Cyclic polymers 2ed 2002 semlyen

Introduction: Cyclic Polymers - the first 40 yearsby J Anthony Semlyen, University of York, UK Circular DNA by Alexander V Vologodskii, New York University, USA Cyclic Peptides by John S

CYCLIC POLYMERS

CYCLIC POLYMERS

(Second edition)

Edited by

J ANTHONY SEMLYEN

University of York, U.K.

KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

Introduction: Cyclic Polymers - the first 40 years

by J Anthony Semlyen, University of York, UK

Circular DNA

by Alexander V Vologodskii, New York University, USA

Cyclic Peptides

by John S Davies, University of Wales, Swansea, UK

Cyclic Oligosaccharides and Polysaccharides

by Shinichi Kitamura, Kyoto Prefectural University, Japan

Cyclic Polysiloxanes

by Stephen J Clarson, University of Cincinnati, USA

Cyclic Oligomers of Polycarbonates and Polyesters

by Daniel J Brunelle, GE Corporate Research and Development,

Schenectady, NY, USA

Large Crown Ethers, Cyclic Polyethers and Cyclic Block Copolyethers

by Colin Booth and Colin Price, University of Manchester, UK

Large Cyclic Esters and Ether-Esters

by Barry R Wood and S Caroline Hamilton, University of York, UK

Cyclomer Technology for High Performance Polymers

by Yong Ding and Allan S Hay, McGill University, Canada

Organic Cyclic Polymers

by Jacques Roovers, National Research Council, Canada

Neutron Scattering and Nuclear Magnetic Resonance Investigations ofCyclic Polymers

by Peter C Griffiths, Cardiff University, UK

by Harry W Gibson and Eric J Mahan, Virginia Polytechnic Institute

and State University, USA

Oligomeric and Polymeric Catenanes

by David A Leigh and Richard A Smith, University of Warwick, UK

Cyclic Inorganic Oligomers and Polymers

by Ionel Haiduc, Universitatea “Babes-Bolyai”, Cluj-Napoca, Romania

Cyclisation and the Formation, Structure and Properties of Polymer

by Mustapha Benmouna, Institut de Physique et Chimie, Tlemcen,

Algeria and Ulrich Maschke, Université des Sciences et Technologies

de Lille, France

415561

601

699

741vi

The first edition of “Cyclic Polymers” was published by Elsevier AppliedScience Publishers in 1986 It consisted often chapters reviewing the advancesthat had been made in a new area of Polymer Science, where macromoleculesare based on large ring molecules rather than long chain molecules There havebeen important developments in the subject since then and this second edition of

“Cyclic Polymers” describes many of them in sixteen chapters dealing withboth synthetic and biological cyclic polymers

In this second edition, some of the developments in cyclic polymer chemistryover the past forty years are outlined in Chapter 1 and fundamental differencesbetween the properties of large ring molecules and long chain molecules arediscussed In the following three chapters, the importance of cyclic structures inbiological macromolecular science is strikingly demonstrated by detailedreviews of circular DNA, cyclic peptides and cyclic oligosaccharides andpolysaccharides The preparation and properties of synthetic cyclic polymersare covered in a series of chapters on cyclic polysiloxanes, large cyclic ethersand ether-esters and other organic cyclic polymers The importance of ring-opening polymerization reactions to produce commercially valuable linear

polymers is emphasised in chapters on cyclic polycarbonates and cyclic

polyesters and on cyclic oligomers used to prepare high performance polymers.Cyclic inorganic oligomers and polymers are described in a separate chapter Afundamental difference between large ring and long chain molecules is theability of the rings to form catenanes and rotaxanes and advances in both theseare described in separate chapters in this book Two techniques that have beenfound to be particularly useful for investigating synthetic cyclic polymers areneutron scattering and nuclear magnetic resonance spectroscopy and a chapter isdevoted to outlining the applicability of both these methods Another chapterdescribes the role of cyclization in network formation Finally, theoreticalaspects of cyclic polymer properties are reviewed in the last chapter of the book.Two aspects of the subject of large ring molecules and cyclic polymers areespecially noteworthy The first is its multidisciplinary nature, so that cyclicpolymers may be based on organic, inorganic or biological macromolecules andsubjects such as chemistry, physics, biology, materials science, engineering andcomputer science are all involved The second aspect results from the uniquetopology of large ring molecules, which has been exploited so effectively bynature (for example with circular DNA, which supercoils, catenates and formspermanent knots in different natural living systems)

vii

A most encouraging feature of this new, rapidly developing subject of cyclicpolymers has been the world-wide cooperation it has engendered The editor ofthis book alone has had the privilege of working with over 150 coworkers andcoauthors drawn from five continents The authors contributing to this bookcome from Algeria, Canada, England, France, Japan, Roumania, the UnitedStates of America and Wales Scientific research is one activity where co-operation rather than competition can result not only in greater materialachievement but also it can enhance international understanding

The editor of this book would like to thank all the authors who have contributed

to it so effectively, as well as his coworkers at York, Dr Barry Wood and DrCaroline Hamilton, for their dedicated help in its final preparation

With more and more large ring molecules and cyclic polymers being prepared,characterised and investigated year by year in biological as well as syntheticmacromolecular systems, the future of the subject of cyclic polymers seemswell assured as we advance into the twentyfirst century

J Anthony Semlyen

University of York 1999

Professor Alexander VologodskiiNew York University

Department of ChemistryNew York

NY 10003USA

Dr John S DaviesUniversity College SwanseaDepartment of ChemistrySingleton Park

Swansea

W Glamorgan SA2 8PPWales

Professor Shinichi KitamaraKyoto Prefectural UniversityDepartment of Agricultural Chemistry

606 ShimogamoKyoto

Japan

Dr Stephen J ClarsonCollege of Engineering

644 Baldwin HallUniversity of Cincinnati

OH 45221-0018USA

ix

Dr Colin Booth and Professor Colin PriceDepartment of Chemistry

University of ManchesterOxford Road

ManchesterM13 9PLUK

Dr Barry R Wood and Dr S Caroline HamiltonDepartment of Chemistry

University of YorkYork YO 10 5DDUK

Professor Allan S Hay and Dr Yong DingDepartment of Chemistry

McGill University

801 Sherbrook St WMontreal

PQ H3A 2K6CanadaProfessor Jacques RooversInstitute for Environmental ChemistryNational Research Council of CanadaOttawa K1A 0R6

