen to demonstrate
this technique. In this approach, various terpyridine end-functionalized polymer side chains were synthesized by reversible addition-fragmentation chain-transfer polymerization (RAFT) with a
terpyridine-functionalized RAFT agent. As the first step, the polymerterpyridineRuCl3 monocomplex was obtained from the terpyridine end-functionalized polymer precursor by refluxing with
RuCl3. Subsequently, the metallo-supramolecular cyclic brush
polymers were prepared by reacting the terpyridine-Ru(III)
mono-complex onto the cyclic backbone under reducing conditions (ethanol, N-ethylmorpholine), to produce the stable bis(terpyridine)–Ru(II) linkage connecting the polymer side chains and
the cyclic polynorbornene main chains (Fig. 5). Because of the
inherent metal-containing molecular structure, these novel supramolecular hybrid cyclic brush polymers provide an ideal model for
direct imaging of single polymer chains by TEM. As shown in Fig. 5,
a representative TEM image clearly shows the cyclic molecular
topology with an average diameter of ca. 40 nm. This is the first
time that TEM has been used to directly image the molecular
topology of cyclic polymers.
6. Grafting from
In 2012, we reported the third method for the preparation of
cyclic brush polymers based on REMP by combining it with the
grafting from technique [36]. The formation of cyclic brush polymers with polyester side chains was selected as a model system
to demonstrate this approach (Fig. 6). The functional norbornenebased monomer M2 was designed to have a hydroxyl side group
which was used as the initiating group for ring-opening polymerization of cyclic ester monomers. Following cyclic polymer formation, a series of cyclic brush polymers were subsequently prepared
with various polyester side chains, such as poly(d-valerolactone)
(PVL), poly(L-lactide) (PLLA), and poly(e-caprolactone) (PCL).
Employing similar ideas, when the cyclic macroinitiators from
REMP have various initiating sites, such as alkyl bromide or thiocarbonylthio groups, controlled polymerization techniques like
ATRP or RAFT coupled with grafting from generated cyclic brush
43
polymers. As a result, this initial concept, based on the combination of REMP and the grafting from technique, is expected to become a powerful toolbox for the preparation of functionalized
cyclic polymers and brushes with varied molecular architectures
and functionalities.
7. Cyclic gels
The properties of gels are determined by their inherent chemical structures. Two excellent examples have demonstrated the
general rule that manipulating the cross-linking structure significantly alters the final gel characteristics. Topological (TP) gels
[38], developed by Okumura used a-cyclodextrin (a-CD) and
poly-(ethylene glycol) (PEG). By chemically cross-linking the RCD component, ‘figure-of-eight’ cross-linkages were generated that
allowed ‘sliding’ of the PEG chains. This structural topology gave
good tensile strength and a large swelling capacity to the gel. Another case is the double network (DN) gels [39], developed by Gong
that contain a highly cross-linked, stiff poly(2-acrylamido-2-methylpropanesulfonic acid) network and a loosely cross-linked
poly(acrylamide) network.
Structural defects in networks, such as loops and dangling chain
ends, deteriorate gel properties as they do not contribute to the
elasticity of the crosslinked networks. Eliminating these defects
is difficult, if not impossible using, many traditional employed
methods for network formation, including radical copolymerization of monomers (with a small amount of cross-linker), or the direct cross-linking of linear polymer chains. In contrast, if cyclic
polymer chains are used rather than their linear counterparts,
then, in theory, networks without dangling chain ends could be
formed when more than two cross-linkages per polymer chain
are constructed. This should result in gels with improved properties. Inspired by this idea, we prepared cyclic gels from chemically
cross-linking cyclic polymers [40].
REMP was used to synthesize cyclic polymer building block because it not only produces polymers with high molecular weight,
but also generates large quantities of pure cyclic polymers. Cyclic
and linear poly(5-acetoxy-1-cyclooctene) (PACOE) were then prepared by REMP and ring-opening metathesis polymerization
(ROMP) respectively (Fig. 7). To eliminate the influence of molecular conformation on the gel formation and properties, the cyclic
and linear PACOE were designed to have similar molecular weight
(Mn,cyclic = 595,700;
Mn,linear = 571,000)
and
polydispersity
(PDIcyclic = 1.77; PDIlinear = 1.70). The remaining internal double
bonds enabled facile cross-linking. As shown in Fig. 8, the classical
thiol-ene chemistry was utilized to initiate radical formation and
the subsequent carbon-centered radical coupling cross-linked the
Fig. 5. Synthesis of metallo-supramolecular cyclic brush polymers by the combination of REMP and grafting onto technique [37].
44
K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47
Fig. 6. Synthesis of cyclic brush polymers by combination of REMP and grafting from technique [36].
Table 1
Gel fraction, swelling ratio in different organic solvents, and shear modulus of cyclic
and linear gels in Xylene [40].
