Reactive & Functional Polymers 80 (2014) 40–47

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Reactive & Functional Polymers

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Cyclic polymers as a building block for cyclic brush polymers and gels

http://dx.doi.org/10.1016/j.reactfunctpolym.2014.01.0121381-5148/Published by Elsevier B.V.

⇑ Corresponding authors at: Department of Polymer Science and Engineering,University of Massachusetts, Amherst, MA 01003, USA (K. Zhang). Fax: +1 413 5450082.

E-mail addresses: kzhang@iccas.ac.cn (K. Zhang), tew@mail.pse.umass.edu (G.N.Tew).

Ke Zhang a,b,⇑, Gregory N. Tew a,⇑a Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003, USAb 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 a b s t r a c t

Article history:Available online 12 February 2014

Keywords:Cyclic polymerBrush polymerPolymer gelRing-Expansion metathesis polymerizationClick chemistry

Cyclic polymers, as one of the oldest topological polymers, are undergoing resurgence. This is largelyascribed to the significant achievements in modern polymer chemistry. The novel ring-expansion tech-niques have conveniently produced varied cyclic polymers with highly topological purity and on largescales, which should facilitate their use in the near future. Beyond the monocyclic molecular conforma-tions, the combination of controlled polymerization techniques and click chemistry have established arobust strategy for preparing cyclic polymers with more complex architectures, such as theta, eight,and tadpole shapes. This diversification in cyclic polymer composition and conformation significantlybroadens interest in the cyclic polymers. However, compared to the synthesis achievements, the explo-ration of cyclic polymer property and application are lagging behind. Recently, we explored the ring-expansion metathesis polymerization on various functional ring-strained olefin monomers to producecyclic functional polymers, which were then used as the building blocks to fabricate cyclic brush poly-mers and cyclic gel materials and will be discussed here.

Published by Elsevier B.V.

1. Introduction Alternatively, ring-expansion is based on the insertion of monomer

Cyclic polymers have received significant attention recently withseveral important reviews [1–7] and a specific cyclic polymer sym-posium celebrated at the 245th ACS national meeting. Due to theirunique endless molecular topology, cyclic polymers have markedlydifferent characteristics from their linear counterparts, including asmaller hydrodynamic volume and radius of gyration, lower meltviscosity, higher thermostability, and increased rate of crystalliza-tion [1–7].

To date, known synthetic strategies for cyclic polymers can begeneralized into two categories: ring-closure and ring-expansionmethods both have advantages and disadvantages. In the ring-clo-sure method, cyclic polymers are prepared by applying highly effi-cient coupling chemistry to end-functionalized linear telechelicpolymers. Progress in combining click chemistry with controlledpolymerization has increased the viability of this method for syn-thesizing cyclic polymers with controlled molecular weight andlow polydispersity, but there are still limitations. To assure hightopological purity, ring-closure reactions typically require diluteconditions leading to reduced yields. In addition, it is difficult toobtain cyclic polymers with high molecular weight by this method.

units into an activated cyclic chain. Because the cyclic polymers re-main intact during the whole ring-expansion process, this methodcan produce high molecular weight cyclic polymers with high pur-ity, even from concentrated solutions or the bulk. One disadvan-tage is that it can be difficult to control the molecular weightand polydispersity of the resultant cyclic polymers. The ring-clo-sure and ring-expansion methods have enabled the preparationof many types of cyclic polymers, including cyclic polystyrene[8], polymethacrylate [9], polyacrylate [10] polyacrylamide [11]polyester [12] polycyclooctene [13] and polynorbornene [14]. Inaddition, cyclic polymers with complex architectures have beendeveloped, including those with theta [15], eight [15], tadpole[16], block [17], and brush [18–20] shapes. The various syntheticmethods, polymer types, and various cyclic architectures havebeen thoroughly described in recent reviews [1–7].

As a complement in this tutorial review, we mainly focus on therecent progress on cyclic polymers from our research group fo-cused on ring-expansion metathesis polymerization (REMP) toproduce the functional cyclic polyolefins, which were then usedas the universal building blocks to prepare cyclic brush polymersand cyclic gels.

2. Ring-expansion metathesis polymerization

REMP was firstly reported by Grubbs and coworkers in 2002[13]. Following significant contributions from Bielawski, Boydston,

Fig. 2. Cyclic Ru-alkylidene catalysts and appropriate cyclic olefin monomers.

