Polymers
1. Introduction
Prior to the early 1920's, chemists doubted the existence of
molecules having molecular weights greater than a few thousand. This
limiting view was challenged by Hermann Staudinger,
a German chemist with experience in studying natural compounds such as
rubber and cellulose. In contrast to the prevailing rationalization of
these substances as aggregates of small molecules, Staudinger proposed
they were made up of macromolecules composed of 10,000 or more atoms. He formulated a polymeric structure for rubber,
based on a repeating isoprene unit (referred to as a monomer). For his
contributions to chemistry, Staudinger received the 1953 Nobel Prize.
The terms polymer and monomer were derived from the Greek roots poly (many), mono (one) and meros (part).
Recognition that polymeric macromolecules make up many important
natural materials was followed by the creation of synthetic analogs
having a variety of properties. Indeed, applications of these materials
as fibers, flexible films, adhesives, resistant paints and tough but
light solids have transformed modern society. Some important examples of
these substances are discussed in the following sections.
2. Writing Formulas for Polymeric Macromolecules
The repeating structural unit of most simple polymers not only
reflects the monomer(s) from which the polymers are constructed, but
also provides a concise means for drawing structures to represent these
macromolecules. For polyethylene, arguably the simplest polymer, this is
demonstrated by the following equation. Here ethylene (ethene) is the
monomer, and the corresponding linear polymer is called high-density
polyethylene (HDPE). HDPE is composed of macromolecules in which n
ranges from 10,000 to 100,000 (molecular weight 2*105 to 3 *106 ).
If Y and Z represent moles of monomer and polymer respectively, Z is approximately 10-5 Y. This polymer is called polyethylene rather than polymethylene, (-CH2-)n,
because ethylene is a stable compound (methylene is not), and it also
serves as the synthetic precursor of the polymer. The two open bonds
remaining at the ends of the long chain of carbons (colored magenta) are
normally not specified, because the atoms or groups found there depend
on the chemical process used for polymerization. The synthetic methods
used to prepare this and other polymers will be described later in this
chapter.
Unlike simpler pure compounds, most polymers are not composed of
identical molecules. The HDPE molecules, for example, are all long
carbon chains, but the lengths may vary by thousands of monomer units.
Because of this, polymer molecular weights are usually given as
averages. Two experimentally determined values are common:
Mn
, the number average molecular weight, is calculated from the mole
fraction distribution of different sized molecules in a sample, and
Mw
, the weight average molecular weight, is calculated from the weight
fraction distribution of different sized molecules. These are defined
below. Since larger molecules in a sample weigh more than smaller
molecules, the weight average M
w is necessarily skewed to higher values, and is always greater than M
n. As the weight dispersion of molecules in a sample narrows, M
w approaches M
n, and in the unlikely case that all the polymer molecules have identical weights (a pure mono-disperse sample), the ratio M
w / M
n becomes unity.
The influence of different mass distributions on Mn and Mw may be examined with the aid of
a simple mass calculator.
To use this device Click Here. |
|---|
Many polymeric materials having chain-like structures similar to
polyethylene are known. Polymers formed by a straightforward linking
together of monomer units, with no loss or gain of material, are called
addition polymers or
chain-growth polymers. A listing of some important addition polymers and their monomer precursors is presented in the following table.
