Review on Making Polymer Macro Cycles

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Synthetic approaches for the preparation of cyclic polymers

Boyd A. Laurent and Scott M. Grayson*

Received 9th February 2009

First published as an Advance Article on the web 17th April 2009

DOI: 10.1039/b809916m

Despite decades of studies devoted to the unique physical properties and potential applications

of cyclic polymer topologies, their exploration has remained limited because of synthetic

inefficiencies and acyclic impurities. Many recently developed synthetic techniques offer efficient

routes to well-defined cyclic macromolecules to answer this need. This tutorial review aims to

provide a concise overview of the most significant synthetic contributions in this field, and

highlight the relative advantages and disadvantages of each approach.

1. Introduction

Tailored control of polymer architecture has been a goal of

polymer chemists since it was first understood that a polymer’s

physical properties are inherently dependent on its nanoscopic

architecture. Synthetic exploration of a number of architec-

tures such as linear polymers, polymer brushes, star polymers,

ladder polymers, dendrimers, hyperbranched polymers, and

network polymers has enabled a detailed understanding of

specifically how covalent architecture affects their observed

macroscopic properties. The effect of a ‘‘continuous’’ cyclic

topology on polymer properties is of significant interest because

the end-groups of non-cyclic architectures have demonstrated

a significant role in their material properties. However, a

detailed physical understanding of cyclic polymers has been

limited largely by synthetic complications. In addition to

difficulties in preparing large scales of cyclic polymers, most

methods yield materials with at least trace amounts of linear

polymer impurities, which can jeopardize the validity of physical measurements. For these reasons, better synthetic

techniques that yield high-purity cyclic materials have been

sought.

During the first synthetic explorations of cyclization

reactions in small organic molecules, Paul Ruggli,1 and later

Karl Ziegler et al.2

demonstrated that high dilution could beused to favor the formation of cyclics. This results from the

fact that even under high dilution, when intermolecular reactions

are disfavored, the effective molarity of reactive groups for the

cyclization reaction remains high because they are covalently

tethered to each other. While small rings (3–4 covalent bonds)

are disfavored due to Baeyer strain, intermediate rings

(5–6 covalent bonds) are favored because of low strain, and

slightly larger rings (7–13 covalent bonds) are disfavored due

to Pitzer and transannular strain; the conformational flexibility

in significantly larger rings results in negligible strain energies.

As early as 1935, Ruzicka predicted that the increasing

entropic penalties expected for larger cyclization reactions

would become a significant complicating factor.3 However,

the discovery and structural determination of cyclic peptides,

such as gramicidin S,4 and cyclic DNA,5 have since verified the

synthetic feasibility of cyclic macromolecules.

The first examples of synthetic cyclic polymers were prepared

via the ring-chain equilibrium of poly(dimethylsiloxanes) and

polyesters.6 These early synthetic methods typically yielded

Department of Chemistry, Tulane University, New Orleans,LA 70118, USA. E-mail: [email protected];Fax: +1 (504) 865-5596; Tel: +1 (504) 862-8135

Boyd A. Laurent (left) and Scott M. Grayson (right)

Boyd A. Laurent earned his BS in Chemistry in 2005 fromLouisiana State University in Baton Rouge, LA, USA where heworked in the labs of Professor Graca Vicente and Professor KevinSmith. He is currently a Louisiana Board of Regents fellow, pursuing a PhD in Chemistry at Tulane University under the

guidance of Professor Scott M. Grayson. His research is focused on the exploration and synthesis of novel polymer architecturesbased on cyclic polymer substrates.Scott M. Grayson received a BS from Tulane University (1996)and a PhD in Chemistry from the University of California,Berkeley (2002), studying the role of polymer architecture fordrug delivery under Jean M. J. Fre chet. Following post-doctoral studies in the laboratories of C. Grant Willson at the University of Texas, he accepted a position as an assistant professor in thedepartment of Chemistry at Tulane University in New Orleans. Hespent his first semester on ‘‘hurricane sabbatical’’ at Washington

University in St. Louis and returned to Tulane by January of 2006 to continue his research, focusing on the synthesis, characterization,and application of complex, yet well-defined polymer architectures with a particular focus on cyclic polymers.

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2009,38

, 2202–2213 This journal isc

The Royal Society of Chemistry 2009

TUTORIAL REVIEW www.rsc.org/csr | Chemical Society Reviews

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impure materials, with significant amounts of linear

by-products. The differences in physical properties between

the desired cyclic product and linear by-products have enabled

the isolation of macrocyclic fractions. Most commonly, if the

impurities have a significant difference in molecular weight,

fractional precipitation has been used to isolate cyclic materials.

In addition, because the cyclic polymers have a reduced hydro-

dynamic volume, preparative gel permeation chromatography(GPC) provides an effective means for isolating minute

quantities of polymer macrocycle. Alternatively, Beckham and

coworkers have taken advantage of the ‘‘open-ends’’ present in

the linear by-products to form inclusion complexes between

poly(ethylene oxide) and a-cyclodextrin, which enables the

selective precipitation of the linear impurities.7 In addition,

the use of liquid chromatography at the critical condition

(LCCC) has proved to be a very effective technique for isolating

cyclic polymers particularly at higher molecular weights, as

samples are separated by architecture and functionality rather

than molecular weight.8

While these and other rigorous methods of purification have

aided in the isolation of cyclic polymers, and have provided

the first physical data verifying the unique properties of

polymer macrocycles, improved synthetic approaches have

been developed to enable the production of cyclic molecules

with narrow polydispersities and vastly improved cyclic purity.