K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47 41

and Xia in the Grubbs’s research group, REMP has become a pow-erful method for preparing cyclic polyolefins [13,14,19,21–23].Fig. 1 shows the key mechanistic steps involved in REMP. Initiationoccurs via the cyclic olefin monomer inserting into the Ru-alkyl-idene bond to expand the cyclic catalyst ring. Propagation thenproceeds by a repeated insertion of monomer into the growing cyc-lic polymer. As monomer is consumed, a competing intramolecularchain transfer reaction decreases the molecular weight of thegrowing cyclic polymer, broadens its polydispersity, and generatescyclic polymers. An additional catalyst release process can producethe original catalyst and more catalyst-free cyclic polymer.

To date, two series of cyclic Ru-alkylidene catalysts (UC seriesand 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 sizeand the N-heterocyclic carbene (NHC) ligand electronics signifi-cantly impact the catalyst efficiency and the polymerizationbehavior as well. The catalyst with larger ring leads to faster poly-merization rates and lower catalyst release from cyclic polymer.The saturated NHC ligand endows the catalyst with much fasterpolymerization rate than that of the unsaturated NHC ligand. Fur-thermore, various cyclic olefin monomers have been successfullypolymerized by REMP. As shown in Fig. 2, they are composed ofnot only the commercial available cyclic olefins and their simplederivatives having a small functional group, but also those havingbulky side groups such as dendrons and even polymers.

As is typical of ring-expansion approaches, REMP can producelarge amounts of pure cyclic polymers at high polymerization con-centrations. Furthermore, REMP is unique in that it can obtain cyc-lic polymers with the highest reported molecular weight. Thismakes it a good candidate to prepare cyclic polymer buildingblocks for the fabrication of cyclic materials, such as the nano-scalecyclic brush polymers and the macro-scale cyclic gels.

3. Cyclic brush polymers

Brush polymers are composed of a long polymer backbone withdensely grafted polymer side chains. Due to the steric repulsion be-tween densely grafted bulky side chains, these macromoleculeshave an extended, rigid worm-like conformation when the back-bone is much longer than the side chains [24,25]. It is this uniquemolecular conformation that renders brush polymers as a uniquebuilding block for generating advanced nanomaterials, such as cre-ating well-defined single-molecular hybrid nano-objects and forfabricating periodic nanomaterials with large domain spacings byself-assembly [26]. In addition to the linear morphology, themolecular shapes of brush polymers can be enriched by manipulat-

Fig. 1. Key mechanistic steps involved in REMP [22].

ing the backbone topology [27–30]. When cyclic polymers are cho-sen as backbones, the novel cyclic brush polymers are designed tobear a donut-like molecular topology.

Although a few reports have been published on synthesizingcyclic brush polymers [18,31–33], the preparation of brush poly-mers demonstrating stiff toroidal macromolecular topologies hasremained elusive [13,23,34,35]. REMP is unique here because itmeets many of the necessary requirements for building the back-bone of cyclic brush polymers. Inspired by this, REMP has beensuccessfully utilized to prepare cyclic brush polymers by Grubbs’and Tew’s research groups independently [14,19,20,36,37]. Apply-ing the three traditional routes to linear brush polymers, [graftingthrough (polymerization of macromonomers), grafting onto (cou-pling of side chains to backbone), and grafting from (polymeriza-tion of side chains from a backbone macroinitiator)], cyclic brushpolymers have been prepared by REMP.

4. Grafting through

The first report to synthesize and visualize cyclic dendronizedpolymers by the grafting through technique was published bythe Fréchet and Grubbs’ research groups in 2009 [14]. A norborn-ene-based macromonomer was designed with a second generationdendron side chain. When using the SC-5 or UC-6 as catalyst, thecyclic dendronized polymers were obtained by REMP (Fig. 3A).The donut-like molecular topology was visualized by AFM with adiameter of 30–40 nm (Fig. 3B). Following the same concept, theyfurther investigated the direct REMP of norbornene-basedmacromonomers with a bulky polymer side chain (Fig. 3C) [19].A series of cyclic brush polymers were then produced with ultra-high molecular weight and various polymer side chains, includingpolystyrene, polylactide, and poly(n-butyl acrylate). In this work,the macromonomers were efficiently prepared by controlled poly-merization techniques and click chemistry. For a moderate to highpolymerization conversion of these bulky macromonomers, themore active SC-series catalysts were required. The donut-likemolecular topology was visualized by AFM and most cyclic brushpolymers have diameters in the range of 100–180 nm (Fig. 3D),which is much larger than that from the cyclic dendronized poly-mers. The increased molecular size is presumably caused by the

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) thecorresponding AFM characterization (reproduced and adapted from Ref. [19]. Copyright 2011 Wiley).