Some Common Addition Polymers
| Name(s) | Formula | Monomer | Properties | Uses |
|---|
Polyethylene
low density (LDPE) | –(CH2-CH2)n– | ethylene
CH2=CH2 | soft, waxy solid | film wrap, plastic bags |
Polyethylene
high density (HDPE) | –(CH2-CH2)n– | ethylene
CH2=CH2 | rigid, translucent solid | electrical insulation
bottles, toys |
Polypropylene
(PP) different grades | –[CH2-CH(CH3)]n– | propylene
CH2=CHCH3 | atactic: soft, elastic solid
isotactic: hard, strong solid | similar to LDPE
carpet, upholstery |
Poly(vinyl chloride)
(PVC) | –(CH2-CHCl)n– | vinyl chloride
CH2=CHCl | strong rigid solid | pipes, siding, flooring |
Poly(vinylidene chloride)
(Saran A) | –(CH2-CCl2)n– | vinylidene chloride
CH2=CCl2 | dense, high-melting solid | seat covers, films |
Polystyrene
(PS) | –[CH2-CH(C6H5)]n– | styrene
CH2=CHC6H5 | hard, rigid, clear solid
soluble in organic solvents | toys, cabinets
packaging (foamed) |
Polyacrylonitrile
(PAN, Orlon, Acrilan) | –(CH2-CHCN)n– | acrylonitrile
CH2=CHCN | high-melting solid
soluble in organic solvents | rugs, blankets
clothing |
Polytetrafluoroethylene
(PTFE, Teflon) | –(CF2-CF2)n– | tetrafluoroethylene
CF2=CF2 | resistant, smooth solid | non-stick surfaces
electrical insulation |
Poly(methyl methacrylate)
(PMMA, Lucite, Plexiglas) | –[CH2-C(CH3)CO2CH3]n– | methyl methacrylate
CH2=C(CH3)CO2CH3 | hard, transparent solid | lighting covers, signs
skylights |
Poly(vinyl acetate)
(PVAc) | –(CH2-CHOCOCH3)n– | vinyl acetate
CH2=CHOCOCH3 | soft, sticky solid | latex paints, adhesives |
cis-Polyisoprene
natural rubber | –[CH2-CH=C(CH3)-CH2]n– | isoprene
CH2=CH-C(CH3)=CH2 | soft, sticky solid | requires vulcanization
for practical use |
Polychloroprene (cis + trans)
(Neoprene) | –[CH2-CH=CCl-CH2]n– | chloroprene
CH2=CH-CCl=CH2 | tough, rubbery solid | synthetic rubber
oil resistant |
3. Properties of Macromolecules
A comparison of the properties of polyethylene (both LDPE & HDPE)
with the natural polymers rubber and cellulose is instructive. As noted
above, synthetic HDPE macromolecules have masses ranging from 105 to 106
amu (LDPE molecules are more than a hundred times smaller). Rubber and
cellulose molecules have similar mass ranges, but fewer monomer units
because of the monomer's larger size. The physical properties of these
three polymeric substances differ from each other, and of course from
their monomers.
• HDPE is a rigid translucent solid which softens on heating
above 100º C, and can be fashioned into various forms including films.
It is not as easily stretched and deformed as is LDPE. HDPE is insoluble
in water and most organic solvents, although some swelling may occur on
immersion in the latter. HDPE is an excellent electrical insulator.
• LDPE is a soft translucent solid which deforms badly above 75º
C. Films made from LDPE stretch easily and are commonly used for
wrapping. LDPE is insoluble in water, but softens and swells on exposure
to hydrocarbon solvents. Both LDPE and HDPE become brittle at very low
temperatures (below -80º C). Ethylene, the common monomer for these
polymers, is a low boiling (-104º C) gas.
• Natural (latex)
rubber
is an opaque, soft, easily deformable solid that becomes sticky when
heated (above. 60º C), and brittle when cooled below -50º C. It swells
to more than double its size in nonpolar organic solvents like toluene,
eventually dissolving, but is impermeable to water. The C
5H
8 monomer isoprene is a volatile liquid (b.p. 34º C).
• Pure
cellulose,
in the form of cotton, is a soft flexible fiber, essentially unchanged
by variations in temperature ranging from -70 to 80º C. Cotton absorbs
water readily, but is unaffected by immersion in toluene or most other
organic solvents. Cellulose fibers may be bent and twisted, but do not
stretch much before breaking. The monomer of cellulose is the C
6H
12O
6 aldohexose D-glucose. Glucose is a water soluble solid melting below 150º C.
To account for the differences noted here we need to consider the nature of the aggregate macromolecular structure, or morphology,
of each substance. Because polymer molecules are so large, they
generally pack together in a non-uniform fashion, with ordered or
crystalline-like regions mixed together with disordered or amorphous
domains. In some cases the entire solid may be amorphous, composed
entirely of coiled and tangled macromolecular chains. Crystallinity
occurs when linear polymer chains are structurally oriented in a uniform
three-dimensional matrix. In the diagram on the right, crystalline
domains are colored blue.
Increased crystallinity is associated with
an increase in rigidity, tensile strength and opacity (due to light
scattering). Amorphous polymers are usually less rigid, weaker and more
easily deformed. They are often transparent.