The aim of this tutorial review is to highlight the most successful

synthetic advances in the preparation of well-defined cyclic poly-

mers as well as to show the evolution of the field as a result of

modern advances in synthetic polymerization techniques. A

broader perspective of cyclic oligomers and polymers, including

detailed discussion of the physical and theoretical aspects, has been

reviewed elsewhere.9 While a diverse range of more complex cyclic

topologies such as tadpoles, figure-eights, theta-shaped,

mannacles, and sun-shaped polymers have been reported, these

polymeric architectures will not be discussed as they are outsidethe fundamental scope of this review (Fig. 1). Likewise, while there

is a diversity of polymerization and coupling chemistries that

have been applied to preparing cyclic polymers,10 this review

attempts to focus on the scope and limitation of those syntheses

believed to be the most representative of the general techniques,

and those believed to be the most versatile for future

investigations.

2. Synthetic approach

Synthetic strategies for the formation of cyclic polymers can be

divided into two main categories: (1) ring-closure techniques

(Fig. 2) and (2) ring-expansion techniques (Fig. 3). The ring-

closure method involves the coupling of a linear polymer’s

end-groups to yield a cyclic polymer while the ring-expansion

technique involves the insertion of cyclic monomer units into

an activated cyclic chain. The synthetic aspects of both

approaches will be discussed in detail below.

The verification of a polymer macrocycle’s cyclic architec-

ture is critical to judge the success of a synthetic route. A

number of methods have been used to verify the formation of

cyclic polymers based on the known differences in theirphysical properties both in solution and in bulk, relative to

linear analogues. Polymer macrocycles exhibit a longer

retention time by GPC compared to linear analogues due to

their more compact cyclic topology, and therefore exhibit

Fig. 1 Representations of more complex cyclic polymer topologies.

Fig. 2 Schematic representation of the three most common ring-

closure techniques for the preparation of cyclic polymers.

Fig. 3 Schematic representation of the ring-expansion technique for

the preparation of cyclic polymers.

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smaller hydrodynamic radii. This technique, when compared

with a more direct method of determining molecular weight

(e.g. MALDI-TOF MS) is the most common technique for

verifying cyclization. In addition, the glass transition (T g)

temperatures of cyclic polymers typically increase with mole-

cular weight faster than their corresponding linear polymers

due to a decrease in chain mobility of the cyclic species.

Likewise, the intrinsic viscosities of cyclic polymers are con-sistently lower than linear analogues and their self-diffusion

coefficients, as measured by pulsed-gradient spin echo NMR

techniques,11 resemble those of smaller linear polymers. It has

also been shown that cyclic poly(styrene) exhibits an enhanced

fluorescence emission intensity relative to its linear analogues

at molecular weights less than 20 kDa.12 Although no single

analytical technique can definitively prove a polymer’s cyclic

topology, a combination of multiple techniques can provide

strong evidence.

2.1 Ring-closure techniques

The first successful approach developed for the preparation of

relatively pure cyclic polymers utilized ring-closure techniques.These ring-closure techniques can be broken into two major

categories: (1) the ring-closure of homodifunctional polymers

and (2) the ring-closure of heterodifunctional polymers. Initial

investigations focused primarily on the homodifunctional

ring-closure of living dianionic polymers due to their synthetic

accessibility. One of the critical features of all ring-closure

techniques is that they require high ‘‘Ruggli–Ziegler’’ dilution

to favor intramolecular cyclization over intermolecular

coupling. For homodifunctional couplings that are bimolecular

in nature, it is difficult to prepare extremely high purity cyclic

polymers. This results from the fact that the first coupling

requires an intermolecular reaction whereas the second

coupling involves an intramolecular reaction and typically

optimized conditions that favor one, disfavor the other. On

the other hand, with appropriately efficient unimolecular

coupling reactions, either with homodifunctional or hetero-

difunctional polymers, this approach can yield high purity

cyclic macromolecules under high dilution.

2.1 (i) Ring-closure of a,x-homodifunctional polymers

Homodifunctional bimolecular coupling. Since the bimolecular

cyclization reaction of homodifunctional polymers is first order

in both polymer and the bimolecular coupling agent, the use of

exact stoichiometric quantities of reagents is crucial (Fig. 4). If

an excess of the bifunctional linking agent is added to the

homodifunctional polymer, this can result in both ends of a

polymer end-group reacting with different linking agentspreventing cyclization (Case I). If insufficient bifunctional

linking agent is used, the product will be dominated by linear

polymeric dimers (Case II).

Even with 1:1 stoichiometry, excluding these two side

reactions is difficult. This complication is elucidated by a

detailed examination of the competitive kinetics between

intramolecular cyclization and intermolecular oligomerization

(Fig. 5). The first step in the formation of cyclic polymers is the

reaction between the polymer and the linking agent (Rc1),

however, once the intermediate is formed, the rates of secondary

couplings (Rc2, Rc2’) and oligomerizations (Rolig) compete

with the rate of cyclization (Rcyc). Because the rate of each

reaction is merely the product of the rate constants ( k) and

the concentration of the reactants, the concentration of thebis-functional polymer and coupling agent must be kept low to

favor cyclization over the many by-products. However, in

order to provide sufficient precursor for the cyclization, there

must be an appreciable concentration of the starting materials.

Therefore, unless a synthetic procedure can be developed that

provides a vastly larger rate constant for the cyclization

relative to the by-products, linear contaminants are inevitable.

As a result, tedious purification techniques must be employed

to isolate pure cyclic product.

Because living anionic polymerization allows for the

formation of living polymers with well-defined molecular

Fig. 4 Schematic representation of the by-products formed by

inexact stoichiometries during the bimolecular homodifunctional

cyclization.

Fig. 5 Schematic representation of the competing coupling reactions

and by-products formed during the bimolecular homodifunctional

cyclization.