42 K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47

longer and significantly extended backbone conformation. In addi-tion to the cyclic topology, linear contaminants were occasionallyobserved by AFM in this method (Fig. 3D), however, it was demon-strated that they were caused by mechanical scission of the cyclicbackbone during AFM characterization.

5. Grafting onto

The combination of REMP and the grafting onto technique forthe formation of cyclic brush polymers was firstly developed inour group in 2011 [20]. In this approach (Fig. 4), a functional nor-bornene-based monomer (M1) was designed to have an activatedpentafluorophenol ester side group. When applying REMP on M1(Fig. 4(i)), high molecular weight functional cyclic polymers (cyclicPoly-1) were produced bearing activated ester side groups on eachrepeat unit. The highly efficient nucleophilic substitution of acti-vated esters by amines enabled cyclic Poly-1 to be used as a tem-plate for preparing cyclic brush polymers when coupling withamine end-functionalized polymer side chains. As an example to

Fig. 4. Synthesis of cyclic brush polymers from the combination of REMP and

demonstrate this concept, monofunctional poly(ethylene glycol)(PEG) with an amine end group and with an average of 13 repeatunits was chosen as the coupling polymer side chains (Fig. 4(ii)).A high grafting density of >95% was confirmed in this case. Theconformation of the resultant cyclic brush polymers was character-ized by AFM and a cyclic morphology with an outer diameter of ca.30 nm was observed.

Considering the difficulty of preparing amine end-functional-ized polymers, we derivatized the cyclic Poly-2 template with aclickable alkyne side group on each repeat unit. Given that azideend-functionalized polymers are conveniently obtained by atom-transfer radical polymerization (ATRP), cyclic Poly-2 was easilyfunctionalized with various polymer side chains when employingthe Cu(I)-catalyzed azide-alkyne click chemistry. A cyclic polymerbrush containing polystyrene (PS) side chains was prepared toillustrate the concept (Fig. 4(iii)). PS-N3 was obtained by substitut-ing the terminal bromine of PS-Br from ATRP by an azide moiety.The highly efficient Cu(I)-catalyzed azide-alkyne click chemistryresulted in quantitative grafting density.

grafting onto technique and the corresponding AFM characterization [20].

K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47 43

Most recently, we explored the use of supramolecular chemis-try techniques to prepare cyclic brush polymers by combiningREMP and the grafting onto technique [37]. In order to include or-ganic/inorganic hybrid concept as well, the metal–ligand supramo-lecular interaction was chosen as the building unit for thepreparation of cyclic brush polymers. Due to the outstanding che-lating ability to various transition-metal ions, the terpyridine li-gand was introduced as the side group on each repeat unit of thecyclic Poly-3 (Fig. 5). The classic two-step terpyridine-Ru(III)/Ru(II)supramolecular chemistry was specifically chosen to demonstratethis technique. In this approach, various terpyridine end-function-alized polymer side chains were synthesized by reversible addi-tion-fragmentation chain-transfer polymerization (RAFT) with aterpyridine-functionalized RAFT agent. As the first step, the poly-mer�terpyridine�RuCl3 monocomplex was obtained from the ter-pyridine end-functionalized polymer precursor by refluxing withRuCl3. Subsequently, the metallo-supramolecular cyclic brushpolymers were prepared by reacting the terpyridine-Ru(III)mono-complex onto the cyclic backbone under reducing condi-tions (ethanol, N-ethylmorpholine), to produce the stable bis(ter-pyridine)–Ru(II) linkage connecting the polymer side chains andthe cyclic polynorbornene main chains (Fig. 5). Because of theinherent metal-containing molecular structure, these novel supra-molecular hybrid cyclic brush polymers provide an ideal model fordirect imaging of single polymer chains by TEM. As shown in Fig. 5,a representative TEM image clearly shows the cyclic moleculartopology with an average diameter of ca. 40 nm. This is the firsttime that TEM has been used to directly image the moleculartopology of cyclic polymers.

