6 Ionic polymers
B.N. HENDY
6.1 Introduction
Ionic polymers contain ions which are chemically bound within their
structure. The term is not meant to apply to polymers that have been made
by ionic methods, nor to those that, fortuitously, have ionic end-groups. We
will discuss mainly the types which fall within the scope of conventional
organic polymer chemistry. Ionic polymers are specialist materials, some of
which have limited, though important, commercial applications. Many are
modifications of conventional polymers but a few are uniquely designed
structures. Those containing few ions are thermoplastics called ionomers:
those containing many ions are water-soluble and called polyelectrolytes.
6.2 Classification
Ionic polymers may be classified according to the nature of their bound ions,
specifically the type of ion, its position within the structure and the amount
present along a given length of polymer chain. They may be further classified
according to the nature of the counter ion, or by the nature of the supporting
polymeric backbone.
6.2.1 Type of bound ion
The bound ion may be either an anion or a cation. Most commonly it is the
former but both types can occur together and the polymer is then said to
be ampholytic. In each case the polyion may be strong or weak according
to the degree to which it ionizes.
Table 6.1 Examples of bound ions
Type of polyion Nature of polyion
Strong Intermediate Weak
Anion Sulphonate Phosphonate Carboxylate
- S0 3 -P0 3 H- -CO 2
Cation Quaternary Amine
-NR; -NH;
, IONIC POLYMERS 111
6.2.2 Position of the bound ion
The bound ion can be pendant to the polymer's covalent backbone or it can
be integral or enchained. Pendant ions, such as in a polysulphonate, are the
most common. Ionenes have integral ions.
ionene
polySLtlhonate
R
1+
1 _
-N-
1
5°3 R
pendant integral
6.2.3 Amount of bound ion
The polymer may contain a few bound ions per unit mass or it may contain
many. The ion content, or charge density, may be expressed by relating the
number of ionic bonds to that of the covalent network bonds thus leading
to a scale of values ranging from zero for compounds such as polyethylene
to 100 for ionic crystals like sodium chloride, but it is more informative to
quote the equivalent weight of the parent polyacid or polybase. It is
particularly useful to express the ionic content as being the number of
equivalents in each kilogram of the polymer. A copolymer consisting of an
ionic monomer unit with a non-ionic is frequently characterized by the
amount of the ionic unit it contains as a percentage. Conventional organic
polymers can be modified with ions to cover a large range of ion contents,
as shown in Table 6.2.
6.2.4 Type of counter ion
The counter ions neutralize the charges on the bound ions and may be
grouped into three types: (a) univalent, (b) di- or trivalent and (c) polymeric.
The properties of an ionic polymer are profoundly affected by which type
of counter ion it contains, as will be described later. Polymers with polymeric
Table 6.2 Ionic contents of organic ionic polymers
Equiva-
Designation Chemical type Formula Mol % Weight % lentsfkg
CH 3
1
Ionomer copolymer -(CH 2 CH 2 )49 C- 2 6 0.7
1
COO-
Polyelectrolyte homopolymer -CH 2 CH- 100 100 14
I
COO-
, 112 SPECIALTY POLYMERS
counter ions are often called polyelectrolyte complexes, polysalts, polyion
complexes, simplexes, or coacervates.
6.2.5 The backbone
Almost any conventional polymer can be modified with ions to form an
ionic polymer of the pendant type. Ionic polymers of the integral type, like
the ionenes, are specialized structures whose backbones can exist only in the
ionic form. In either case the polymer may be thermoplastic or elastomeric,
and it may have other features like cross-linking or branching.
6.3 Synthesis
Specific syntheses will be described later but certain generalities will be
discussed here. Ionic polymers can be synthesized by any of the methods
applicable to conventional polymers but the choice of method will be
influenced as much by the ionic function as by the type of backbone. The
methods can be divided mainly into three types:
(a) Direct synthesis
(b) Post-functionalization of a standard pre-formed polymer
(c) Post-functionalization of a special pre-formed polymer
Direct synthesis is often used for polyelectrolytes. Typically, an aqueous
solution of an ionic vinyl monomer (e.g. acrylic acid) is homopolymerized
in aqueous solution. However, direct synthesis of less ionic polymers, which
involves copolymerizing ionic with non-ionic monomers, is often not so
straightforward because the two types of monomer are so different in their
physical and copolymerization characteristics. Copolymerizations in aqueous
emulsion or suspension can, for instance, be spoiled because the ionic
monomer separates from its non-ionic comonomer by dissolving in the
aqueous phase. Examples of direct copolymerization include ethylene with
small amounts of methacrylic acid in the gaseous phase and butadiene with
small amounts of methacrylic acid in aqueous emulsion at low pH to suppress
the solubility of the acid monomer.
Post-functionalization of standard pre-formed polymers is a frequently
used synthetic method provided that the polymer is reactive. Examples are
the suI phonation of polystyrene and the grafting on to polybutadiene of
thioglycollic acid using free radicals. In some post-functionalization reactions
there can be problems in achieving a uniform fully reacted product. This
problem arises because the covalent polymeric substrate, the reactant and
the ionic polymeric product have very different solubilities so that care is
required to achieve a homogeneous reproducible reaction.