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weights and low polydispersities, this approach has been used

extensively to produce precursors for the bimolecular

homodifunctional coupling. Three research groups, those of

Ho ¨ cker,13 Rempp,14 and Vollmert,15 published nearly simul-

taneous reports for the preparation of cyclic polymers via

this route. In these procedures, the authors used sodium

naphthalenide (1) to generate a bis-anionic initiator (2) for

the polymerization of styrene. The anionic end-groups (3) were

eventually terminated by the addition of a,a0-dihalo- p-xylene

(4) linking agent to produce the corresponding ring-polystyrenes

(5) (Fig. 6). However, after adding 1 equiv. of linker, thepresence of styryl anion was still evident by colorimetric

means. Therefore, to enable the facile separation of the presumed

linear by-products, excess difunctional linking agent was

added to functionalize all remaining linear chain-ends,

followed by the addition of high molecular weight living

anionic polymer. The resultant disparity in molecular weight

between the high molecular weight by-products and relatively

low molecular weight cyclic polymers allowed facile separation

of the cyclic and linear compounds via fractionation. In these

initial reports, cyclic poly(styrene) with molecular weights

between 3 kDa and 60 kDa was synthesized with poly-

dispersity indices typically below 1.2 but the yields of cycliza-

tion were often low (o50%). GPC analysis of the reactionproduct showed the expected shift to longer retention times

when compared to the corresponding linear precursors. Since

these first reports, similar techniques have been used to

synthesize cyclic poly(styrene) with molecular weights up to

450 kDa.16

A number of groups have utilized analogous techniques to

prepare cyclic polymers with a diversity of monomers including

styrenics, vinylpyridines, ethylene oxides, and dienes. Typically,

anionic polymerization is initiated by reaction of sodium

naphthalenide (1) with either styrene or 1,1-diphenylethene

to produce bifunctional initiators 2 and 6 respectively.

Bifunctional living initiators 7 and 8 can also be formed by

reacting alkyl lithium nucleophiles with the alkene groups of

1,2-bis(isopropenyl-4-phenyl)ethane (9) or 1,3-bis(1-phenyl-

ethylenyl)benzene (10) (Fig. 7). Different electrophilicbifunctional linking agents that have also been used for the

ring-closure step include dichlorodimethylsilane (11), dichloro-

methane (12), and a,a0-dichloro- or a,a0-dibromo- p-xylene

(13). In addition, reaction of the dianionic polymer chain with the

1,2-bis(isopropenyl-4-phenyl)ethane (9) and 1,3-bis(1-phenyl-

ethylenyl)benzene (10) linking agents generates relatively

stable dianions which can either be quenched or reacted with a

different monomer to yield more complex cyclic topologies such

as tadpoles or figure-eight polymers. Since these initial publi-

cations, a number of groups have used this approach to obtain

a diversity of cyclic homopolymers as well as block copoly-

mers. Some examples of these polymer compositions include

cyclic poly(butadiene), poly(2-vinylpyridine), poly(isoprene),and cyclic block copolymers comprised of poly(styrene)-b-

poly(dimethylsiloxane), poly(styrene)-b-poly(2-vinylpyridine),

poly(styrene)-b-poly(ethylene oxide), poly(propylene oxide)-

b-poly(ethylene oxide), poly(styrene)-b-poly(isoprene), and

poly(butadiene)-b-poly(styrene). Further details for the

synthesis of these cyclic polymers and other complex

architectures using anionic polymerization has been

thoroughly reviewed by Hadjichristidis et al .17

In order to minimize the linear impurities produced during

the bimolecular coupling, Ishizu et al. investigated a bi-phasic

coupling reaction. The poly(styrene) bis-anion (3) was reacted

Fig. 6 The first syntheses of cyclic poly(styrene) using anionic poly-

merization and a bimolecular homodifunctional coupling with

dihalo- p-xylene linking agents.

Fig. 7 Structures of common bifunctional initiators used to generate

dianionic living polymers as well as common linking agents used for

ring-closure.


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with a large excess of 1,4-dibromobutane so that the

dihalogenated linear polymer intermediate (14) could be isolated,

purified, and characterized before cyclization. The ring-closure

to yield 15 was performed by the reaction of a stoichiometric

amount of 1,6-diaminohexane with the bromo-end-capped

poly(styrene) polymer (Fig. 8).18 In contrast to previous

methods, the authors confined the reaction to the organic-

aqueous interface by utilizing the solubility differences of both

components. The bromo-end-capped poly(styrene) (14) was

dissolved into a mixture of toluene and DMSO while the

1,6-diaminohexane coupling agent was dissolved into a basic

aqueous phase. The authors demonstrated extremely high

cyclization efficiencies (490%) to yield cylic poly(styrene)

(15) with linear precursor concentrations as high as 10À3 M.

Previous coupling strategies required much lower concentra-

tions of 10À5 M and 10À7 M in order to repress intermolecular

coupling and achieve similar yields of cyclic polymer. This

observed result is presumably caused by the reduced rate of the

intermolecular coupling because it must occur at the interface

between the two phases while the rate of cyclization is not

inhibited because of its intramolecular nature. In a subsequentpublication, the authors extended this technique to synthesize

cyclic block copolymers that consist of poly(styrene)-b-poly-

(isoprene) and compared the phase segregation properties

in the bulk of these materials to that of linear tri-block

analogues.19 As predicted, the covalently forced loop

configuration leads to a narrower lamellar spacing than the

linear analogues.