Three factors that influence the degree of crystallinity are:
i) Chain length
ii) Chain branching
iii) Interchain bonding
The importance of the first two factors is nicely illustrated by the
differences between LDPE and HDPE. As noted earlier, HDPE is composed of
very long unbranched hydrocarbon chains. These pack together easily in
crystalline domains that alternate with amorphous segments, and the
resulting material, while relatively strong and stiff, retains a degree
of flexibility. In contrast, LDPE is composed of smaller and more highly
branched chains which do not easily adopt crystalline structures. This
material is therefore softer, weaker, less dense and more easily
deformed than HDPE. As a rule, mechanical properties such as ductility,
tensile strength, and hardness rise and eventually level off with
increasing chain length.
The nature of cellulose supports the above analysis and demonstrates
the importance of the third factor (iii). To begin with, cellulose
chains easily adopt a stable rod-like conformation. These molecules
align themselves side by side into fibers that are
stabilized by inter-chain hydrogen bonding
between the three hydroxyl groups on each monomer unit. Consequently,
crystallinity is high and the cellulose molecules do not move or slip
relative to each other. The high concentration of hydroxyl groups also
accounts for the facile absorption of water that is characteristic of
cotton.
Natural rubber is a completely amorphous polymer. Unfortunately, the
potentially useful properties of raw latex rubber are limited by
temperature dependence; however, these properties can be modified by
chemical change. The cis-double bonds in the hydrocarbon chain provide
planar segments that stiffen, but do not straighten the chain. If these
rigid segments are completely removed by hydrogenation (H2
& Pt catalyst), the chains lose all constrainment, and the product
is a low melting paraffin-like semisolid of little value. If instead,
the chains of rubber molecules are slightly cross-linked by sulfur
atoms, a process called vulcanization which was discovered by
Charles Goodyear in 1839, the desirable elastomeric properties of rubber
are substantially improved. At 2 to 3% crosslinking a useful soft
rubber, that no longer suffers stickiness and brittleness problems on
heating and cooling, is obtained. At 25 to 35% crosslinking a rigid hard
rubber product is formed. The following illustration shows a
cross-linked section of amorphous rubber. By clicking on the diagram it
will change to a display of the corresponding stretched section. The
more highly-ordered chains in the stretched conformation are
entropically unstable and return to their original coiled state when
allowed to relax (click a second time).
On heating or cooling most polymers undergo thermal transitions that
provide insight into their morphology. These are defined as the
melt transition, Tm , and the
glass transition, Tg .
Tm is the temperature at which crystalline domains lose their structure, or melt. As crystallinity
increases, so does Tm.
Tg is the temperature below which amorphous domains lose the structural mobility of the polymer
chains and become rigid glasses. |
Tg often depends on the history of the sample,
particularly previous heat treatment, mechanical manipulation and
annealing. It is sometimes interpreted as the temperature above which
significant portions of polymer chains are able to slide past each other
in response to an applied force. The introduction of relatively large
and stiff substituents (such as benzene rings) will interfere with this
chain movement, thus increasing Tg (note polystyrene below).
The introduction of small molecular compounds called plasticizers into
the polymer matrix increases the interchain spacing, allowing chain
movement at lower temperatures. with a resulting decrease in Tg.
The outgassing of plasticizers used to modify interior plastic
components of automobiles produces the "new-car smell" to which we are
accustomed.
Tm and Tg values for some common addition polymers are listed below. Note that cellulose has neither a Tm nor a Tg.
| Polymer | LDPE | HDPE | PP | PVC | PS | PAN | PTFE | PMMA | Rubber |
|---|
| Tm (ºC) | 110 | 130 | 175 | 180 | 175 |
>200 | 330 | 180 | 30 |
|---|
| Tg (ºC) | _110 | _100 | _10 | 80 | 90 | 95 | _110 | 105 | _70 |
|---|
Rubber is a member of an important group of polymers called elastomers.
Elastomers are amorphous polymers that have the ability to stretch and
then return to their original shape at temperatures above Tg.
This property is important in applications such as gaskets and O-rings,
so the development of synthetic elastomers that can function under
harsh or demanding conditions remains a practical goal. At temperatures
below Tg elastomers become rigid glassy solids and lose all
elasticity. A tragic example of this caused the space shuttle Challenger
disaster. The heat and chemical resistant O-rings used to seal sections
of the solid booster rockets had an unfortunately high Tg near 0 ºC. The unexpectedly low temperatures on the morning of the launch were below this Tg, allowing hot rocket gases to escape the seals.