6. Grafting from

In 2012, we reported the third method for the preparation ofcyclic brush polymers based on REMP by combining it with thegrafting from technique [36]. The formation of cyclic brush poly-mers with polyester side chains was selected as a model systemto demonstrate this approach (Fig. 6). The functional norbornene-based monomer M2 was designed to have a hydroxyl side groupwhich was used as the initiating group for ring-opening polymer-ization of cyclic ester monomers. Following cyclic polymer forma-tion, a series of cyclic brush polymers were subsequently preparedwith 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 fromREMP have various initiating sites, such as alkyl bromide or thio-carbonylthio groups, controlled polymerization techniques likeATRP or RAFT coupled with grafting from generated cyclic brush

Fig. 5. Synthesis of metallo-supramolecular cyclic brush polymers

polymers. As a result, this initial concept, based on the combina-tion of REMP and the grafting from technique, is expected to be-come a powerful toolbox for the preparation of functionalizedcyclic polymers and brushes with varied molecular architecturesand functionalities.

7. Cyclic gels

The properties of gels are determined by their inherent chemi-cal structures. Two excellent examples have demonstrated thegeneral rule that manipulating the cross-linking structure signifi-cantly alters the final gel characteristics. Topological (TP) gels[38], developed by Okumura used a-cyclodextrin (a-CD) andpoly-(ethylene glycol) (PEG). By chemically cross-linking the R-CD component, ‘figure-of-eight’ cross-linkages were generated thatallowed ‘sliding’ of the PEG chains. This structural topology gavegood tensile strength and a large swelling capacity to the gel. An-other case is the double network (DN) gels [39], developed by Gongthat contain a highly cross-linked, stiff poly(2-acrylamido-2-meth-ylpropanesulfonic acid) network and a loosely cross-linkedpoly(acrylamide) network.

Structural defects in networks, such as loops and dangling chainends, deteriorate gel properties as they do not contribute to theelasticity of the crosslinked networks. Eliminating these defectsis difficult, if not impossible using, many traditional employedmethods for network formation, including radical copolymeriza-tion of monomers (with a small amount of cross-linker), or the di-rect cross-linking of linear polymer chains. In contrast, if cyclicpolymer chains are used rather than their linear counterparts,then, in theory, networks without dangling chain ends could beformed when more than two cross-linkages per polymer chainare constructed. This should result in gels with improved proper-ties. Inspired by this idea, we prepared cyclic gels from chemicallycross-linking cyclic polymers [40].

REMP was used to synthesize cyclic polymer building block be-cause it not only produces polymers with high molecular weight,but also generates large quantities of pure cyclic polymers. Cyclicand linear poly(5-acetoxy-1-cyclooctene) (PACOE) were then pre-pared by REMP and ring-opening metathesis polymerization(ROMP) respectively (Fig. 7). To eliminate the influence of molecu-lar conformation on the gel formation and properties, the cyclicand 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 doublebonds enabled facile cross-linking. As shown in Fig. 8, the classicalthiol-ene chemistry was utilized to initiate radical formation andthe subsequent carbon-centered radical coupling cross-linked the

by the combination of REMP and grafting onto technique [37].

Fig. 6. Synthesis of cyclic brush polymers by combination of REMP and grafting from technique [36].

Fig. 7. Synthesis of cyclic PACOE from REMP and linear PACOE from ROMP [40].

Table 1Gel fraction, swelling ratio in different organic solvents, and shear modulus of cyclicand linear gels in Xylene [40].

Runa GF (%)b Q (mL/g)c G (Pa)d

DCM THF Benzene Anisole Xylene

Cyclic gels1e 51.3 126.2 112.8 110.1 101.3 94.74 1112f 47.5 124.1 104.5 103.8 100.6 87.39 723g 35.7 199.6 160.5 157.9 150.8 125.4 ND

Linear gels4e 74.3 40.95 38.56 37.14 34.62 31.16 39795f 59.8 71.92 61.29 62.96 58.25 51.46 10286g 42.9 174.3 146.4 137.9 132.9 116.8 ND

a For runs 1–3, gels were obtained from cyclic PACOE; for runs 4–6, gels wereobtained 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.

44 K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47

internal double bonds in PACOE, leading to gel formation. By thiscross-linking condition, the gels were conveniently obtained inair at room temperature in just a few minutes.