Oike et al. provided an alternative solution for minimizing

linear by-products by using electrostatic interactions to template

the cyclization.20 Using anionic polymerization and subsequent

end-group modification, polymers with reactive cationic termini

can be prepared. Small molecule anions such as di-carboxylates

and di-sulfonates readily form dimeric ion pairs in tetrahydro-furan and template their subsequent coupling to form cyclic

polymer. Because of the strength of the electrostatic attraction,

high dilution can minimize the interaction between polymer

chains (reducing Rc2, Rc20, Rolig), while the ionic pairing main-

tains a high rate of initial coupling and cyclization (Rc1, Rcyc).

The authors initially demonstrated this technique using

poly(tetrahydrofuran) polymers end-capped with strained

N -phenylpyrrolidinium groups and bifunctional carboxylate

anions to pre-assemble the cyclic ion-pair precursors (16). After

the assembly process was completed at the desired concentration

of polymer, the system was heated, triggering the nucleophilic

attack of the carboxylate on the pyrrolidinium salts to produce

the neutral cyclic diester species (17) (Fig. 9). The authors have

shown this to be a highly efficient process at concentrations as

high as 4.6 Â 10À5 M for poly(tetrahydrofuran) of 4.3 kDa.

Characterization of the product by GPC verified its smaller

hydrodynamic radius while viscosity measurements determinedthe inherent viscosity ratio [Zcyc.]/[Zlin.] to be 0.647, which is in

agreement with previously reported values. The authors have

since extended this same methodology to successfully produce

cyclic poly(ethylene oxide)s, cyclic poly(styrene)s,21 and a diverse

library of cyclic polymer architectures, but the molecular weights

of the produced polymers were generally limited to below 5 kDa.

Homodifunctional unimolecular coupling. An alternative

approach that overcomes the complications of the bimolecular

coupling reaction while using homodifunctional polymers

involves the direct coupling reaction between identical

polymer end-group functionalities. In this unimolecular case,

dilution of the linear precursors represses the oligomerization,but will not reduce the rate of intramolecular coupling since

the reactive groups are tethered to one another. Therefore,

if a high efficiency homocoupling reaction is applied to this

approach, very well-defined cyclic polymers can be made while

suppressing oligomeric by-products.

One early example of an efficient homocoupling cyclization

reaction was demonstrated by Tezuka and Komiya using ring-

closing metathesis of allyl end-functionalized polymers.22

Initially, poly(tetrahydrofuran) was polymerized, end-capped

with allyl groups, and cyclized under ultra-dilute conditions with

Grubb’s Ru-based metathesis catalyst. The authors found that

Fig. 8 The interfacial condensation cyclization technique employed

by Ishizu and co-workers to minimize intermolecular oligomerization

during bimolecular coupling.18

Fig. 9 Cyclization via ionic pre-assembly followed by a covalent

fixation used by Tezuka and co-workers to minimize intermolecular

oligomerization during bimolecular coupling.20

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concentrations below 4 Â 10À5 M were necessary to assure

cyclization over oligomerization. MALDI-TOF MS verified a

shift in mass corresponding to the loss of ethylene (À28 Da)

while GPC verified a reduced hydrodynamic volume. This

coupling technique is also compatible with atom transfer radical

polymerization (ATRP) if Keck couplings were used to install

the required alkene end-groups on both bromo-functionalized

polymer chain ends. Acrylate monomers were polymerized from

a bifunctional ATRP initiator (18) and then the halide end-

groups were converted with allyl tributyl tin (19) to the desired

allylic end-groups (20).23 Ring-closing metathesis to form cyclic

poly(methylacrylate) (21) was performed using analogous

conditions as described for the poly(THF) (Fig. 10). The cyclic

topology was again confirmed by the expected shift to longer

GPC retention times relative to linear precursors, and by the

characteristic loss of ethylene in the MALDI-TOF massspectra.

An additional unimolecular coupling technique that has been

applied to cyclize homodifunctional polymers is the reversible

oxidation of thiol terminated polymers to yield a disulfide

linkage. Monteiro and coworkers utilized reversible addition

fragmentation chain transfer (RAFT) polymerization from a

bifunctional initiator to synthesize a linear poly(styrene) with a

molecular weight of 3.7 kDa and a polydispersity of 1.1.24

During aminolytic removal of the dithioester end-groups, the

corresponding poly(styrene) di-thiols (22) were oxidized by

either aerial oxidation or by a strong oxidizing agent (Fe(III)Cl3)

to form the cyclic disulfides (23) using a slow feed addition

(2.4 Â 10À4

M final concentration) in order to favor theintramolecular cyclization product (90%) (Fig. 11). Alternatively,

if the oxidative coupling was performed at higher concentra-

tion or without the slow feed process, the reaction yielded

predominantly long linear oligomers of the poly(styrene)

precursor with smaller amounts of cyclic product. In addition,

both the cyclic and oligomeric products could be readily

converted back to the original linear precursors by reduction

with Zn. This approach is unique in that the polymer

sample can be rapidly and reversibly converted from cyclic

poly(styrene) to linear poly(styrene) by controlling both the

redox environment and concentration.

2.1 (ii) Ring closure of a,x-heterodifunctional polymers.

a,o-Heterodifunctional polymers, those bearing differentfunctional groups at opposite chain-ends, provide an effective

route to cyclic polymers as long as complimentary end-groups

can be efficiently installed and coupled at high dilution. A

significant advantage of this process over the bimolecular

homodifunctional coupling procedure is that the complimentary

functionalities are tethered to one another, eliminating

problems resulting from inexact stoichiometries. This

technique also avoids the need for an initial intramolecular

coupling and therefore high dilution can be used to prevent

intermolecular oligomerization without slowing the rate of

cyclization (Rcyc) (Fig. 12). A final minor advantage over the

unimolecular homodifunctional approach is that because hetero-

difunctional polymer precursors contain different end-groups,

the effective concentration of complimentary functional

groups on different chains is reduced by half, further slowing

the rate of intermolecular oligomerization (Rolig). However,

this approach is more synthetically demanding as it involves

Fig. 10 The homodifunctional unimolecular cyclization of poly-

(methyl acrylate) used by Tezuka and co-workers via an olefin

metathesis coupling.23

Fig. 11 Reversible cyclization and ring-scission utilizing the

reversibility of thiol/disulfide oxidation and reduction by Montiero

and co-workers.24

Fig. 12 Schematic representation of the competing coupling

reactions during unimolecular heterodifunctional cyclization.