4. Regio and Stereoisomerization in Macromolecules
Symmetrical monomers such as ethylene and tetrafluoroethylene can
join together in only one way. Monosubstituted monomers, on the other
hand, may join together in two organized ways, described in the
following diagram, or in a third random manner. Most monomers of this
kind, including propylene, vinyl chloride, styrene, acrylonitrile and
acrylic esters, prefer to join in a head-to-tail fashion, with some
randomness occurring from time to time. The reasons for this
regioselectivity will be discussed in the synthetic methods section.
If the polymer chain is drawn in a zig-zag fashion, as shown above,
each of the substituent groups (Z) will necessarily be located above or
below the plane defined by the carbon chain. Consequently we can
identify three configurational isomers of such polymers. If all the
substituents lie on one side of the chain the configuration is called isotactic. If the substituents alternate from one side to another in a regular manner the configuration is termed syndiotactic. Finally, a random arrangement of substituent groups is referred to as atactic. Examples of these configurations are shown here.
Many common and useful polymers, such as polystyrene,
polyacrylonitrile and poly(vinyl chloride) are atactic as normally
prepared. Customized catalysts that effect stereoregular polymerization
of polypropylene and some other monomers have been developed, and the
improved properties associated with the increased crystallinity of these
products has made this an important field of investigation. The
following values of Tg have been reported.
| Polymer | Tg atactic | Tg isotactic | Tg syndiotactic |
|---|
| PP | –20 ºC | 0 ºC | –8 ºC |
|---|
| PMMA | 100 ºC | 130 ºC | 120 ºC |
|---|
The properties of a given polymer will vary considerably with its
tacticity. Thus, atactic polypropylene is useless as a solid
construction material, and is employed mainly as a component of
adhesives or as a soft matrix for composite materials. In contrast,
isotactic polypropylene is a high-melting solid (ca. 170 ºC) which can
be molded or machined into structural components.
Synthesis of Addition Polymers
All the monomers from which addition polymers are made are alkenes or
functionally substituted alkenes. The most common and thermodynamically
favored chemical transformations of alkenes are
addition reactions. Many of these addition reactions are known to proceed in a stepwise fashion by way of
reactive intermediates,
and this is the mechanism followed by most polymerizations. A general
diagram illustrating this assembly of linear macromolecules, which
supports the name
chain growth polymers, is presented here. Since
a pi-bond in the monomer is converted to a sigma-bond in the polymer,
the polymerization reaction is usually exothermic by 8 to 20 kcal/mol.
Indeed, cases of explosively uncontrolled polymerizations have been
reported.
It is useful to distinguish four polymerization procedures fitting this general description.
• Radical Polymerization The initiator is a radical, and the propagating site of reactivity (*) is a carbon radical.
• Cationic Polymerization The initiator is an acid, and the propagating site of reactivity (*) is a carbocation.
• Anionic Polymerization The initiator is a nucleophile, and the propagating site of reactivity (*) is a carbanion.
• Coordination Catalytic Polymerization The initiator is a transition metal complex, and the propagating site of reactivity (*) is a terminal catalytic complex.
1. Radical Chain-Growth Polymerization
Virtually all of the monomers described above
are subject to radical polymerization. Since this can be initiated by
traces of oxygen or other minor impurities, pure samples of these
compounds are often "stabilized" by small amounts of radical inhibitors
to avoid unwanted reaction. When radical polymerization is desired, it
must be started by using a radical initiator, such as a peroxide
or certain azo compounds. The formulas of some common initiators, and
equations showing the formation of radical species from these initiators
are presented below.
By using small amounts of initiators, a wide variety of monomers can
be polymerized. One example of this radical polymerization is the
conversion of styrene to polystyrene, shown in the following diagram.
The first two equations illustrate the
initiation process, and the last two equations are examples of
chain propagation. Each monomer unit adds to the growing chain in a manner that generates the most
stable radical. Since carbon radicals are
stabilized by substituents
of many kinds, the preference for head-to-tail regioselectivity in most
addition polymerizations is understandable. Because radicals are
tolerant of many functional groups and solvents (including water),
radical polymerizations are widely used in the chemical industry.