Canada

Dr Peter C GriffithsSchool of Chemistry and Applied ChemistryUniversity of Wales Cardiff

PO Box 912Cardiff CF1 3TBWales

xiChapter 12

Dr David A Leigh and Mr Richard SmithDepartment of Chemistry

University of WarwickCoventry

CV4 7ALUKProfessor Ionel HaiducFacultatea de ChimieUniversitatea “Babes-Bolyai”

RO-3400Cluj-NapocaRoumaniaProfessor R F T Stepto and Dr David J R TaylorManchester Materials Science Centre

University of Manchester & UMISTGrosvenor Street

ManchesterM1 7HSUKProfessor Ulrich MaschkeLaboratoire de Chimie MacromoleculaireUA-CNRS No 351

Université des Sciences et Technologies de LilleF-59655 Villeneuve d’ Ascq Cedex

FranceProfessor Mustapha BenmounaUniversity Aboubakr BelkaidInstitute of Physics

Bel Horizon BP119

13000 TlemcenAlgeria

Contributors Biographies

Authors are listed in the order in which they appear in the book

J Anthony Semlyen was a Gibbs Scholar, Salters Scholar and Domus

Senior Scholar at Merton College, Oxford until 1962 and a Research Lecturer at Christ Church, Oxford in 1963 and 1967 He obtained his

MA, BSc and DPhil degrees at Oxford under the supervision of Dr Courtenay S G Phillips From 1964 to 1966, he was a Fulbright Scholar with Professor Paul J Flory at Stanford University, California Since

1967, he has been at the University of York He has edited Cyclic Polymers (Elsevier Applied Science, 1986), co-edited Siloxane Polymers (Prentice Hall, 1993) with Dr Stephen J Clarson and edited Large Ring Molecules (John Wiley & Sons, 1996) His research interests are large

ring molecules and cyclic polymers and he has published research papers, reviews and books on these topics with the active collaboration of over

150 co-workers and co-authors drawn from five continents

Alexander Vologodskii received his PhD in Molecular Biophysics at

Moscow Physical Technical Institute in 1975 He spent the next 16 years

in the Institute of Molecular Genetics, Moscow In 1985 he received the higher Russian scientific degree, Doctor of Science, at Moscow University He was a visiting scientist at University of California at Berkeley in 1992-1993 and now is Research Professor in the Department

of Chemistry at New York University His research interests include conformational properties of nucleic acids, particularly properties of

circular DNA molecules He authored the book Topology and Physics of Circular DNA (1992) and more than 70 research papers.

John S Davies received his BSc and PhD degrees from the University of

Wales, Swansea, before post-doctoral work with John C Sheehan at MIT, Cambridge Massachusetts He returned to Swansea as junior lecturer in

1964, joining the research team of Professor Cedric Hassall on peptide antibiotics and general peptidomimetic work This led to setting up his own research group on the synthesis/modifications of cyclic peptides and cyclodepsipeptides, carbohydrate derivatives on amino acids, and an interest in the chiral analysis of peptides He has reviewed annually for a number of years, the literature on cyclopeptides and cyclodepsipeptides for the Royal Society of Chemistry’s Specialist Periodical Reports on

Amino Acids, Peptides and Proteins He is currently the Senior Editor of

xiii

this Specialist Periodical Report, and Senior Lecturer in the Department

of Chemistry at Swansea.

Shinichi Kitamura received his PhD in Agricultural Chemistry from

Kyoto University in 1984 From 1985 to 1987 he was a Postdoctoral Fellow at Yale University with Professor Julian M Sturtevant He is now Lecturer in Chemistry of Biological Macromolecules at Kyoto Prefectural University His research interests include bioactive carbohydrates, protein-carbohydrate interactions, and biochemical applications of calorimetry.

Stephen J Clarson is a Yorkshireman by birth but currently resides in the

Cincinnati area with his wife, their two daughters, four Himalayan Persians and two Mini Lops He obtained his DPhil in Chemistry at the

University of York, England in 1985 where he studied with Dr Tony Semlyen He then spent the summer at the Institute of Macromolecular Chemistry, Prague before taking up a postdoctoral appointment with Professor James Mark In the spring of 1988 he joined the faculty of theDepartment of Materials Science and Engineering at the University ofCincinnati and is currently Assistant Dean for Educational Development

in the College of Engineering Dr Clarson has received numerous awards for his teaching including the Neil Wandmacher Excellence in Teaching

Award in 1993 for ‘Most Outstanding Teacher in the College of Engineering’, the TEXNIKOI Award in 1992 for ‘Outstanding Teaching

and Service to the College of Engineering’ and the Engineering Tribunal Award, for the ‘Outstanding Teacher of the Quarter’, Spring Term, 1992 and Spring Term, 1995 Dr Clarson has also published over one hundred

technical articles and co-authored the text ‘Siloxane Polymers’ with Dr

Tony Semylen, which was published in 1993 Due for publication in

1999 is the text ‘Silicones and Silicone-Modified Materials’ The research carried out in his group has led to a number of inventions and he holds three US patents His current scientific research interests include polymer synthesis, organosilicon chemistry, cardiovascular biomaterials and plasma polymerization.

Daniel J Brunelle received a BS degree from Emory University in 1970.

He received his MS and PhD degrees from The John Hopkins University

in 1972 and 1974 He then carried out postdoctoral research on total

synthesis and synthetic methodology with Professor E J Corey at Harvard University for two years After joining GE Corporate Research and Development in 1977, he began work on a number of organic and polymer-chemistry problems Specific areas of interest include synthesis and ring-opening polymerization of cyclic oligomers, high temperature phase transfer catalysis, mechanisms of catalysis, and new methods of

polymer formation, with special emphasis on polycarbonates, and

polyetherimides He is an inventor on over 75 patents, has published

more than 70 research papers, and was the editor of the volume “Ring

Opening Polymerization,” published in 1993.

Colin Booth completed his PhD in Chemistry under the direction of

Professor Geoffrey Gee at the University of Manchester in 1956 After

working with R L Scott at UCLA he spent four years with Shell Chemical

Co, Synthetic Rubber Division, California, before returning to Manchester as Research Fellow, subsequently to join the teaching staff.

His present position is Reader in Polymer Science He was editor, with

Colin Price of Volume 1 (Polymer Characterisation) and 2 (Polymer Properties) of Comprehensive Polymer Science (1989), reflecting his lifelong interest in this area His present research interests include the aqueous solution properties of block copolyethers (including the gel state), particularly the effect of block and chain architecture, and the solid state properties of uniform cyclic oligo(oxyethylene)s (large unsubstituted crown ethers) and of cyclic poly(oxyethylene)s.