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complimentary end-group functionalities, which usually require

a near quantitative post-polymerization end-group trans-

formation. Incomplete end-group transformations will reduce

cyclization yields and generate linear impurities.

The earliest work utilizing the heterodifunctional cyclization

technique was performed by Schappacher and Deffieux, by

utilizing the living nature of cationic 2-chloroethyl vinyl ether

(CEVE) polymerizations to synthesize extremely well-defined

linear polymer precursors with polydispersities typically less

than 1.2 (Fig. 13).25 The polymerization was initiated from a

styrene functionalized vinyl ether (24) via the addition of

hydroiodic acid followed by a ZnCl2 catalyzed polymerization.

The iodo end-group (25) can be efficiently abstracted by the

addition of SnCl4 and the resultant terminal carbocation

coupled to the opposite styrenic end-group to form the

cyclized polymer with a stabilized benzylic cation. Addition

of an alkoxide to the cation yields a stable cyclic polymer ( 26).

Crude GPC traces confirmed the generation of significantamounts of the cyclic product and smaller amounts of an

intermolecular condensation product (B20%), which ranged

in molecular weights from 1 kDa to 3 kDa. The glass transi-

tion temperatures (T g) were also used as evidence for the cyclic

structure since cyclic polymers are predicted to demonstrate a

higher glass transition temperature when compared to their

linear counterparts. Since this initial publication, Deffieux and

coworkers have successfully carried out the synthesis of figure-

eight, tadpole, and theta-shaped poly(CEVE) polymers

through similar polymerization and coupling chemistry.

Rique-Lurbet et al . then extended this work to the living

anionic polymerization of styrene using the previously described

cyclization chemistry because of its demonstrated efficiency.26

Because styrene was used as the monomer, the styrenic end-

group had to be added post-polymerization in order to prevent

cross reactivity. This was achieved by the anionic polymeriza-

tion of styrene from a protected diethyl acetal followed by

quenching the reactive end-group first with 1,1-diphenylethene

followed by p-chloromethylstyrene. The iodo functionality

was then added to the diethyl acetal end-group by reaction

with trimethylsilyl iodide before employing the analogous

cyclization procedure with SnCl4 as described above. In the

case of styrene, molecular weights ranged from 2 kDa to

12 kDa and polydispersities for both the linear and cyclic

polymers were narrow (o1.2). This synthetic approach affords

efficient yields of high purity cyclic polymer (495%) without

tedious purification techniques such as fractionation or

preparative GPC. The ratio of the Mn’s (Mn,cyc/Mn,lin) calculated

by GPC, when calibrated against linear poly(styrene)

standards, was determined to be 0.85 which is in agreement

with measurements previously reported for cyclic poly(styrene)

prepared via bimolecular coupling. In addition, this techniquehas been amenable to the preparation of block copolymers

comprising of both poly(styrene) and poly(CEVE).

A variety of alternative coupling chemistries have been

successfully applied to polymers generated via living anionic

polymerization for ring-closure following the hetero-

difunctional approach. Using linear poly(styrene) precursors

containing a-geminal diethyl ether and o-diol functionalities,

Schappacher and Deffieux have successfully carried out an

acid-catalyzed transacetalization reaction under high dilution

to produce the analogous cyclic polymer with B85% yield.27

Schappacher and Deffieux also used another similar acetaliza-

tion reaction between the pendant alcohols and pendant vinyl

ethers on relatively short ‘‘A’’ and ‘‘C’’ blocks of an ABC

triblock copolymer to form extremely high molecular weight

cyclic poly(CEVE) (84 kDa). The poly(CEVE) ‘‘B’’ block was

then grafted with poly(styrene) chains to form cyclic polymer

brushes.28 Atomic force microscopy images of the cyclic

polymer brushes provided the first clear visual evidence of

the ring-shaped polymers, as well as identified smaller

amounts of the expected linear and tadpole by-products.

Kubo et al. prepared cyclic poly(styrenes) through the use of

a high efficient amide bond forming reaction.29 The authors

initiated the polymerization by reacting ortho-ester 27 with

lithium followed by polymerization of styrene and quenching

the active anion with 2,2,5,5-tetramethyl-1-(3-bromopropyl)-

1-aza-2,5-disilacyclopentane (28) to give the requisite linear

protected poly(styrene) precursor (29) (Fig. 14). Hydrolysisafforded the a-carboxylic acid and the o-amino functionalities

(30) which were coupled via activation of the carboxylic acid

with 1-methyl-2-chloropyridinium iodide (31) to give the cyclic

poly(styrene) (32). In subsequent publications, the amide

linkage was reduced to the amine in order to enable

Fig. 13 Cyclization of poly(CEVE) via the ring-closing addition to a

terminal carbocation used by Schappacher and Deffieux.25

Fig. 14 Cyclization via amide bond formation by activation of

carboxylic acid end-group performed by Kubo et al.29

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subsequent functionalization by the same activated carboxylic

acid approach to yield both tadpole and figure-eight

poly(styrene)s.

