Table Of ContentAdvances in Polymer Science
Fortschritte red Hochpolymeren-Forschung
Volume 91
Edited by
H.-J. CANTOW, Freiburg i. Br. • G. DALL'AsTA, Cesano Maderno • J. D. ,YRREF
Madison • H. FUJITA, Osaka. M. GORDON, Colchester. W. KERN, Mainz
.G ,ATTAN Milano • S. ,ARUMAKO Kyoto • C. G. ,REGREBREVO Ann Arbor
G. V. ScHuLz, Mainz • W. P. SLICHTER, Murray Hill • A. ,J.N AMREVATS Leiden
J. K. STILLE, Iowa City
With 37 Figures
Springer-Verlag Berlin Heidelberg New York 1975
Editors
Prof. Dr. CANTOW, HANS-JOACHIr~ Institut fiir Makromolekulare Chemie der Universit~it, 7800 Frei-
burg i. Br., Stefan-Meier-Str. 31, BRD
Prof. Dr. GINO DALL'ASTA, SNIA VISCOSA--Centro Sperimentale, Cesano Maderno (MI), Italia.
Prof. Dr. JOHN D. FERRY, Department of Chemistry, The University of Wisconsin, Madison 6,
Wisconsin 53706, U.S.A.
Prof. Dr. HmOSHI FUJITA, Osaka University, Department of Polymer Science, Toyonaka, Osaka, Japan
Prof. Dr. MANFRED GORDON, University of Essex, Department of Chemistry, Wivenhoe Park,
Colchester C04 3 SQ, England
Prof. Dr. KERN, WEANER Institut fiir Organische Chemie der Universit/it, 6500 Mainz, BRD
Prof. Dr. Giuuo NATTA, lstituto di Chimica lndustriale del Politecnico, Milano, Italia
Prof. Dr. OKAMURA, SEIZO Department of Polymer Chemistry, Kyoto University, Kyoto, Japan
Prof. Dr. SELRAHC G. ,REGREBREVO The University of Michigan, Department of Chemistry, Ann Arbor,
Michigan 48 104, U.S.A.
Prof. Dr. GONTER SCHULZ, VlCTOR Institut far Physikalische Chemie der Universitiit, 6500 Mainz,
BRD
Dr. WILLIAM P. SLICHTER, Bell Telephone Laboratories Incorporated, Chemical Physics Research
Department, Murray Hill, New Jersey 07971, U.S.A.
Prof. Dr. JAN ALBERT ,NAMREVATS Chem. Laboratoria der Rijks-Universiteit, afd. Fysische Chemie I,
Wassenaarseweg, Postbus 75, Leiden, Nederland
Prof. Dr. JOHN K. ,ELLITS University of Iowa, Department of Chemistry, Iowa City, U.S.A.
ISBN 3-540-07460-0 Springer-Verlag Berlin Heidelberg New York
ISBN 0-387-07460-0 Springer-Verlag New York Heidelberg Berlin
Library of Congress Catalog Card Number 61-642
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Contents
Stable Organic Cation Salts: Ion Pair Equilibria and Use in
Cationic Polymerization
YNOHTNA HTIWDEL and DIVAD .C NOTGNIRREHS
The Cationic Isomerization Polymerization of
3-Methyl-l-butene and 4-Methyl-l-pentene
HPESOJ P. YDENNEK and NHOJ .E NOTSNHOJ 75
Grafting on Polyamides
ASIOLE .B ONAM and ADNANREF M. .B OHNITUOC 79
Rigid Rods and the Characterization of Polyisocyanides
KNARF HCILLIM 711
Stable Organic Cation Salts:
Ion Pair Equilibria and use in Cationic Polymerisation
A. LEDWITH
Donnan Laboratories, University of Liverpool, Liverpool L69 3BX, England
D. C. SHERRINGTON
Department of Pure and Applied Chemistry,
University of Strathclyde, Glasgow G1 1XL, Scotland
Table of Contents
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
II. The Nature and Properties of Stable Organic Cation Salts . . . . . . . . . . 3
A. Stabilisation of Organic Cations . . . . . . . . . . . . . . . . . . . . 3
B. General Value in Polymerisation Systems . . . . . . . . . . . . . . . . 4
C. Conductance and Ion Pair Equilibria . . . . . . . . . . . . . . . . . . 6
C.1. Theoretical Considerations . . . . . . . . . . . . . . . . . . . . 6
C.2. Relevant Results . . . . . . . . . . . . . . . . . . . . . . . . . 11
III. Application of Stable Organic Cations to Kinetic Studies of Vinyl Polymerisations. 81
A. General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
B. N-Vinylcarbazole and Related Monomers . . . . . . . . . . . . . . . . 21
C. Alkyl Vinyl Ethers . . . . . . . . . . . . . . . . . . . . . . . . . . 23
D. p-Methoxystyrene and Cyclopentadiene . . . . . . . . . . . . . . . . . 25
E. o- Divinylbenzene . . . . . . . . . . . . . . . . . . . . . . . . . . 27
IV. Application of Stable Organic Cations to Kinetic Studies of Ring Opening Poly-
merisations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
A. General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
B. Tetrahydrofuran . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
C. Thietans and Thiiranes (Cyclic Sulphides) . . . . . . . . . . . . . . . . 33
D. Azetidines and Aziridines . . . . . . . . . . . . . . . . . . . . . . . 36
E. Cyclic Acetais . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