Colin Price obtained his PhD in Chemistry from the University of

Manchester in 1964 He then joined the teaching staff at Manchester where he is now Professor of Polymer Chemistry and Head of the Department of Chemistry He has published some 200 papers on polymer

synthesis, characterisation and properties He is the joint editor of

Volumes 1 and 2 of Comprehensive Polymer Science For his work on

the microstructure, supramolecular structure and phase behaviour of block

copolymers in bulk and solution, and for studies on rubber elasticity, he received the 1991 RSC Award for Macromolecules and Polymers His present extensive programme of research on copolymers is carried out in collaboration with other members of the Manchester Polymer Centre, and

with groups in industry and other universities, both in the UK and abroad.

He is also engaged in research on new routes to aqueous polymer

dispersions including water borne zirconium ionomers.

Barry R Wood graduated in 1986 with a BSc (Hons) degree in Applied

Chemistry from Thames Polytechnic (now the University of Greenwich).

He obtained an MSc degree in Polymer Science at North London Polytechnic (now the University of North London) and a PhD degree

from Brunel University in 1990 working with Professor Michael Folkes

on electrically conducting colloidal dispersed gold-polymer composites.

He has spent the last nine years as a research fellow at the University of York working with Dr J A Semlyen preparing and studying large cyclic

esters and ether-esters.

S Caroline Hamilton gained her first degree in Chemistry at the

University of York and an MSc with distinction in Materials Technology from Napier University in Edinburgh She spent six months at BNFL, Springfields in the solid oxide fuel cell group She obtained her DPhil degree at the University of York, where she investigated large ring esters and ether-esters under the supervision of Dr Tony Semlyen Her hobbies include playing field hockey and computers With Dr Barry Wood, she runs a part-time computer business (“Ecky Thump Computers”).

Allan S Hay was born in Edmonton, Alberta, Canada Dr Hay received

a BSc and MSc in Chemistry from the University of Alberta (1950, 1952) In 1955 he received his PhD in Organic Chemistry from the University of Illinois and then joined the staff of the GE Research

$1 billion per year In 1968 he became manager of the Chemical

Laboratory in the Research and Development Center of GE In 1980 he

was appointed to the position of Research and Development Manager, Chemical Laboratories where he directed the work of 220 scientists and

engineers engaged in exploratory chemistry, process chemistry, chemical

engineering, polymer physics and engineering, electrochemistry, biotechnology, and electronic materials In September 1987, Dr Hay

accepted the GE/NSERC Industrial Research Chair of Polymer Chemistry

at McGill University Research on oxidative coupling chemistry has continued, however, the major focus of the research has been: (1),

synthesis of amorphous, thermooxidatively stable polymers with very

high glass transition temperatures which remain soluble and processable;

(2), the development of novel methods for crosslinking these soluble

polymers to make them insoluble, which makes them attractive as materials for use in the aerospace and electronic industries; (3), synthesis

of novel cyclic precursors to some of these high performance polymers,

which potentially makes them attractive as high temperature adhesives, coatings, and as matrix resins for advanced composites Dr Hay was

elected a Fellow of the Royal Society of London in 1981 and received the Society of Plastics Engineers International Award in 1975 In 1971 he

was elected a Coolidge Fellow of General Electric In 1984 he received

the Achievement Award of the Industrial Research Institute and in 1985 was made a Chemical Pioneer of the American Institute of Chemists He received the Carothers Award in 1985 In 1987 he was awarded an

honorary DSc degree from his alma mater, the University of Alberta In

1992 he was made an Honorary Professor, Dalian University of Science

and Technology, Dalian, China In 1997 he was awarded the Tomlinson Chair in Chemistry He is the author of more than 250 publications and

patents.

Yong Ding is a polymer scientist at Innovative Membrane Systems,

Praxair Inc, Norwood, MA He obtained his PhD degree in Polymer

Chemistry with Professor Allan S Hay from McGill University Then he moved to the University of Arizona, working with Professor H K HallJr

as a postdoctoral fellow His research activities include the synthesis, characterization and application of polymers, and oligomers.

Jacques Roovers received his PhD in Polymer Chemistry from the

University of Louvain with G Smets (1962) After postdoctoral work

with S Bywater in Ottawa and at the University of Louvain, he joined the National Research Council in Canada in 1966 In 1977-1978, he took a

sabbatical with W W Graessley at Northwestern University He did work

on anionic polymerization and is interested in the use of anionic

polymerization for the synthesis of model polymers with long-chain architectural features Emphasis is placed on the relation between the

large-scale structure of polymers and its effect on their dilute solution

properties and melt theology He is presently also adjunct professor at the

University of Athens.

Peter Griffiths obtained his BSc(Hons) degree in Chemistry from the

University College of North Wales in 1988 He then moved to Bristoland under the guidance of Professor T Cosgrove, undertook his PhDdegree in the area of polymer diffusion After postdoctoral periods in Bristol and then at the Royal Institute of Technology, Stockholm workingwith Professor P Stilbs, he took up his present position in Cardiff in 1985.His research interests include polymer solutions, polymer-surfactant interactions and the structure and dynamics of adsorbed polymer layers.

Harry W Gibson was born in Syracuse, NY, USA and grew up in the

Adirondack Mountains of northern New York State, close to the Canadian border and ca 60 miles from Montreal He received his BS (1962, with distinction) and PhD (1966) degrees in Chemistry from Clarkson University where his thesis under the direction of Professor Frank D Popp was based on alkaloid syntheses From 1965-66 he was a postdoctoral research associate of Professor Ernest L Eliel at the University of Notre Dame (IN), where he carried out stereochemical studies, primarily on acyclic molecules From 1966-69 he was employed as a research chemist

at Union Carbide Corporation in Tarrytown, NY, where he did research

on uses of formic acid and carried out kinetic and mechanistic studies ofreactions of alcohols with epoxides From 1969-1984 he advanced up the technical ladder to Senior Scientist at Xerox Corporation in Webster, NY and was involved in studies of liquid crystals, triboelectric charging, photoconduction and dark conduction and the synthesis of materials for

use in these technologies In 1984 he joined Signal Corporation in Des

Plaines, IL, which later merged to become Allied Signal, as Senior

Research Scientist; there he was concerned with polymers for printed

wiring boards and membranes In 1986 he assumed his present position

as Professor of Chemistry at VPI&SU Professor Gibson has published

more than 275 original research papers, chapters and reviews, is listed as inventor on 32 US patents and has delivered more man 145 invited lectures worldwide Currently his research interests include rotaxanes,

polyrotaxanes, hyperbranched and dendritic polymers, self assembly

processes, liquid crystalline materials and “living” radical

polymerizations During his 12 years in academia Professor Gibson has

supported and supervised the research of 26 undergraduates, 12 PhD, 8

MS graduates and 16 postdoctoral fellows in his laboratories Currently 1 undergraduate, 3 PhD students and 2 postdoctoral fellows work under his direction.