One elegant approach addressing the removal of linear

impurities was developed by the Semlyen group by growing

the linear precursors from a solid support.30 The 11-bromo-

undecanoate monomer was bound to a tetraalkyl ammonium

functionalized support via an electrostatic interaction.Polymerization of the monomer could occur, but polymer

product would only be released from the solid support

upon neutralization via cyclization. This approach however,

involves a step-growth mechanism which led to low degrees

of polymerization (DP = 5–15), and high polydispersities.

The authors used this method to synthesize cyclic polymers

of 11-undecanoic acid, however fractionation was still

necessary to produce pure cyclic materials with narrow

polydispersities.

Controlled radical polymerization (CRP) approaches,

such as nitroxide mediated polymerization (NMP), atom

transfer radical polymerization (ATRP), and reversible

addition-fragmentation chain transfer (RAFT) polymerization,

are particularly attractive for preparing cyclic polymers

because of the ease with which end-group functionality can

be modified as well as their low PDI and broad functional

group compatibility.

Lepoittevin et al. utilized NMP to form a linear polymer

precursor which contained an a-alcohol functionality from the

stable nitroxide radical (4-hydroxy-TEMPO) (33) and an

o-carboxylic acid from the initiator 4,4 0-azobis(4-cyanovaleric

acid) (34).31 Activation of carboxylic acid functionalized

polymer 35 following the procedure of Kubo et al.,29 enabled

the ring-closure to yield the macromolecular lactone 36

(Fig. 15). However, this procedure was only efficient for the

production of low molecular weight cyclic poly(styrene)

(o4 kDa) and yielded increasing amounts of oligomeric con-taminants in larger polymers. This was attributed to the

thermal instability of the nitroxide group present in the

backbone of the cyclic polymer.

A particularly versatile technique for the preparation of

cyclic polymers is the combination of controlled radical

polymerization and the 1,3-Huisgen dipolar cycloaddition

‘‘click’’ reaction to provide a highly efficient yet functional

group tolerant synthetic procedure. Laurent and Grayson first

reported the combined use of CRP and ‘‘click’’ during their

synthesis of cyclic polystyrenes by a combination of ATRP

and the ‘‘click’’ coupling of complementary azide and alkyne

end-groups.32 Polymerization from an alkyne functionalized

initiator provides a benzyl bromide end-functionalized (37)

polymer which can be efficiently transformed to the requisite

azide functionality (38) (Fig. 16). The polymer was subsequently

cyclized via a drop-wise addition of the polymer to a Cu(I)

catalyst to yield high purity cyclic polymers containing a

triazole linkage (39). Characterization by MALDI-TOF MS,

GPC, 1H NMR, and FT-IR suggests that with the appropriate

rate of addition, negligible amounts of linear oligomer are

produced foregoing the need for rigorous purification. Theversatility of ATRP has also enabled the preparation of

cyclic poly(N -isopropylacrylamide)33 as well as cyclic block

co-polymers consisting of poly(methyl acrylate-b-styrene).34

The combination of RAFT polymerization and the ‘‘click’’

cyclization also provides an efficient route to cyclic polymers.

Winnik and coworkers prepared cyclic poly(N -isopropylacryl-

amide) via RAFT initiation from an azide functionalized chain

transfer agent (40), followed by aminolysis of trithiocarbonate to

yield a thiol terminated polymer (Fig. 17).35 An alkyne

functionality was then incorporated by the Michael addition of

an a,b-unsaturated propargyl acrylate ester (41) to yield a

bisfunctional polymer with complimentary functionalities at

opposite ends (42). The ‘‘click’’ cyclization reaction was thenperformed by the in situ reduction of Cu(II)SO4 by sodium

ascorbate in aqueous solution under extremely dilute conditions

to afford cyclic poly(N -isopropylacrylamide) (43) with molecular

weights up to 19 kDa and excellent control over polydispersity

(o1.2). Goldmann et al. used an alternative end-group modifica-

tion approach to produce cyclic poly(styrene) from an azide

functionalized RAFT agent. The end-group was exchanged

by quenching with an excess of propargyl functionalized

azo-bis(4-cyano valeric acid). The ‘‘click’’ cyclization was then

performed using analogous procedures described above to yield

the corresponding cyclic poly(styrene).36

Fig. 15 Synthesis of cyclic poly(styrene) by Lepoittevin et al. via

activation of the terminal carboxylic acid and subsequent coupling

with pendant hydroxyl on the initiator.31

Fig. 16 Synthesis of cyclic poly(styrene) via a combination of ATRP

and ‘‘click’’ coupling by Laurent and Grayson.32


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2.2 Ring-expansion techniques

Ring-expansion polymerizations typically involve a catalyst orinitiator that yields a growing cyclic polymer chain, held

together by a relatively labile bond (e.g. organometallic or

electrostatic). Propagation by insertion of new monomer into

this weak bond is driven by thermodynamic factors, such as

ring strain in the monomer. The cyclic polymer will either

retain this initiating species, or in some cases, expel the catalyst

by a back-biting or ‘‘intramolecular chain transfer’’ reaction.

The critical advantage of the ring-expansion technique is

that high dilution is not required to yield cyclic polymers. As a

result this technique, when optimized, is amenable to larger

scale syntheses. Also, because the cyclic structure is maintained

throughout propagation, high molecular weight polymers can

be easily prepared without the entropic penalty associated withthe ‘‘ring-closure’’ approaches. In addition, the propagating

cyclic structure will prevent the formation of linear

by-products, as long as the monomer and initiator are

rigorously purified to remove any linear contaminants. The

significant complication of this approach is that the formation

of stable cyclic polymer is based on inherent rates of

polymerization, depolymerization, and back-biting; therefore,

the catalyst and reaction components must be carefully

selected to ensure high molecular weight, low polydispersity,

and (if desired) complete removal of the catalyst from the

cyclic product.