F. Oxazolines and Oxazines . . . . . . . . . . . . . . . . . . . . . . . 43
V. Effects of Counter-Ion on the Properties of Polymeric Products . . . . . . . . 46
A. Molecular Weights . . . . . . . . . . . . . . . . . . . . . . . . . . 46
B. Molecular Weight Distributions . . . . . . . . . . . . . . . . . . . . 49
C. Stereochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
VI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
VII. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
I. Introduction
Initiation of cationic polymerisation involves the generation of a positively
charged species with its attendant counterion.
AB--+A+B - '
The charged electrophile can then either add to a susceptible monomer
molecule producing a positively charged active centre
+
+ + A HC------~2HC ---, ACH2--CH
t I
R R
+ A +X~ -~ A--)(~
,"tO less often, it can accept an electron, again yielding a propagating cation
+ + A CH2=CH ~ "A + CH2--CH . +
I
R R
The process of charge generation takes place in situ with Lewis Acid initiators
(TIC14, SnC14, BF3, SbCI5, A1C13, A1R3 etc.) and often requires a bimolecular
reaction involving two neutral components, e.g.
rB1A2 3 ~ AIBr~ + 2rB1A (selfionisation)
+ 3 BF H20 ~-- + + H BFaOH- .
Such pre-initiation equilibria as these, though of diminished importance in
the large scale production of polymers, are of significance in any detailed kinetic
and mechanistic study of polymerisation reactions. A knowledge of equilibrium
constants (or forward rate constants) would be necessary for a complete kinetic
understanding, and in general such data are not readily available, though some
advances to this end have been made (1-5). Not only do Lewis Acids give rise to
these kinetic problems, but also to chemical problems regarding the necessity or
otherwise for co-catalyst molecules, and, in this regard, Kennedy )6( has recently
tried to crystallise current thinking. Experiments involving low molecular weight
Organic Stable Cation Salts 3
model systems have also gone some way to providing an understanding of such
catalyst-cocatalyst complexities (7-9).
Further complication arises concerning the physical nature of + A B- and that
of the growing polymeric species ~ B-. It is now well known that any
organic salt can exist in many forms in an appropriate solvent, depending on the
prevailing conditions. The degree and nature of association of + A with B-,
and of each with solvent molecules, can very considerably, and some detailed
knowledge of the liquid state is necessary for a comprehensive analysis of any
solution reaction involving ions. The pioneering work in this field was carried
out by the late S. Winstein and his research workers between 1953-1963 [for a
tribute to Winstein see Ref. (t0)]. Extensive kinetic studies of solvolytic reactions
led these workers to propose the existence in solution of various types of ionic
aggregate, and to quantify their differing reactivities. Complementary studies of
carbanion reactions by Cram and co-workers (11) confirmed the generality of
Winstein's suggestions. Characterisation of similar phenomena in ionic poly-
merisations has been most readily achieved by many studies of anionic poly-
merisations, with notable contributions by Szwarc (12), Schulz (t3), and latterly
Ise (14).
In cationic polymerisation no generalised picture has yet emerged, though the
use of stable organic cation salts (15) has made a useful contribution.
II. The Nature and Properties of Stable Organic Cation Salts
.A noitasilibatS of cinagrO snoitaC
To describe any particular organic species as "stable" requires of course,
that there should be some well defined reference point for comparison. Further-
more, the actual "stability" of the species in question will vary according to the
nature of the reaction medium (where appropriate), the counter-ion, and all the
factors (vide infra) affecting ion-pair dissociation. In some respects therefore, it is
perhaps inappropriate to describe any reactive organic intermediate as "stable"
without specifying the basis for the classification. However, it is highly con-
venient---although thermodynamically imprecise--to describe as "stable" those
organic cation salts which may be conveniently isolated in crystalline forms,
under normal laboratory conditions.