Runa
GF (%)b
Q (mL/g)c
G (Pa)d
DCM
THF
Benzene
Anisole
Xylene
Cyclic gels
1e
51.3
2f
47.5
g
3
35.7
126.2
124.1
199.6
112.8
104.5
160.5
110.1
103.8
157.9
101.3
100.6
150.8
94.74
87.39
125.4
111
72
ND
Linear gels
4e
74.3
5f
59.8
6g
42.9
40.95
71.92
174.3
38.56
61.29
146.4
37.14
62.96
137.9
34.62
58.25
132.9
31.16
51.46
116.8
3979
1028
ND
a
For runs 1–3, gels were obtained from cyclic PACOE; for runs 4–6, gels were
obtained from linear PACOE.
b
GF: Gel fractions.
c
Q: Swelling ratios.
d
G: Shear moduli, where ND means ‘not determined’.
e
The initial PACOE concentration was 84 mg/mL.
f
The initial PACOE concentration was 42 mg/mL.
g
The initial PACOE concentration was 21 mg/mL.
Fig. 7. Synthesis of cyclic PACOE from REMP and linear PACOE from ROMP [40].
internal double bonds in PACOE, leading to gel formation. By this
cross-linking condition, the gels were conveniently obtained in
air at room temperature in just a few minutes.
Table 1 and Fig. 9 compared the differences in gel formation and
properties between the cyclic and linear gels. Several general rules
can be extracted from these data as follows:
1. Compared to conventional networks formed by cross-linking
linear polymer chains, cyclic gels had a lower gel fraction
(GF), statistic cross-link density (l), and shear modulus (G), a
larger statistical mesh size (n), and a higher swelling ratio (Q)
at the same initial PACOE concentration (C0). These differences
became more pronounced with increasing C0 (Table 1 and
Fig. 9A–D).
2. For the linear gels, as expected, the increment of C0 increased in
GF, l, and G, and decreased in n and Q [41–45]. The cyclic gels
demonstrated similar behavior at low C0; however, at higher
C0, the increase of C0 led to the unusual increase of GF, Q, G,
and n simultaneously (Fig. 9A–D).
3. At the same preparation state (same C0), the swelling ability and
maximum strain at break of the cyclic gels were always greater
than those of the linear gels, and these differences became more
pronounced as C0 increased (Fig. 9E and F).
Fig. 8. Preparation of cyclic and linear gels from the thiol-ene chemistry.
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K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47
A
240
B
Cyclic gels
Linear gels
210
180
2.5
Q ratio
150
Q
Cyclic gels/Linear gels
3.0
120
2.0
1.5
90
60
1.0
30
20
30
40
50
60
70
80
20
90
30
40
C
D
Cyclic gels
Linear gels
1.2
50
60
70
80
90
70
80
90
C 0 (mg/mL)
C 0 (mg/mL)
Cyclic gels
Linear gels
70
60
0.8
ξ (nm)
µ (µmol/mL)
1.0
0.6
50
0.4
40
0.2
30
0.0
20
30
40
50
60
70
80
90
20
C 0 (mg/mL)
E
30
40
50
60
C 0 (mg/mL)
F
0
0
Stress (KPa)
-20
-30
Cyclic gels
Linear gels
Mooney-Rivlin Fit
-40
-1.0
-0.8
-0.6
-0.4
-0.2
Stress (KPa)
-5
-10
-10
-15
-20
-25
Cyclic gels
Linear gels
Mooney-Rivlin Fit
-30
-35
-0.8
0.0
-0.6
G
-0.2
0.0
H 700
210
180
150
600
2
-0.31 (R =0.68)
120
500
90
2
-0.80 (R =0.99)
ξ (nm)
Q'
-0.4
Strain
Strain
2
-0.58 (R =0.99)
400
60
300
Cyclic gels
Linear gels
10
20
30
40 50 60
Φ0
Cyclic gels
Linear gels
0.005
0.01
0.015
0.02 0.025
ν2,s
Fig. 9. Swelling ratio (Q) (A), the ratio of Q (B), statistical cross-link density (l) (C), and statistical mesh size (n) (D) between cyclic and linear gels as function of the initial
PACOE concentration (C0). Stress–strain curves for cyclic and linear gels prepared at C0 of 84 mg/mL (E) and 42 mg/mL (F). (G) Volumetric equilibrium swelling ratio (Q0 ) as a
function of C0 for cyclic and linear gels and (H) statistical network mesh size (n) as a function of equilibrium polymer volume fraction (t2,s) for cyclic and linear gels [40].
4. Although the linear gels follow the theoretical scaling predictions
well [45,46], the behavior of cyclic gels strongly deviated from the
description of the classical gel scaling laws (Fig. 9G and H).