To see an animated model of the radical chain-growth polymerization of vinyl chloride
In principle, once started a radical polymerization might be expected
to continue unchecked, producing a few extremely long chain polymers.
In practice, larger numbers of moderately sized chains are formed,
indicating that chain-terminating reactions must be taking place. The
most common termination processes are
Radical Combination and
Disproportionation.
These reactions are illustrated by the following equations. The growing
polymer chains are colored blue and red, and the hydrogen atom
transferred in disproportionation is colored green. Note that in both
types of termination two reactive radical sites are removed by
simultaneous conversion to stable product(s). Since the concentration of
radical species in a polymerization reaction is small relative to other
reactants (e.g. monomers, solvents and terminated chains), the rate at
which these radical-radical termination reactions occurs is very small,
and most growing chains achieve moderate length before termination.
The relative importance of these terminations varies with the nature
of the monomer undergoing polymerization. For acrylonitrile and styrene
combination is the major process. However, methyl methacrylate and vinyl
acetate are terminated chiefly by disproportionation.
Another reaction that diverts radical chain-growth polymerizations from producing linear macromolecules is called chain transfer.
As the name implies, this reaction moves a carbon radical from one
location to another by an intermolecular or intramolecular hydrogen atom
transfer (colored green). These possibilities are demonstrated by the
following equations
Chain transfer reactions are especially prevalent in the high
pressure radical polymerization of ethylene, which is the method used to
make LDPE (low density polyethylene). The 1º-radical at the end of a
growing chain is converted to a more stable 2º-radical by hydrogen atom
transfer. Further polymerization at the new radical site generates a
side chain radical, and this may in turn lead to creation of other side
chains by chain transfer reactions. As a result, the morphology of LDPE
is an amorphous network of highly branched macromolecules.
2. Cationic Chain-Growth Polymerization
Polymerization of isobutylene (2-methylpropene) by traces of strong
acids is an example of cationic polymerization. The polyisobutylene
product is a soft rubbery solid, Tg = _70º C,
which is used for inner tubes. This process is similar to radical
polymerization, as demonstrated by the following equations. Chain growth
ceases when the terminal carbocation combines with a nucleophile or
loses a proton, giving a terminal alkene (as shown here).
Monomers bearing cation stabilizing groups, such as alkyl, phenyl or
vinyl can be polymerized by cationic processes. These are normally
initiated at low temperature in methylene chloride solution. Strong
acids, such as HClO4 , or Lewis acids containing traces of
water (as shown above) serve as initiating reagents. At low
temperatures, chain transfer reactions are rare in such polymerizations,
so the resulting polymers are cleanly linear (unbranched).
3. Anionic Chain-Growth Polymerization
Treatment of a cold THF solution of styrene with 0.001 equivalents of
n-butyllithium causes an immediate polymerization. This is an example
of anionic polymerization, the course of which is described by the
following equations. Chain growth may be terminated by water or carbon
dioxide, and chain transfer seldom occurs. Only monomers having anion
stabilizing substituents, such as phenyl, cyano or carbonyl are good
substrates for this polymerization technique. Many of the resulting
polymers are largely isotactic in configuration, and have high degrees
of crystallinity.
Species that have been used to initiate anionic polymerization
include alkali metals, alkali amides, alkyl lithiums and various
electron sources. A practical application of anionic polymerization
occurs in the use of superglue. This material is methyl 2-cyanoacrylate,
CH2=C(CN)CO2CH3. When exposed to water, amines or other nucleophiles, a rapid polymerization of this monomer takes place.
4. Ziegler-Natta Catalytic Polymerization
An efficient and stereospecific catalytic polymerization procedure
was developed by Karl Ziegler (Germany) and Giulio Natta (Italy) in the
1950's. Their findings permitted, for the first time, the synthesis of
unbranched, high molecular weight polyethylene (HDPE), laboratory
synthesis of natural rubber from isoprene, and configurational control
of polymers from terminal alkenes like propene (e.g. pure isotactic and
syndiotactic polymers). In the case of ethylene, rapid polymerization
occurred at atmospheric pressure and moderate to low temperature, giving
a stronger (more crystalline) product (HDPE) than that from radical
polymerization (LDPE). For this important discovery these chemists
received the 1963 Nobel Prize in chemistry.