David Leigh carried out his PhD studies on novel macrocyclic

trichothecenes in the research group of J Fraser Stoddart at the University

of Sheffield from 1984-1987 He then spent two years investigating carbohydrate-protein interactions as a Research Associate with David Bundle at the National Research Council of Canada (NRC) laboratories in Ottawa before returning to the UK to a Lectureship at the University of Manchester Institute of Science and Technology (UMIST) in 1989 In

1996 he was promoted directly to Reader and in October 1998 hesimultaneously took up the Chair in Synthetic Chemistry and an EPSRC

Advanced Research Fellowship in the Centre for Supramolecular and Macromolecular Chemistry at the University of Warwick His research interests include novel molecular architectures and their applications in materials.

Richard A Smith obtained his BSc(Hons) degree in Chemistry from the

University of York in 1996 He men joined the research group of Professor David Leigh at the University of Manchester Institute of

Science and Technology (UMIST) to begin his PhD on developing a

synthetic route towards a “true” polycatenane ([n]catenane) In 1998 he

moved to the University of Warwick with Professor Leigh to complete his final year.

Ionel Haiduc is Professor at “Babes-Bolyai” University, in Cluj-Napoca,

Roumania He obtained his PhD in Moscow with Professor K A

Andrianov with a thesis in Organosilicon Chemistry, was a Fulbright Postdoctoral Fellow with Professor Henry Gilman at Iowa State University (1966-1968) and with Professor R Bruce King at the University of Georgia, Athens, Georgia (1971-1972) He was Visiting Professor at Instituto de Quimica, Universidad Nacional Autonoma de

Mexico (1993-1994), University of Texas at El Paso (1997) and

Universidad de Santiago de Compostela, Spain (1998) He received a

Humboldt Fellowship for a reasearch visit at Universitat Magdeburg, Germany (1997) and the Gauss Professorship of the Akademie der

Wissenschaften in Gottingen, Germany (1998) He also received visiting

grants from the National Science Foundation (USA, 1992), European Community (Spain 1993) and British Council (United Kingdom, 1995)

and a NATO Cooperative Research Grant (United Kingdom, 1997) He

authored or co-authored several books (including The Chemistry of Inorganic Ring Systems, 1970, The Chemistry of Inorganic Homo- and Heterocycles, 1987; Basic Organometallic Chemistry, 1985, Organometallics in Cancer Chemotherapy, 1989, 1990 and Supramolecular Organometallic Chemistry, 1998 in press) and more than

250 research papers and several chapters in some multi-authored books His interests cover inorganic ring systems, Main Group Organometallic and coordination chemistry, organophosphorus and organoarsenic ligands and Supramolecular Organometallic chemistry He participated in an extensive international collaboration with colleagues from United Kingdom, Germany, Spain, Mexico, Belgium, United States of America, Brazil, Canada, France, which resulted in numerous joint publications.

After the anti-communist revolution in Roumania (December 1989) he was elected and served as Rector (President) of “Babes-Bolyai”

University (1990-1993), and in 1998 was elected Vicepresident of the Roumanian Academy.

Robert Stepto obtained his BSc degree in Chemistry (with Special

Honours in Physical Chemistry) and his PhD degree studying the

Diffusion of Polymers in Solution from the University of Bristol He

moved to UMIST in 1961 as a Research Fellow in Polymer Crosslinking

in the Department of Polymer and Fibre Science He was subsequently

appointed to the teaching staff and eventually to Professor of Polymer

Science in the Materials Science Centre, UMIST He was awarded a DSc

by the University of Manchester in 1987 for his work on Statistical Studies in Polymer Science Robert Stepto’s research activities are in the area of the physical chemistry of polymers including polymerisation statistics, intramolecular reaction and gelation; the formation, structure and properties of polymer networks; polymer solutions and mixtures, covering diffusion, scattering, phase behaviour and intrinsic viscosity; computational studies of polymer chain behaviour; and the thermoplastic processing of starch and starch materials He has well over 200 research publications to his name and has recently edited a book on Polymer

Networks He received an Interphex Award for Innovation in production

in the Pharmaceutical Industry for his work on starch processing He is

currently the European Editor of Computational and Theoretical Polymer Science, Secretary of the Polymer Networks Group, Chairman of the

IUPAC Commission on Macromolecular Nomenclature and President of the Macromolecular Division of IUPAC.

Vice-David Taylor earned a BSc(Hons) in Polymer Science and Technology

(Chemistry) at the University of Manchester Institute of Science and Technology (UMIST) During a subsequent period of employment in the

plastics processing industry, David also continued his theoretical research into the molecular origins of elastomeric behaviour, eventually leading to

the award of an MSc degree in 1995 He then returned to UMIST as a Research Associate, and has since been engaged in theoretical studies of polymer network formation, structure and properties, under the guidance

of Professor Bob Stepto In 1996 David was a visiting scientist in the Polymer Group at Biosym/MSI (now Molecular Simulations Inc) in San Diego.

Ulrich Maschke studied Chemistry at the University of Mainz

(Germany) where he received his Diploma in 1989 working on

miscibilities of polymer mixtures In 1992 he finished his PhD thesis (Static and dynamic properties of polymer melts studied by neutron

scattering) at the Max-Planck-Institut für Polymerforschung in Mainz with Professor B Ewen and Professor E W Fischer In the fell of 1992 he

joined the French National Center of Scientific Research (CNRS) as a Research Associate (Chargé de Recherche) at the laboratory of macromolecular chemistry at the University of Lille, France His current research interests include the synthesis and analysis of materials

composed of liquid crystals and polymers (polymer dispersed liquid crystals) He is working together with Professor Mustapha Benmouna and his group of the University of Tlemcen (Algeria).