Kricheldorf and Lee carried out the initial work on ring-

expansion polymerization using lactide monomers and cyclic

tin oxide catalyst.37 Modifying their previous work on

generating linear polymers from the alkoxide ligands of tin

catalyst, the authors demonstrated the synthesis of cyclic

polymers using a cyclic tin initiator (Fig. 18). Initially, the

authors successfully polymerized both cyclic b-butyrolactone

and cyclic e-caprolactone using the cyclic tin oxide initiator

2,2-dibutyl-5,5-dimethyl-1,3-dioxa-2-stannane (44). In this

process, propagation occurs by the insertion of monomer into

the tin–oxygen bond to produce a ring-expanded macrocycle

(45). The resultant macrocyclic architectures were verified by

generation of the linear analogues via competitive ligand

exchange between the alkoxide ligands on the tin species

within the polymer macrocycle, and dimercaptoethane.

Characterization using GPC demonstrated a shorter retention

time for the linear by-product and 1H NMR verified theremoval of the tin initiator to yield hydroxyl end-groups.

Analogous techniques have been used by Kricheldorf and

co-workers to prepare a variety of complex cyclic polymer

topologies based on lactone and lactide monomers.38

One disadvantage of this approach is that the hydrolytically

labile tin oxide bond is present in the cyclic topology of the

product.

In order to stabilize the macrocyclic products, a number of

groups have developed techniques for removal of the tin

catalyst while retaining the cyclic topology. Kricheldorf and

co-workers demonstrated that a bis-functional phthalate

thioester can replace the tin oxide initiator after polymeriza-

tion to yield a more stable polymer macrocycle.39

Aring-insertion/ring-elimination process allows the esterifica-

tion of both of the alkoxide ligands on the tin catalyst to

afford the corresponding diester linkage without intermediate

ring-opening. An alternative macrocyclic stabilization

approach was developed by Li et al. in which a macrocyclic

block co-polymer was prepared from a similar cyclic tin initiator

(46), caprolactone and a short photo-crosslinkable lactone

block with acrylate side-chains (47).40 After preparing the

cyclic block copolymer (48) and UV crosslinking, the tin

oxide initiator can be purposely hydrolyzed while maintaining

the integrity of the macrocyclic structure (49) (Fig. 19). This

Fig. 17 Synthesis of cyclic poly(N -isopropylacrylamide) via RAFT

polymerization and subsequent ‘‘click’’ cyclization by Winnick and

co-workers.35

Fig. 18 First example of the ring-expansion polymerization via

b-butyrolactone insertion into a cyclic tin initiator by Kricheldorf

and co-workers.37

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procedure produced pure cyclic poly(e-caprolactone) with a

molecular weight of 24 kDa and 1.45 polydispersity. No

inter-macrocycle crosslinking was noted during the UV cross-

linking process, as long as the concentration remained below

2.8 Â 10À4M.

Another early example of ring-expansion polymerization

was provided by Shea and co-workers, utilizing insertion of

methylene units into the carbon–boron bonds of 50 by the

polyhomologation of dimethylsulfoxonium methylide.41

Thisprocess occurs by the attack of the methylidene on the electron

deficient boron center followed by a concerted migration of

the alkyl group adjacent to the borane to the methylide (51)

and release of dimethylsulfoxide. Because polyhomologation

can occur along any of the carbon–boron bonds, the sterically

bulky thexyl group was attached on the boron center to direct

methylene insertion only along the two less hindered

carbon–boron bonds, yielding cyclic poly(methylene)s (52)

(Fig. 20). In these studies, the molecular weights of

poly(methylene)s generated were below 2 kDa, but poly-

dispersity indices were also low (o1.2). Attempts to replace

the boron atom with a ketone functionality showed only

modest yields (30%).

Bielawski et al. demonstrated an elegant application of ring-

opening metathesis catalysts to enable cyclic ring-expansion by

use of a cyclic Ru catalyst (Fig. 21).42 In this process, the cyclic

Ru catalyst (53) inserts itself into the unsaturated alkene of

cyclooctene (54) to initiate the ring-expansion polymerization.

Propagation occurs through repetition of the Ru insertion

into additional monomer; however, the polymerization is

complicated by two competing reactions. As the monomer is

consumed, the likelihood of depolymerization increases.

Likewise, at low monomer concentration, the Ru catalyst

can undergo intramolecular chain transfer to regenerate thecyclic catalyst and an inactivated cyclic poly(octenamer) (55).

Initial reports described the synthesis of cyclic poly(octenamer)

with molecular weights up to 1200 kDa and polydispersities

typically around 2.0. Additionally, the authors showed similar

results in the cases of both 1,5-cyclooctadiene and 1,4,9-trans-

cis-trans-cyclododecatriene, producing cyclic poly(1,4-buta-

diene) with molecular weights of 2.3 kDa to 145 kDa.