Ziegler-Natta catalysts are prepared by reacting certain transition
metal halides with organometallic reagents such as alkyl aluminum,
lithium and zinc reagents. The catalyst formed by reaction of
triethylaluminum with titanium tetrachloride has been widely studied,
but other metals (e.g. V & Zr) have also proven effective. The
following diagram presents one mechanism for this useful reaction.
Others have been suggested, with changes to accommodate the
heterogeneity or homogeneity of the catalyst. Polymerization of
propylene through action of the titanium catalyst gives an isotactic
product; whereas, a vanadium based catalyst gives a syndiotactic
product.
Copolymers
The synthesis of macromolecules composed of more than one monomeric
repeating unit has been explored as a means of controlling the
properties of the resulting material. In this respect, it is useful to
distinguish several ways in which different monomeric units might be
incorporated in a polymeric molecule. The following examples refer to a
two component system, in which one monomer is designated A and the other B.
| Statistical Copolymers | Also
called random copolymers. Here the monomeric units are distributed
randomly, and sometimes unevenly, in the polymer chain: ~ABBAAABAABBBABAABA~. |
| Alternating Copolymers | Here
the monomeric units are distributed in a regular alternating fashion,
with nearly equimolar amounts of each in the chain: ~ABABABABABABABAB~. |
|---|
| Block Copolymers | Instead
of a mixed distribution of monomeric units, a long sequence or block of
one monomer is joined to a block of the second monomer: ~AAAAA-BBBBBBB~AAAAAAA~BBB~. |
|---|
| Graft Copolymers | As the name suggests, side chains of a given monomer are attached to the main chain of the second monomer: ~AAAAAAA(BBBBBBB~)AAAAAAA(BBBB~)AAA~. |
|---|
1. Addition Copolymerization
Most direct copolymerizations of equimolar mixtures of different
monomers give statistical copolymers, or if one monomer is much more
reactive a nearly homopolymer of that monomer. The copolymerization of
styrene with methyl methacrylate, for example, proceeds differently
depending on the mechanism. Radical polymerization gives a statistical
copolymer. However, the product of cationic polymerization is largely
polystyrene, and anionic polymerization favors formation of poly(methyl
methacrylate). In cases where the relative reactivities are different,
the copolymer composition can sometimes be controlled by continuous
introduction of a biased mixture of monomers into the reaction.
Formation of alternating copolymers is favored when the monomers have
different polar substituents (e.g. one electron withdrawing and the
other electron donating), and both have similar reactivities toward
radicals. For example, styrene and acrylonitrile copolymerize in a
largely alternating fashion.
Some Useful Copolymers
| Monomer A | Monomer B | Copolymer | Uses |
|---|
| H2C=CHCl | H2C=CCl2 | Saran | films & fibers |
| H2C=CHC6H5 | H2C=C-CH=CH2 | SBR
styrene butadiene rubber | tires |
| H2C=CHCN | H2C=C-CH=CH2 | Nitrile Rubber | adhesives
hoses |
| H2C=C(CH3)2 | H2C=C-CH=CH2 | Butyl Rubber | inner tubes |
| F2C=CF(CF3) | H2C=CHF | Viton | gaskets |
A terpolymer of acrylonitrile, butadiene and styrene, called ABS rubber, is used for high-impact containers, pipes and gaskets.
2. Block Copolymerization
Several different techniques for preparing block copolymers have been
developed, many of which use condensation reactions (next section). At
this point, our discussion will be limited to an application of anionic
polymerization. In the anionic polymerization of styrene described above, a reactive site remains at the end of the chain until it is quenched. The unquenched polymer has been termed a living polymer,
and if additional styrene or a different suitable monomer is added a
block polymer will form. This is illustrated for methyl methacrylate in
the following diagram.
Condensation Polymers
A large number of important and useful polymeric materials are not
formed by chain-growth processes involving reactive species such as
radicals, but proceed instead by conventional functional group
transformations of polyfunctional reactants. These polymerizations often
(but not always) occur with loss of a small byproduct, such as water,
and generally (but not always) combine two different components in an
alternating structure. The polyester Dacron and the polyamide Nylon 66,
shown here, are two examples of synthetic condensation polymers, also
known as step-growth polymers. In contrast to chain-growth
polymers, most of which grow by carbon-carbon bond formation,
step-growth polymers generally grow by carbon-heteroatom bond formation
(C-O & C-N in Dacron & Nylon respectively). Although polymers of
this kind might be considered to be alternating copolymers, the
repeating monomeric unit is usually defined as a combined moiety.