Mustapha Benmouna earned a BS degree in Physics at the University of

Algiers in 1969 and in Electrical Engineering in 1971 in Paris He

obtained a Master’s degree in 1977 and a PhD in 1979 at the University

of Michigan (Ann Arbor, USA) in 1979 with Professor Ziya A Akcasu In

1984, he earned a PhD in Physics at the University of Strasbourg (France) with Professor Henri Benoit working on the static and dynamic scattering properties of polymer mixtures and copolymers He joined the University

of Tlemcen in Algeria in 1981 as an associate professor and was

appointed full professor in 1986 He was visiting professor of several universities in Europe (University of East Anglia, England, 1986;

University of Konstanz, Germany, 1987; University of Strasbourg,

France, regularly from 1984 – to date, University of Lille, France, 1996

and 1998) and United States (University of Michigan, 1986; National

Institut of Standards and Technology, Gaithersburg, Maryland and the University of Maryland, 1995, Tulane University, New Orleans, 1995) Since 1990, he became a regular visiting professor of the Max-Planck- Institut für Polymerforschung (Mainz, Germany) collaborating with Professor E W Fischer, Professor Thomas A Vilgis, Professor Bernd

Ewen and Professor Adam Patkowski Recently, he joined the group of

Professor Kurt Kremer as a visiting professor Since 1995, he became

interested in the properties of polymer dispersed liquid crystals and

started a programme of collaboration between the University of Tlemcen

and the University of Lille collaborating with Dr Ulrich Maschke and Professor Xavier Coqueret This collaboration is supported by both the

CNRS in France and the Ministry of Education in Algeria.

CHAPTER 1 INTRODUCTION : CYCLIC POLYMERS - THE FIRST 40 YEARS

J Anthony Semlyen,

University of York, UK

1.1 Preparation and characterisation of cyclic oligomers

and polymers

1.1.1 Early investigations of large cyclics

Some 40 years ago in November 1959, the author was given a tutorialassignment on synthetic polymers His tutor at Merton College, Oxford, Dr

Courtenay Phillips recommended Paul Flory’s book“The Principles of Polymer

Chemistry”[1] The book describes how polymeric materials such as plastics,

rubbers and fibres are constructed from long chains of covalently bound atoms.The resultant long chain molecules may consist of tens of thousands or evenhundreds of thousands of atoms This was established by Hermann Staudinger

in his macromolecular hypothesis, despite the opposing views that polymericmaterials were either ring molecules or were colloidal aggregates of smallermolecules held together by secondary forces in micellar structures (see, for

example, Ref [1] [2]) While reading “The Principles of Polymer Chemistry”,

the author noticed that the largest well characterised ring molecules described inthe book had only 24 skeletal bonds Further reading showed that there was agap in chemistry waiting to be developed, namely the preparation,characterisation, investigation and possible application of large ring moleculeswith more than 30 skeletal bonds as well as synthetic cyclic polymers with morethan 100 skeletal bonds The author’s main aim in chemistry since that time was

to try and establish this new subject area both in collaboration with andalongside other research chemists in this general area The first edition of

“Cyclic Polymers ” was published some 27 years later in 1986 [3] In the book,

14 authors described the advances that had been made in the chemistry of bothsynthetic and biological cyclic polymers

The subject of cyclic polymers effectively began with a paper by Jacoband Wollman [4] about 40 years ago in 1958, who concluded that the geneticmap of bacterial chromosomes of Escherichia coli showed circularity Furtherevidence for the existence of circular DNA was soon established [5] [6] and finalconfirmation came from electron microscopy of øX174 DNA [7] (see chapter 7

in Ref [3]) The discovery of large cyclic molecules in natural biologicalsystems is described in Ref [3] as well as in a more recent book [8] in chapters

by Andrzej Stasiak, Harold Scheraga and David Brant and also in this book

J A Semlyen (ed.), Cyclic Polymers, Second Edition, 1–46.

© 2000 Kluwer Academic Publishers Printed in the Netherlands.

1

At the time when the first papers on circular DNA molecules were beingpublished, there was an upper limit of about 30 skeletal bonds for wellcharacterised large synthetic ring molecules The pioneering researches ofWallace Carothers on high polymeric substances is described in his collectedpapers [9] They show how important new polymers including linear polyamidesand linear polyesters were prepared and characterised In addition, a number oflarge ring molecules were prepared for the first time such as many-memberedcyclic esters and anhydrides An example of the largest is the macrocyclic ester,decamethylene octadecanedioate (Figure 1), which has 30 skeletal bonds

Figure 1 - The chemical structure and model of decamethylene octadecanedioate.

In 1962, a review of the stereochemistry of many-membered rings waspublished [10] describing a wide range of large ring molecules including a 30-membered macrocyclic polyene [11] (Figure 2) and the first catenane with itsinterlocked ring system [12] (Figure 3)

Despite the important advances that had been made in the chemistry oflarge organic ring molecules following the characterisation of benzene as the firstring compound in 1865 (Figure 4), it was developments in siloxane andpolysiloxane chemistry and in chromatographic techniques that resulted in thepreparation of narrow molar mass fractions of the first cyclic polymer, allowingfor detailed investigations of its properties

Figure 2 - The chemical structure and molecular model of a 30 membered macrocyclic

polyene

Figure 3 - The chemical structure and molecular model of the first catenane.

Figure 4 - The chemical structure and molecular model of the benzene ring.

1.1.2 Large rings in polysiloxane systems

The first report on the production of ring and chain species inpoly(dimethylsiloxane) (PDMS) ring-chain equilibrates was made by Scott [13]

in 1946 and in 1962 Hartung and Camiolo [14] published measured values forthe concentrations of the cyclics (with ) in PDMSequilibration reactions in solution in xylene Carmichael and his coworkers[15][16] extended these studies and were able to analyse cyclicdimethylsiloxanes with values of using gas-liquid chromatography Allthese investigations culminated in the work of Brown and Slusarczuk [17] whocarried out PDMS ring-chain equilibration reactions in the bulk and in solution

in toluene In their base catalysed solution reaction, the siloxane concentrationwas and the temperature was 383 K