Typically, polydispersities were lower for low molecular

weight polymers (Mn = 2.3 kDa, PDI = 1.6) and increased

with higher molecular weight polymers (Mn = 145 kDa,

PDI = 1.8). High molecular weights were noted in the order

of minutes; however, once a significant amount of monomer

had been consumed, back-biting reactions led to a rapiddecrease in molecular weight of the polymer as well as an

increase in polydispersity.43 This technique has yielded some

of the largest molecular weight cyclic polymers to date. With

an appropriately tuned catalyst, the rates of polymerization,

depolymerization, and intramolecular chain transfer can be

optimized to yield well-defined cyclic polymers. Recent

optimization by Boydston et al. investigated the role of the

size of the catalyst heterocycle as well as the electronics of the

N -heterocyclic carbene ligand (saturated vs. unsaturated) on

the Ru center.44 The inherent rates of propagation, intra-

molecular chain transfer, and catalyst stability were studied

Fig. 19 UV crosslinking by Je ´ rome and co-workers of the shortacrylate functionalized A block within a cyclic ‘‘ABA’’ triblock-

copolymer to stabilize the macrocycle and allow removal of the tin

initiator.40

Fig. 20 Polyhomologation by Shea and co-workers of cyclic boranesused to form cyclic poly(methylene).41

Fig. 21 Olefin metathesis by Grubbs and co-workers utilizing a cyclic

Ru-catalyst to produce macrocyclic poly(octene).42,44


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in the polymerization of cyclooctene and these studies suggest

that cyclic Ru complexes with 5- and 6-carbon heterocycles as

well as saturated N -heterocyclic carbene ligands to be the best

catalysts for balancing these kinetic factors and producing

pure cyclic poly(olefins).

Kudo et al. extended their previous synthesis of linear

poly(ethylenesulfide) polymers from a thiocarbamate initiator

to cyclic analogues using the ring-expansion of a cyclic

thiocarbamate initiator.45 Using the cyclic thiazolidine-2,4-

dione (56) as an initiator, phenoxypropylenesulfide (57) was

polymerized via a tetrabutylammonium chloride catalyst to

yield well-defined cyclic poly(sulfide)s (58) with polydispersities

around 1.3 for molecular weights below 8 kD (Fig. 22).

However, at higher molecular weights, polydispersities increased

substantially (B1.9).

Recently, Waymouth and co-workers have reported cyclic

polymers by utilizing N -heterocyclic carbenes to polymerize

both lactide and lactone monomers in the absence of the

alcohol initiators typically used for preparing linear

poly(esters).46 Ring-expansion polymerization occurs via

N -heterocyclic carbene (59) attack of the carbonyl of the

lactide yielding a zwitterionic active polymer chain (60). When

sufficient monomer has been consumed by the growing

polymer chain, the proximity of the alkoxide anion to the

imidazole cation favors eventual back-biting to yield a cyclicpoly(lactide) (61) while regenerating the N -heterocyclic

carbene catalyst (59) (Fig. 23). Through this technique, the

authors were able to synthesize extremely pure cyclic polymers

of both lactide and b-butyrolactone ranging in molecular

weights from 7 kDa to 26 kDa. In all cases, extremely narrow

polydisperse materials were produced (e.g. o1.3). As a result

of the electrostatic attraction, the monomer concentration

during polymerization could be relatively high (0.6 M)

while still yielding high purity cyclic polymers without linear

by-products.

Because controlled radical polymerizations involve a rever-

sible homolytic bond cleavage, polymerization, and then

radical recombination, this polymerization technique is alsoamenable to ring-expansion polymerization if a cyclic initiator

is used. In order to prevent the formation of linear impurities,

the active radicals on opposite chain-ends must recombine in

an intramolecular fashion after propagation to reform the

macrocycle. Pan and co-workers have demonstrated such a

system using a modified RAFT technique wherein 60Co g-rays

were used to induce polymerization of methyl acrylate from a

cyclic dithioester initiator (62) at a temperature (À30 1C) low

enough to prevent intermolecular recombination of active

radicals.47 This contrasts traditional RAFT polymerizations

which involve a thermal cleavage of the thioester which in turn

leads to diffusion of the chain ends and the potential for

intermolecular recombination of radicals. This g-ray technique

was used to prepare high purity cyclic poly(methyl acrylate)

(63) with molecular weights of 8 kDa and polydispersities

typically around 1.3 (Fig. 24). Because of the monomer

versatility of controlled radical polymerizations, block

co-polymerization of N -isopropylacrylamide and methyl

acrylate was also achieved to produce cyclic ABA block

co-polymers. This approach is particularly attractive because

it allows for a diverse number of olefin monomers includingacrylates, methacrylates, and styrenics and is amenable to a

range of side chain functionalities.

Fig. 22 Ring-expansion polymerization of thiiranes by Kudo et al .

from a cyclic thiocarbamate initiator.45

Fig. 23 Zwitterionic ring expansion polymerization of lactide usinga N -heterocyclic carbene catalyst performed by Waymouth and

co-workers.46

Fig. 24 Ring-expansion of a cyclic dithioester initiator used by Pan

et al. to produce cyclic poly(methyl acrylate).47

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3. Conclusion and future outlook

Significant advances in synthetic methodologies promise to

improve the availability of a wide-range of cyclic polymers for

further study. In particular, the application of controlled

radical polymerization techniques and highly efficient coupling

reactions, such as the Huisgen 1,3-dipolar cycloaddition,

enable the preparation of high purity cyclic polymers. While

success of this technique requires quantitative end-group

functionalization and highly dilute conditions, the approach

enables the incorporation of a broad diversity of monomer

and sidechain functionalities onto the cyclic product. On the

other hand, using catalytic ring-expansion techniques, if the

relative rates of each competing reaction (intiation, propagation,

and back-biting) are optimized, offers a scalable route to cyclic

macromolecules with high molecular weight as well as control

over polydispersity.

With further optimization of the above techniques, access to

high purity cyclic polymers should provide the required data

to further our understanding of the physical properties of

cyclic topologies, and polymers in general. In addition, the

inclusion of functionality along the backbone of cyclicpolymers provides access to a diversity of more complex

topologies. These cyclic substrates are of particular interest

where the attachment of functional moieties onto the cyclic

topology is expected to have a unique effect on their synergistic

relationship.

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Review on Making Polymer Macro Cycles

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