Unequivocal assignments of relative thermodynamic stabilities of a wide range
of carbocations may be made from measurements of heats of formation in super
acids such as HSO3F/SbF5 (16) and also from cyclotron resonance studies (t7)
and, whilst these studies lie properly outside the scope of this review, it is worth
noting that electron deficient carbon (i.e. a carbocation) is inherently an unstable
species from both a thermodynamic and a kinetic point of view. To increase the
stability of such ions it is necessary to delocalise the positive charge by sub-
stitution of suitable stabilising groups at the positive site. This charge delocalisa-
tion effectively spreads out the charge concentration in the ion so that no single
carbon atom is excessively cationic. Simultaneously, the peripheral charge
4 .A and D. Ledwith .C notgnirrehS
distribution acts to increase the size of the ion, an important stabilising factor
especially in solution.
The size of the phenyl ring coupled with its ability to delocalise charge by a
resonance mechanism, primarily to its ortho- and para-positions, makes it an
extremely efficient group for the stabitisation of trigonally hybridised (sp )2
carbon. Thus replacement of the hydrogen atoms in methyl cation (CH~) by
phenyl groups gives the indicated sequence of increasing carbocation stabilities:
~HC < PhCH~ + Ph2CH
< < PhaC + .
It will be obvious that replacement of hydrogen atoms in one or other of the
phenyl rings of triphenyl methyl cation (PhaC ÷) by electron releasing conjugating
substituents such as RO-, R2N-, RS-, aryl, etc. will increase still further the
carbocation stability. Hetero atom effects (e.g. from O, N, S) are even more
manifest in carbocations having such substituents attached directly to the electron
deficient site as in
carboxonium ROCHe-*-~ ~C~-)~(R 2 H and
immonium R2N--CH + 2 ~ H~R~-N2R + 2 derivatives,
and, more obviously still, in oxonium (R30+), sulphonium (R3S+), and quaternary
ammonium (R4N +) cations.
A special case of carbocation stability arises where the cation complies with
the Hfickel (4n + 2) rule governing aromatic structures. Of these, the best known
and most useful is the cycloheptatrienyl cation, more frequently referred to as the
tropylium ion. For an informative and wide ranging account of structures,
stabilities, properties and reactions of almost every type ocfa rbocation, reference
should be made to the series of monographs edited by Olah and Schleyer (18).
B. General Value Polymerisation in Systems
Preformed stable organic cation salts have been used to a variety of ends in
the investigation of cationic polymerisations, the most obvious being the use of
salts of triphenylmethyl (Ph3C +) and cyctoheptatrienyl (C~H~) carbocations as
initiators of vinyl and ring opening polymerisations (I5). For the latter systems,
use of more thermodynamically stable species such as oxonium (Et30 ÷) and
sulphonium (G-*Et) ions has proved to be even more important.
Very often these initiations avoid the difficulties presented by the more
common protonic and aprotonic acid catalysts, in that they are chemically well
defined and allow a quantitative initiation reaction to proceed without com-
plicating side reactions or the need for co-catalysts and promoters. Furthermore,
since these salts are stable Crystalline solids, physicochemical analysis of their
solutions can be undertaken, in parallel with kinetic investigations of polymerisa-
tion. For example, ultraviolet/visible absorption spectrophotometry can be used
to monitor directly the concentration of species such as Ph3 C÷ in solution (19-23).
Cation Organic Stable Salts
Proton NMR spectroscopy can similarly be used to monitor the concentrations
of such salts, and has the advantage of being able to elucidate structures simul-
taneously (24,25). Independent conductimetric measurements can provide
immediate information on the state of ionic aggregation in solution, in particular
providing quantitative data concerning the proportion of free solvated ions to
those existing as ion paired entities (26).
dK
@@ @ (soN.)+ 0 (soN.).
Ion pairs Free ions
While the latter measurements, strictly speaking, evaluate the state of dissociation
of the initiator ions only, they also provide at least a guide as to the degree of
dissociation of the propagating polymeric ion pairs. In the case of vinyl polymerisa-
tions, where no "living" cations have been observed to date, direct evaluation of
the dissociation constant Kd, of the growing ion pair is not possible. However,
in a number of cyclic monomer polymerisations "living" characteristics are
observed, and direct measurements have been possible (27).