Under the conditions used by Brown and Slusarczuk nearly all thematerial obtained is cyclic, with only small amounts of high molar mass chainsand the latter are well separated from the cyclic species Furthermore, for thisreaction, in principle the individual cyclic concentrations produced should becalculable by application of the Jacobson-Stockmayer theory [18] and thedistribution of linear species by the Flory relationships for linear condensationpolymers (see Ref 1) Thus, the Brown and Slusarczuk reaction in toluene gave

an opportunity for the preparation of narrow molar mass fractions of cyclicpolymers with an average up to 1000 skeletal bonds, provided that suitableanalytical and preparative techniques were available From the point of view ofcharacterisation, it would also be an advantage to calculate the individualconcentrations of cyclic and linear species in such equilibrates and this was alsoundertaken [19]

Early experiments on gas-liquid chromatography (GLC) were carried out

by James and Martin [20] and the first book on the subject of gaschromatography was written by Courtenay Phillips [21] The technique became

so well developed that it could be used to analyse cyclics with up

to 100 skeletal bonds, when OV17 from the Field Instrument Company is used

as the stationary liquid phase and Embacel is used as the solid support [22] Anexample of the GLC analysis of a cyclic fraction is shown in Figure 5

at York was designed and constructed by David Sympson [25] The results of atypical fractionation are shown in Figure 6 and the dispersities of the individualfractions were all in the range In the higher molar massfractions obtained by preparative GPC, appreciable amounts of linear species arepresent but the rings can be separated from the chains very effectively, asdescribed in Ref [25], Once a whole range of narrow fractions of PDMS cyclicshad been obtained, these first cyclic polymers could be characterised and theirproperties investigated This was done in collaboration with 20 other Universityresearch groups and with Dow Corning Ltd., resulting in a range of publications(see, for example, Ref [26] [27]) Some of these properties will be describedlater in the chapter.

Figure 6 - Analytical gel permeation chromatography of the individual fractions of cyclic

dimethyl siloxanes obtained by preparative GPC The number average number of skeletal

bonds in the fractions range from 40 to 1270.

Figure 7 - Molecular model representation of a cyclic with 100

skeletal bonds.

Cyclic dimethylsiloxanes can also be conveniently produced in cationicpolymerizations as have been described by JulianChojnowski [28] [29] Thepolymerization of the cyclic trimer with trifluoromethanesulphonicacid (‘triflic’ acid) in solution in n-heptane at concentrations of 40wt%may be kinetically or thermodynamically controlled Chains corresponding tothe cyclics are found to obey Gaussian statistics in solution for n > 6,resulting in gradients close to the theoretical values of -1.5 and -2.5 in the logversus log 3n plots for the kinetically and thermodynamically controlledreactions respectively [29] Other cyclic polysiloxanes have been prepared atYork using ring-chain equilibration reactions These systems have included the

[22], [30] and [31] [32])

Recently, a range of per-deuterated cyclic dimethylsiloxaneshave been obtained in our laboratory by a modification andextension of a preparation described by Beltzung and coworkers [33] Thereaction scheme is as follows [34] [35]:

The large cyclics were fractionated by preparative GPC, giving the firstexamples of fully deuterated cyclic polymers The fractions are suitable forfundamental conformational and topological investigations by, for example,neutron scattering and field gradient nuclear magnetic resonance spectroscopy.They were characterised by infrared (IR), nuclear magnetic resonance (NMR)and mass spectroscopic techniques [34] [35]

Following the preparation and characterisation of linear block copolymers ofpolystyrene (PS) and PDMS by several groups (see, for example, Ref [36] [37]),Frank Jones set out to prepare the corresponding cyclic block copolymers in theauthor’s laboratory [38] Taking advantage of the reactive nature of the PDMSblocks, the linear block copolymers were used for ring-chain equilibrationreactions in toluene solution following methods used by Peter Wright for PDMSalone [39] Following solution fractionation and GPC and NMR spectroscopicanalysis, Jones concluded that one of the fractions was cyclic in nature.Reference [40] describes a recent synthesis of macrocyclic PS-PDMS blockcopolymers

1.1.3 Preparation of some organic cyclic polymers

The preparation and characterisation of cyclic PDMS was followed byreports in 1980 of the preparation of cyclic polystyrene by Geiser and Höcker[41] [42], who polymerized styrene using sodium naphthalene as the initiator andtetrahydropyran as the solvent Intramolecular cyclization of the resultant

‘living’ polymers was carried out using Chains in theproduct were separated from the cyclic material by reacting the former with highmolar mass ‘living’ polystyrene in intermolecular reactions Finally, the ringpolystyrenes were isolated by fractionation giving cyclic fractions with number-average molar masses up to ca 24000 Variations in the preparation of cyclicpolystyrene material by a number of different authors (see, for example, Ref.[43] [44]) have been reviewed by Helmut Keul and Hartwig Höcker [45] Some

of the problems associated with the preparation and investigation of high molarmass cyclic polystyrene have been discussed by McKenna and hiscoworkers[46] In recent years, a range of different organic cyclic polymers have beenobtained by a variety of different experimental approaches [45] These includecyclic alkanes with up to 300 skeletal bonds [47] [48], cyclicpolybutadiene [49] and cyclic poly (2-chloroethyl vinyl ether) [50] Otherorganic cyclic oligomers and polymers are reviewed in detail in this book andsome are now commercially available, including cyclic polystyrene and cyclicpoly(methylmethacrylate)

1.1.4 Characterisation of cyclic oligomers

Large ring compounds can be produced in many chemical reactions but

it was only when powerful investigative techniques were developed that itbecame possible to characterise large cyclics and establish their identities Thesetechniques include chromatographic methods such as GLC, GPC and HPLC(high performance liquid chromatography) as well as spectroscopic methodsincluding IR, NMR and several types of mass spectroscopy Most of thesetechniques are now used routinely in chemical research Their power andsophistication have increased steadily over the past 40 years and this has proved

a major factor in the continuing preparation and characterisation of new largering compounds and cyclic polymers Some examples of how these techniquesare applied will now be given In a recent investigation of the preparation andcharacterisation of mixtures of cyclic oligomers ofpoly(butylene terephthalate)(PBT), a number of state-of-the-art techniques were applied to the cyclics

where [51] These cyclics were prepared bysolution ring-chain reactions of PBT in 1, 2-dichlorobenzene usingdibutyl tinoxide as the catalyst The largest rings observed by the analytical techniques had

Figure 8 shows a GPC of the PBT cyclic oligomers that were recoveredfrom the ring-chain reaction by filtering off the insoluble linear polymer and thenremoving the solvent [51] The columns used for the separation were four PL-gelmixed-E columns supplied by Polymer Laboratories Ltd By contrast withthe SX-1 Bio-bead columns (obtained from Biorad Laboratories) used in muchearlier investigations [52][53], the new mixed E-columns allow GPC analysis toproceed much faster as well as producing better resolution In thepoly(decamethylene adipate) system, such columns have been used to analyseindividual cyclic oligomers up to 200 skeletal bonds [54]

uRID-6A refractive Index detector [51].