Many polymers are insufficiently reactive to be post-functionalized, or are
damaged by the reaction conditions, and by-products may be formed which
B.N. HENDY
6.1 Introduction
Ionic polymers contain ions which are chemically bound within their
structure. The term is not meant to apply to polymers that have been made
by ionic methods, nor to those that, fortuitously, have ionic end-groups. We
will discuss mainly the types which fall within the scope of conventional
organic polymer chemistry. Ionic polymers are specialist materials, some of
which have limited, though important, commercial applications. Many are
modifications of conventional polymers but a few are uniquely designed
structures. Those containing few ions are thermoplastics called ionomers:
those containing many ions are water-soluble and called polyelectrolytes.
6.2 Classification
Ionic polymers may be classified according to the nature of their bound ions,
specifically the type of ion, its position within the structure and the amount
present along a given length of polymer chain. They may be further classified
according to the nature of the counter ion, or by the nature of the supporting
polymeric backbone.
6.2.1 Type of bound ion
The bound ion may be either an anion or a cation. Most commonly it is the
former but both types can occur together and the polymer is then said to
be ampholytic. In each case the polyion may be strong or weak according
to the degree to which it ionizes.
Table 6.1 Examples of bound ions
Type of polyion Nature of polyion
Strong Intermediate Weak
Anion Sulphonate Phosphonate Carboxylate
- S0 3 -P0 3 H- -CO 2
Cation Quaternary Amine
-NR; -NH;
, IONIC POLYMERS 111
6.2.2 Position of the bound ion
The bound ion can be pendant to the polymer's covalent backbone or it can
be integral or enchained. Pendant ions, such as in a polysulphonate, are the
most common. Ionenes have integral ions.
ionene
polySLtlhonate
R
1+
1 _
-N-
1
5°3 R
pendant integral
6.2.3 Amount of bound ion
The polymer may contain a few bound ions per unit mass or it may contain
many. The ion content, or charge density, may be expressed by relating the
number of ionic bonds to that of the covalent network bonds thus leading
to a scale of values ranging from zero for compounds such as polyethylene
to 100 for ionic crystals like sodium chloride, but it is more informative to
quote the equivalent weight of the parent polyacid or polybase. It is
particularly useful to express the ionic content as being the number of
equivalents in each kilogram of the polymer. A copolymer consisting of an
ionic monomer unit with a non-ionic is frequently characterized by the
amount of the ionic unit it contains as a percentage. Conventional organic
polymers can be modified with ions to cover a large range of ion contents,
as shown in Table 6.2.
6.2.4 Type of counter ion
The counter ions neutralize the charges on the bound ions and may be
grouped into three types: (a) univalent, (b) di- or trivalent and (c) polymeric.
The properties of an ionic polymer are profoundly affected by which type
of counter ion it contains, as will be described later. Polymers with polymeric
Table 6.2 Ionic contents of organic ionic polymers
Equiva-
Designation Chemical type Formula Mol % Weight % lentsfkg
CH 3
1
Ionomer copolymer -(CH 2 CH 2 )49 C- 2 6 0.7
1
COO-
Polyelectrolyte homopolymer -CH 2 CH- 100 100 14
I
COO-
, 112 SPECIALTY POLYMERS
counter ions are often called polyelectrolyte complexes, polysalts, polyion
complexes, simplexes, or coacervates.
6.2.5 The backbone
Almost any conventional polymer can be modified with ions to form an
ionic polymer of the pendant type. Ionic polymers of the integral type, like
the ionenes, are specialized structures whose backbones can exist only in the
ionic form. In either case the polymer may be thermoplastic or elastomeric,
and it may have other features like cross-linking or branching.
6.3 Synthesis
Specific syntheses will be described later but certain generalities will be
discussed here. Ionic polymers can be synthesized by any of the methods
applicable to conventional polymers but the choice of method will be
influenced as much by the ionic function as by the type of backbone. The
methods can be divided mainly into three types:
(a) Direct synthesis
(b) Post-functionalization of a standard pre-formed polymer
(c) Post-functionalization of a special pre-formed polymer
Direct synthesis is often used for polyelectrolytes. Typically, an aqueous
solution of an ionic vinyl monomer (e.g. acrylic acid) is homopolymerized
in aqueous solution. However, direct synthesis of less ionic polymers, which
involves copolymerizing ionic with non-ionic monomers, is often not so
straightforward because the two types of monomer are so different in their
physical and copolymerization characteristics. Copolymerizations in aqueous
emulsion or suspension can, for instance, be spoiled because the ionic
monomer separates from its non-ionic comonomer by dissolving in the
aqueous phase. Examples of direct copolymerization include ethylene with
small amounts of methacrylic acid in the gaseous phase and butadiene with
small amounts of methacrylic acid in aqueous emulsion at low pH to suppress
the solubility of the acid monomer.
Post-functionalization of standard pre-formed polymers is a frequently
used synthetic method provided that the polymer is reactive. Examples are
the suI phonation of polystyrene and the grafting on to polybutadiene of
thioglycollic acid using free radicals. In some post-functionalization reactions
there can be problems in achieving a uniform fully reacted product. This
problem arises because the covalent polymeric substrate, the reactant and
the ionic polymeric product have very different solubilities so that care is
required to achieve a homogeneous reproducible reaction.
Many polymers are insufficiently reactive to be post-functionalized, or are
damaged by the reaction conditions, and by-products may be formed which