To some extent the importance of preformed cation salts as well defined
initiators has overshadowed some of the other important contributions to the
study of cationic polymerisation. They allow the negative counterion to be well
defined (28, 29), whereas with other initiators, e.g. H20/BF3, it is only possible to
speculate concerning the structure of the counterion. The stability of counterions
can also be assessed in measurements divorced from the polymerisation reaction
(23). Addition of excess chemically inert salts is Useful in changing the counterion
in a polymerisation which in turn can create changes in the rate of propagation,
in the stereochemistry, and in molecular weight and molecular weight distribution
of the polymer product (15, 30, 31). Where the added salt has a counterion in
common with that of the propagating ion pair then disturbance of the dissociative
equilibria of the latter takes place by a normal mass action effect, thus allowing
further quantification of the reactivities of the various ionic species present
(27, 32, 33). There is at least one report of a positive effect of this type being used as a
diagnostic test for the structure of the counterion in a polymerisation initiated by
the combination, hydrogen chtoride-stannic chloride (34).
Finally, highly reactive salts such as triphenylmethyl hexachloroantimonate
provide very convenient scavenging agents for the removal of the last traces of
nucleophilic impurities in polymerisation solvents. Penczek (35) has taken this
further in developing a spectroscopic technique for the determination of trace
quantities of residual water by reaction of the latter with known concentrations
of triphenylmethyl salts, according to the following equilibria:
H
+J
X+C3hP - + H20 ~ Ph3C--O X-
H
Ph~COH - + . HaO+X
6 .A Ledwith and .D Sherrington C.
C. Conductance dna Ion Pair Equifibria
C.1. Theoretical Considerations
Today it is widely accepted that any organic salt, + A B-, may exist in a number
of aggregated forms in solution as shown below.
A +.,.B-...A + + B-
A + •,. B-
and/or
B-...A +...B- + + A
Complex aggregates Triple ions
.9.e quadrupole ions
+ 2A ).vlos( + 2B- ~ (solv.) 2[A + (solv.)B-] 2[A+...B-].
Free solvated ions Loose ion pairs Tight ion pairs
Though the realisation that these various types of aggregate may have quite
different reactivities was recognised relatively recently (36), the concept of ion
association, and in particular ion pairing, was evoked thirty-five years before by
Bjerrum (37). Prior to this, Debye and Hiickel {38) had developed the modern
theory of strong electrolytes introducing the idea of an "ion atmosphere" and its
related potential field around a reference ion. This electrostatic approach allowed
calculation of activity coefficients from a molecular model• Onsager (39) used the
Debye-Hiickel model when describing the movement of an ion in an applied
electric field, and derived the following equation for the conductance )2( of an
ion in a dilute solution of an electrolyte of concentration, C
2 = 20-(P20+ Q)C ~.
P and Q are constants defined by the dielectric constant D and viscosity /r
of the solvent and the prevailing temperature T, and o2 is the limiting conductance
at infinite dilution.
Onsager's equation is satisfactory when all the ions present are free. However,
when the gain in energy in forming a discrete ion pair ® G, i.e. zl z2 e2/aD, is
much greater than thermal energy kT, then such species have a prolonged lifetime
in excess of collisionat magnitude, and their presence must be accounted for in a
theoretical treatment of conductance (Fig. .)1
Fuoss (40) has improved Bjerrum's original treatment (37) of this situation
and, although a number of other sophistications have been introduced, his
formulation (41) is the one most used today. In fact rather fortuitously the relatively
low dielectric constants of solvents employed in organic chemical reactions,
particularly ionic potymerisations, are ideal media for the application of these
theories. The analysis carried out by Fuoss leads not surprisingly to an equation
Stable Organic Cation Salts 7
Solvent
@ @
dielectric
constant D
T
,~, temperature
I I
---T-"
lacitirC hcaorppa ecnatsid
.... "---~ .> g l
E
2
£
I
~c
Distance between ~ns
Fig. .1 Ion pair formation = no. (z of charges; = e electronic charge; = k Boltzmann constant)
whose basic form is similar to that derived by Ostwald for weakly ionised electro-
lytes e.9.
OOCsHC H _..~ CH3CO0- + + H
~2 C
KI= t-c~'
where e, the degree of ionisation, is given by
= 2/20.
2, being the molar conductance at concentration C, and 20 the value at infinite
dilution.
Hence,
1 1 2C
2 o 2 2~Kj
The dissociation of an ion pair can be represented in a very similar manner,
.g.e
B+A - + + ~ A -B