An alternative chromatographic technique to GPC is HPLC and Figure 9shows the trace obtained for the PBT cyclic oligomers prepared as describedabove [51] The HPLC column was a spherisorb S3 0DS2 column with a twosolvent eluent: water (0.1% ethanoic acid) and acetonitrile (0.1% ethanoic acid)

at a flow rate of The instrument was fitted with an ultravioletdetector functioning at a wavelength of 240 nm When analysing a mixture ofcyclic oligomers it is important to establish the identity of the first member of theseries

In the case of the PBT system, the cyclic dimer was isolated using

column chromatography and its identity confirmed by X-ray crystallography [51]

(see Figure 10)

Figure 10 - Representation of the crystal structure of the cyclic dimer

There are a range of spectroscopic techniques available for

characterising cyclic oligomer populations, including NMR and mass

spectroscopy For the PBT rings, two examples of mass spectra are shown in

Figure 11 and Figure 12 [51] Liquid chromatography tandem mass

spectroscopy and 500 MHz NMR spectroscopy are particularly useful for

characterising mixtures of cyclic oligomers

Figure 11 - Fast atom bombardment mass spectrum of cyclic PBT oligomers showing

spectral lines (M+) corresponding to the exact multiples of the repeat unit

Figure 12 - Matrix assisted laser desorption ionization mass spectrum of cyclic PBT

species are also present in the sample.

1.1.5 Cyclics produced in ring-chain equilibration reactions

Between 1969 and 1976, the author and his coworkers published a series

of 14 papers describing large cyclics produced by ring-chain equilibrationreactions [55] Equilibrium cyclic concentrations were then related to thestatistical conformations of the corresponding open chain molecules as described

in section 1.3 Following the preparation of cyclic siloxanes (asdescribed above), an example of a family of cyclic paraffin-siloxanes wasinvestigated [56] by GLC and GPC The equilibrium concentrations of cyclics

with were determined in this way Thestudy illustrated that the conformational characteristics of many chain molecules(in this case short paraffin chains) can be investigated provided that suitablelabile groups are incorporated into the molecular structure (in this case, dimethylsiloxane units)

Monomer-polymer equilibria can be set up in polyether systems (see, forexample, Ref [57] [58]) Poly(tetrahydrofuran) monomer-polymer equilibriawere found to produce no cyclic oligomers under the conditions used in Ref.[59] By contrast, a full range of cyclics with werefound to be present in monomer-polymer equilibrates of poly(1, 3-dioxolane)[60] Bulk and solution polymerisations were carried out by adding borontrifluoride diethyl etherate to the monomer (1,3-dioxolane) alone or to themonomer in solution in dichloroethane The equilibrates were quenched indiethylamine and the cyclics extracted and analysed by GLC, IR, NMR and massspectroscopy

Following earlier investigations [61], the concentrations of the cyclics

with were determined for melt equilibrates of nylon.6[62] The cyclic oligomers were analysed by GPC using sephadex columns fromPharmacia Ltd and glacial ethanoic acid/water as the solvent

Equilibrium ring concentrations were also measured in two aliphaticpolyester [53] and one aromatic polyester system [52] some 25 years ago.Further studies have been carried out more recently (see, for example, Ref [54][63] [64])

Potentially, there is a huge range of large ring compounds that could beprepared in polyether, polyamide and polyester systems Some of these cyclicscould be obtained as pure compounds by preparative GPC or preparative HPLC

It is expected that in the years ahead many new large organic ring compoundswill be made available to chemists in this way

Ring-chain equilibria can also be set up in purely inorganic systems andtwo examples are sodium phosphate melts and liquid sulphur Sodiumphosphate (known as Graham’s salt [65]) may be prepared by heatingsodium dihydrogen phosphate at high temperatures and then rapidly quenchingthe molten polymer [66] The water-soluble product consists of long linearpolyphospbate chains terminated by hydroxyl groups [67] together with 10%w/w cyclics with The equilibrium concentrations of cyclics insuch melts have been measured using paper chromatography as the mainanalytical technique [68] [69] [70]

It is well established that ring-chain equilibria can be set up in liquidsulphur [71] [72] Above a critical transition temperature of 432K, polymericsulphur chains are formed in the molten element [71] Below this temperature,liquid sulphur consists of cyclooctasulphur and a mixture of other cyclics(called Using IR and Raman spectroscopy and HPLC, Steudel and hiscoworkers [73] [74] have established the main constituents of at 388K It is amixture of cyclics containing the small rings and (as ca 3.5% by weight)and ca 1.5% of larger cyclics with x > 8; the remaining 95% by weight beingcyclooctasulphur [73] It is that gives rise to the well-known self-depression

of the freezing point of elemental sulphur discovered by Gernez [75] Although

it was shown that large rings with at least 34 atoms are present at equilibrium inliquid sulphur below 432K, only three have so far been isolated from the moltenelement and these are [76] [77], [77] and [77] From his detailedinvestigations of the unique properties of liquid sulphur, Steudel [74] hasconcluded that the theory of Tobolsky and Eisenberg [72] does not provide a fullexplanation of the behaviour of the element on heating to itsboiling-point Bondrearrangements involving the formation of four-centre transition states are wellknown in inorganic chemistry (see, for example,Ref [78] [79] [80] [81] [82][83]) Such bond interchange reactions in liquid sulphur could lead to ring-ringequilibria in addition to the ring-chain equilibria Further detailed experimentalstudies of this fascinating element would be most welcome (see Section 1.3)

Some of the results of the experimental investigations of cyclicconcentrations in polymeric ring-chain equilibrates outlined in this section will

be discussed from a theoretical point of view in Section 1.3 This describes thecalculation of equilibrium ring concentrations in a range of systems in terms ofthe statistical conformations of the corresponding open chains

1.1.6 Alternative routes to cyclic oligomers

In the first edition of Cyclic Polymers [3], the author reviewed three

methods for preparing mesocyclics that were established more than 50 years ago

The first is the dilution method of Ruggli [84], in which the reactionsystem is diluted so as to favour kinetically-controlled ring formation Anexample is the Thorpe-Ziegler reaction [85] [86]