Table 1 and Fig. 9 compared the differences in gel formation andproperties between the cyclic and linear gels. Several general rulescan be extracted from these data as follows:

1. Compared to conventional networks formed by cross-linkinglinear polymer chains, cyclic gels had a lower gel fraction(GF), statistic cross-link density (l), and shear modulus (G), a

Fig. 8. Preparation of cyclic and linear

larger statistical mesh size (n), and a higher swelling ratio (Q)at the same initial PACOE concentration (C0). These differencesbecame more pronounced with increasing C0 (Table 1 andFig. 9A–D).

2. For the linear gels, as expected, the increment of C0 increased inGF, l, and G, and decreased in n and Q [41–45]. The cyclic gelsdemonstrated similar behavior at low C0; however, at higherC0, 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 andmaximum strain at break of the cyclic gels were always greaterthan those of the linear gels, and these differences became morepronounced as C0 increased (Fig. 9E and F).

gels from the thiol-ene chemistry.

10 20 30 40 50 60

60

90

120

150180210

Cyclic gels Linear gels

Q'

0

-0.80 (R2=0.99)

-0.31 (R2=0.68)

0.005 0.01 0.015 0.02 0.025

300

400

500

600

700

Cyclic gels Linear gels

(nm

)

2,s

-0.58 (R2=0.99)

20 30 40 50 60 70 80 90

1.0

1.5

2.0

2.5

3.0 Cyclic gels/Linear gels

Q ra

tio

C0 (mg/mL)20 30 40 50 60 70 80 90

30

60

90

120

150

180

210

240 Cyclic gels Linear gels

Q

C0 (mg/mL)

20 30 40 50 60 70 80 900.0

0.2

0.4

0.6

0.8

1.0

1.2

µ (µ

mol

/mL)

C0 (mg/mL)

Cyclic gels Linear gels

20 30 40 50 60 70 80 90

30

40

50

60

70 Cyclic gels Linear gels

C0 (mg/mL)

-0.8 -0.6 -0.4 -0.2 0.0-35

-30

-25

-20

-15

-10

-5

0

Stre

ss (K

Pa)

Strain

Cyclic gels Linear gels Mooney-Rivlin Fit

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-40

-30

-20

-10

0

Cyclic gels Linear gels Mooney-Rivlin Fit

Stre

ss (K

Pa)

Strain

ν

ξ(n

m)

ξ

Φ

A B

C D

E F

G H

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 initialPACOE 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 afunction 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].

K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47 45

4. Although the linear gels follow the theoretical scaling predictionswell [45,46], the behavior of cyclic gels strongly deviated from thedescription of the classical gel scaling laws (Fig. 9G and H).

These differences were explained based on the unique molecu-lar topology and its influence on network formation and architec-

ture. Due to the large number of internal double bonds remainingin the PACOE backbone, there is a competition betweenintramolecular and intermolecular carbon centered radical cou-pling. This competition is influenced by C0, but also by the confor-mation of cyclic and linear polymer chains. The mean square radiusof gyration ðhR2

giÞ of the linear polymer is �two times larger than

46 K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47

the corresponding cyclic polymer [13], meaning that cyclic poly-mers are more compact and are more likely to react by intramolec-ular versus intermolecular cross-linking at a given C0 compared totheir linear counterparts. This results in linear gels having a higherGF, l, and G, and a smaller n and Q than the corresponding cyclicgels at each C0.

By varying concentration, it was shown that at lower C0,intramolecular cross-linking reactions dominate over intermolecu-lar ones. Using linear chains (Fig. 10A), this produces loop defectsthat essentially waste the cross-linking points, resulting in lowerl, since the loops do not contribute to the elasticity of the network.When cross-linked at lower C0, linear gels are formed with a largen, which is reflected by a smaller GF and G, and a larger Q. In con-trast, for cyclic polymer chains (Fig. 10B), as intramolecular cross-links effectively reduce the cyclic ring size, the network structureof cyclic gels is similar to that of linear gels obtained in this situa-tion. At lower C0, the differences in the behavior of the networksfrom linear and cyclic PACOE are not as obvious. Upon increasingC0, the overlap of polymer chains in solution increases, generatingmore intermolecular cross-linking reactions. In addition, it isknown that for linear chains this increase in intermolecularcross-links generates a higher l, a tighter network structure, andthus a smaller n [41–45], which can be observed by a larger GFand G, and a smaller Q. The gels made from linear PACOE showedthis behavior (Fig. 10C). However, for cyclic polymer chains(Fig. 10D), the decrease in intramolecular cross-links reduces thenumber of these ‘effectively smaller rings’ so that as moreintermolecular cross-links are formed, the effective cyclic ring size‘‘expands’’ and begins to approach the cyclic polymer’s molecular