These differences were explained based on the unique molecular topology and its influence on network formation and architec-
ture. Due to the large number of internal double bonds remaining
in the PACOE backbone, there is a competition between
intramolecular and intermolecular carbon centered radical coupling. This competition is influenced by C0, but also by the conformation of cyclic and linear polymer chains. The mean square radius
of gyration ðhR2g iÞ of the linear polymer is two times larger than
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K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47
the corresponding cyclic polymer [13], meaning that cyclic polymers are more compact and are more likely to react by intramolecular versus intermolecular cross-linking at a given C0 compared to
their linear counterparts. This results in linear gels having a higher
GF, l, and G, and a smaller n and Q than the corresponding cyclic
gels at each C0.
By varying concentration, it was shown that at lower C0,
intramolecular cross-linking reactions dominate over intermolecular ones. Using linear chains (Fig. 10A), this produces loop defects
that essentially waste the cross-linking points, resulting in lower
l, since the loops do not contribute to the elasticity of the network.
When cross-linked at lower C0, linear gels are formed with a large
n, which is reflected by a smaller GF and G, and a larger Q. In contrast, for cyclic polymer chains (Fig. 10B), as intramolecular crosslinks effectively reduce the cyclic ring size, the network structure
of cyclic gels is similar to that of linear gels obtained in this situation. At lower C0, the differences in the behavior of the networks
from linear and cyclic PACOE are not as obvious. Upon increasing
C0, the overlap of polymer chains in solution increases, generating
more intermolecular cross-linking reactions. In addition, it is
known that for linear chains this increase in intermolecular
cross-links generates a higher l, a tighter network structure, and
thus a smaller n [41–45], which can be observed by a larger GF
and G, and a smaller Q. The gels made from linear PACOE showed
this behavior (Fig. 10C). However, for cyclic polymer chains
(Fig. 10D), the decrease in intramolecular cross-links reduces the
number of these ‘effectively smaller rings’ so that as more
intermolecular cross-links are formed, the effective cyclic ring size
‘‘expands’’ and begins to approach the cyclic polymer’s molecular
weight. Thus, while the classical mesh size is reduced at higher
C0 as expected, the larger ring size of the cyclic polymer chains creates a new mesh spacing, so that statistically, more space is available in the networks formed from cyclic polymers at this
concentration. This leads to the cyclic gels with the non-traditional
trends of GF, G, Q, l and n as a function of C0, and again with the
strong derivation from the classical scaling laws for describing linear gels.
An increasingly popular strategy for modifying the mechanical
and swelling properties of organic polymer gels is the incorporation of inorganic components [47,48]. Taking advantage of the versatility of the metallo-supramolecular chemistry to cross-link
cyclic polymer chains, we recently created novel gels with the
structural merits of both the cyclic gels and organic/inorganic hybrid materials [37]. Such gels are expected to have unique and
interesting properties. The cyclic polymer/inorganic hybrid gels
are produced with inorganic transition-metal ions(II) serving to
cross-link the terpyridine-functionalized cyclic Poly-3 (Fig. 5).
Two model transition metal ions, Fe2+ and Ni2+, were chosen to
demonstrate this concept, and, as shown in Fig. 11A, free-standing
metallo-supramolecular gels were obtained in air at room temperature in less than 5 min, using a molar ratio of 1:5 between metal
ions and terpyridine groups. In addition to the ‘cyclic’ and ‘hybrid’
molecular structure, another appealing feature of these novel gels
is their tunable properties under certain external stimuli, which is
enabled by the characteristic reversibility of the metallo-supramolecular bonds. To demonstrate this, in one example a large excess
of free 2,20 ;60 ,200 -terpyridine was added to the Ni2+ cross-linked
metallo-supramolecular gel and allowed to stand at room temper-
Fig. 10. The ideal network structural units are shown to highlight the most important features in each system. The top two systems correspond to low C0 for linear PACOE (A)
and cyclic PACOE (B), while the bottom two panels correspond to high C0 for linear PACOE (C) and cyclic PACOE (D) [40].
K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47
47
the classical hybrid or double network concepts founded in linear
gels, a series of novel gel materials could be expected with not only
hierarchy network structure but also the advanced gel properties.
Acknowledgments
Generous support was primarily provided by NSF (DMR0820506 and CMMI-0531171), NSFC (21090353 and 21374122).
Partial support was provided from ARO W911NF-09-1-0373 and
ONR N00014-10-1-0348. Shared facilities support also comes from
DMR-0820506. K.Z thanks the Bairen project from CAS for support.
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Fig. 11. Pictures of a metallo-supramolecular cyclic gel and their stimulus–
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8. Conclusion and outlook
Three general methods have been developed for the formation
of cyclic brush polymers by combining REMP with grafting
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These methods illustrate that various cyclic brush polymers can be
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