A second method is the thermal depolymerization route of Carothers andhis coworkers [9] For example, cyclics with up to 30 skeletal bonds wereobtained by heating polymeric esters at 270 °C and 1 mmHg using a tin catalyst

A third method developed by Stoll and Rouvé [87] is believed to involvecyclization on a surface, such as the acyloin condensation on sodium metal

Over the past 40 years, a wide range of cyclic oligomers have beenprepared in kinetically-controlled reactions Some examples are as follows: largecyclic carbonates [88] [89], large cyclic ethers [90] [91], large cyclic esters [51][92] [93] [94] [95] [96], large cyclic ether-esters [97] [98] and macrocyclicoligomers as intermediates for the preparation of high performance polymers[99] [100] Many of these cyclics will be discussed in detail in this book

Cyclic structures are of great importance in biological chemistry as hasbeen emphasised in books previously edited by the author [3] [8] Merrifieldwas awarded a Nobel prize for his synthetic methods in polypeptide chemistry[101], which can be used to synthesise cyclic peptides on solid-phase polymericsupports, that are usually cross-linked polymer gels Mutter [102] has discussedthe influence of peptide residue sequences on polypeptide cyclization reactions,

as described by Alan Tonelli in Ref [3] Synthetic procedures for preparingcyclic oligo-and poly-saccharides are beginning to be established (see, forexample, Ref [103]) An example of a DNA cyclization is phage studied

by Wang and Davidson [104] [105] Many more examples of large ringmolecules and cyclic polymers in biological systems will be described in thisbook

(i) Cyclic oligomers and cyclic polymers have no end-groups, so they can berepresented by simple structural formulae and names For example, cyclicpoly(dimethylsiloxane) is an exact name, whereas in the case of linearpoly(dimethylsiloxane) end-groups should be specified The properties oflinear polymers can be markedly affected by the nature of their end-groups, for example with linear poly(dimethylsiloxane) the surfaceproperties can be different depending on whether the end-groups are

or HO- For some applications, it may be an advantage to have

no end-groups in the polymeric material In this connection, many forms

of DNA are circular [3] [8], thus preventing any possible reactions orinteractions with chain ends Two examples are shown in Figure 13 [131][132] and Figure 14 [132] [133]

Figure 13 - An electron micrograph of a plasmid, which is a circular molecule of DNA about 3µm in circumference It exists apart from the chromosome in a bacterium and replicates on

its own [131].

Figure 14 - An electron micrograph of three closed loops of DNA, each of which is a double helix One strand of each loop is the natural single strand DNA of the bacterial virus

and each loop contains about 5500 base pairs [133].

There are examples of cyclic polypeptides and loops in proteins [3] [8] andrecently large ring molecules have been observed in some polysaccharidesystems, as shown in Figure 15 [134]

Figure 15 - An electron micrograph of a fully ordered cyclic structure of iotacarrageenan.

Figure 16 - Cleavage of a single skeletal bond in a cyclic and linear molecule.

(iii) Skeletal bond interchange reactions of cyclics and linears (as discussed inRef [82] [83]) result in very different consequences, as illustrated inFigure 17 Intramolecular bond interchange of a single ring moleculeresults in two smaller rings, whereas intermolecular bond interchangereactions between two ring molecules results in the formation of one largerring Bond interchange reactions between two long chain moleculesalways result in no change in the number of chemical species

Figure 17 - Skeletal bond Interchange reactions of cyclic and linear molecules.

(iv) The linking together of long chain molecules with reactive groups alongtheir length can result in a whole range of chemical species, whereas bondformation between two of the corresponding cyclic molecules can giveonly a single dimer (as illustrated in Figure 18)

Figure 18 - Linking reactions of linear and cyclic molecules (see text).

(v) Ring-opening polymerization (ROP) reactions are an important application

of large ring molecules The process is illustrated in Figure 19 This showsthe opening of a large cyclic to form the corresponding linear polymer,which in turn reacts with another large cyclic to produce a long, linear chain.Such reactions have been investigated in detail and have found applications,for example, in polycarbonate andpolyester chemistry [88] [89] [136] Theyare proving to be a useful route to very high molar mass linear polymers

Figure 19 - An illustration of the ring opening polymerisation process.

(vi) Differences between the bulk viscosities of large rings and long chains areshown in Figure 20 for the PDMS system [137] [138] Smaller rings aremore viscous than shorter chains and the reverse is the case for the largestspecies studied The cross-over occurs at about 100 skeletal bonds Thelower bulk viscosities of cyclics above the critical molar mass forentanglements are contrary to the expectations of reptation theory [139][140], but may result from fewer entanglements between cyclic molecules

as opposed to linear molecules in the melt (as illustrated in Figure 20).There are also marked differences between the activation energies forviscous flow between ring and chain PDMS species [137]

Figure 20 - The plot shows the logarithm of the bulk viscosity as a function of the logarithm of the number-average number of skeletal bonds for the PDMS system for

rings (—) and chains (- - -) [138].

(vii) Another consequence of the different topologies of ring and chainmolecules is that mixtures of cyclic and linear polymers can have higherbulk viscosities than either component This is illustrated in Figure 21 forPDMS blends The effect is believed to be the result of some threading oflarge rings by long chains creating temporary cross-links in the melt [138]

It has also been observed in polystyrene systems [141]

Figure 21 - Ratio (%) of PDMS ring and chain bulk viscosities (experimental values divided by values calculated assuming that no threading occurs) for blends shown as a

function of the number average number of skeletal bonds [141].

(viii) Topological trapping of large cyclics in networks was outlined in Section1.1.7(c) When PDMS networks are formed in the presence of largePDMS rings, some of the latter become topologically entrapped as shown

in Figure 22 [126] Increasing quantities of cyclics are entrapped as thesizes of the rings increase, so that 95% of cyclics with 500 skeletal bondsare incorporated into the network without the formation of chemicalbonds

(ix) Catenation is a possibility for large ring molecules and cyclic polymers.The first example of a man-made catenane is depicted in Figure 3 [12].The chemistry and topology of catenanes has been discussed previously bySchill [143] and by Jean-Pierre Sauvage and his coworkers in Chapter 5 ofRef 8 A poly[2] catenane with topological bonds in a polymer mainchain has been reported recently [144] Figure 23 shows an electronmicrograph of catenated DNA molecules of a virus, illustrating how naturecan catenate biological macromolecules No comparable structures arepossible for chain molecules