Fig. 10. The ideal network structural units are shown to highlight the most important feaand cyclic PACOE (B), while the bottom two panels correspond to high C0 for linear PAC

weight. Thus, while the classical mesh size is reduced at higherC0 as expected, the larger ring size of the cyclic polymer chains cre-ates a new mesh spacing, so that statistically, more space is avail-able in the networks formed from cyclic polymers at thisconcentration. This leads to the cyclic gels with the non-traditionaltrends of GF, G, Q, l and n as a function of C0, and again with thestrong derivation from the classical scaling laws for describing lin-ear gels.

An increasingly popular strategy for modifying the mechanicaland swelling properties of organic polymer gels is the incorpora-tion of inorganic components [47,48]. Taking advantage of the ver-satility of the metallo-supramolecular chemistry to cross-linkcyclic polymer chains, we recently created novel gels with thestructural merits of both the cyclic gels and organic/inorganic hy-brid materials [37]. Such gels are expected to have unique andinteresting properties. The cyclic polymer/inorganic hybrid gelsare produced with inorganic transition-metal ions(II) serving tocross-link the terpyridine-functionalized cyclic Poly-3 (Fig. 5).Two model transition metal ions, Fe2+ and Ni2+, were chosen todemonstrate this concept, and, as shown in Fig. 11A, free-standingmetallo-supramolecular gels were obtained in air at room temper-ature in less than 5 min, using a molar ratio of 1:5 between metalions and terpyridine groups. In addition to the ‘cyclic’ and ‘hybrid’molecular structure, another appealing feature of these novel gelsis their tunable properties under certain external stimuli, which isenabled by the characteristic reversibility of the metallo-supramo-lecular bonds. To demonstrate this, in one example a large excessof free 2,20;60,200-terpyridine was added to the Ni2+ cross-linkedmetallo-supramolecular gel and allowed to stand at room temper-

tures in each system. The top two systems correspond to low C0 for linear PACOE (A)OE (C) and cyclic PACOE (D) [40].

Fig. 11. Pictures of a metallo-supramolecular cyclic gel and their stimulus–response [37].

K. Zhang, G.N. Tew / Reactive & Functional Polymers 80 (2014) 40–47 47

ature. After �20 days, the free-standing gel became fluid (Fig. 11B).Detailed structure–property studies on these novel gels, specifi-cally their dynamics, are currently underway.

8. Conclusion and outlook

Three general methods have been developed for the formationof cyclic brush polymers by combining REMP with graftingthrough, grafting onto, and grafting from techniques, respectively.These methods illustrate that various cyclic brush polymers can beconveniently produced and that many other molecular combina-tions can be easily envisaged. For example, when using the cyclicbrush polymers with appropriate functional polymer side chainsas single molecule templates, various well-defined cyclic nano-ob-jects such as cyclic hybrid nano-cylinders and cyclic nano-tubescould be expected. In addition, the self-assembly of amphiphiliccyclic brush polymers is likely to produce nanomaterials withattractive morphologies and properties. Unimolecular micelleswith various size and functionality are of considerable interestand should be available through these synthetic methods.

Although we have produced a fundamentally new gel system bycross-linking the cyclic polymers and discovered its unique gel for-mation and properties, research on cyclic gels is just beginning andmany further studies are needed. For example, elaborate cross-linking methods are urgently needed to efficiently manipulatethe intramolecuar and intermolecular cross-link in the formationof cyclic gels, since the intramolecular cross-link effectively re-duces the cyclic ring size and seriously degrades the cyclic gel’sproperties. In addition, by merging the cyclic topology effects with

the classical hybrid or double network concepts founded in lineargels, a series of novel gel materials could be expected with not onlyhierarchy network structure but also the advanced gel properties.

Acknowledgments

Generous support was primarily provided by NSF (DMR-0820506 and CMMI-0531171), NSFC (21090353 and 21374122).Partial support was provided from ARO W911NF-09-1-0373 andONR N00014-10-1-0348. Shared facilities support also comes fromDMR-0820506. K.Z thanks the Bairen project from CAS for support.

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