Examples of naturally occurring condensation polymers are cellulose, the
polypeptide chains of proteins, and poly(β-hydroxybutyric acid), a
polyester synthesized in large quantity by certain soil and water
bacteria. Formulas for these will be displayed below by clicking on the
diagram.
1. Characteristics of Condensation Polymers
Condensation polymers form more slowly than addition polymers, often
requiring heat, and they are generally lower in molecular weight. The
terminal functional groups on a chain remain active, so that groups of
shorter chains combine into longer chains in the late stages of
polymerization. The presence of polar functional groups on the chains
often enhances chain-chain attractions, particularly if these involve
hydrogen bonding, and thereby crystallinity and tensile strength. The
following examples of condensation polymers are illustrative.
Note that for commercial synthesis the carboxylic acid components may
actually be employed in the form of derivatives such as simple esters.
Also, the polymerization reactions for Nylon 6 and Spandex do not
proceed by elimination of water or other small molecules. Nevertheless,
the polymer clearly forms by a step-growth process.
Some Condensation Polymers
| Formula | Type | Components | Tg ºC | Tm ºC |
|---|
| ~[CO(CH2)4CO-OCH2CH2O]n~ | polyester | HO2C-(CH2)4-CO2H
HO-CH2CH2-OH | < 0 | 50 |
 | polyester
Dacron
Mylar | para HO2C-C6H4-CO2H
HO-CH2CH2-OH | 70 | 265 |
 | polyester | meta HO2C-C6H4-CO2H
HO-CH2CH2-OH | 50 | 240 |
 | polycarbonate
Lexan | (HO-C6H4-)2C(CH3)2
(Bisphenol A)
X2C=O
(X = OCH3 or Cl) | 150 | 267 |
~[CO(CH2)4CO-NH(CH2)6NH]n~ | polyamide
Nylon 66 | HO2C-(CH2)4-CO2H
H2N-(CH2)6-NH2 | 45 | 265 |
~[CO(CH2)5NH]n~ | polyamide
Nylon 6
Perlon |  | 53 | 223 |
 | polyamide
Kevlar | para HO2C-C6H4-CO2H
para H2N-C6H4-NH2 | --- | 500 |
 | polyamide
Nomex | meta HO2C-C6H4-CO2H
meta H2N-C6H4-NH2 | 273 | 390 |
 | polyurethane
Spandex | HOCH2CH2OH
 | 52 | --- |
The difference in T
g and T
m between the first
polyester (completely aliphatic) and the two nylon polyamides (5th &
6th entries) shows the effect of intra-chain hydrogen bonding on
crystallinity. The replacement of flexible alkylidene links with rigid
benzene rings also stiffens the polymer chain, leading to increased
crystalline character, as demonstrated for polyesters (entries 1, 2
&3) and polyamides (entries 5, 6, 7 & 8). The high T
g and T
m
values for the amorphous polymer Lexan are consistent with its
brilliant transparency and glass-like rigidity. Kevlar and Nomex are
extremely tough and resistant materials, which find use in bullet-proof
vests and fire resistant clothing.
Many polymers, both addition and condensation, are used as fibers
The chief methods of spinning synthetic polymers into fibers are from
melts or viscous solutions. Polyesters, polyamides and polyolefins are
usually spun from melts, provided the Tm is not too high. Polyacrylates suffer thermal degradation and are therefore spun from solution in a volatile solvent. Cold-drawing is an important physical treatment that improves the strength and appearance of these polymer fibers. At temperatures above Tg,
a thicker than desired fiber can be forcibly stretched to many times
its length; and in so doing the polymer chains become untangled, and
tend to align in a parallel fashion. This cold-drawing procedure
organizes randomly oriented crystalline domains, and also aligns
amorphous domains so they become more crystalline. In these cases, the
physically oriented morphology is stabilized and retained in the final
product. This contrasts with elastomeric polymers, for which the
stretched or aligned morphology is unstable relative to the amorphous
random coil morphology.
By clicking on the following diagram, a
cartoon of these changes will toggle from one extreme to the other. This
cold-drawing treatment may also be used to treat polymer films (e.g.
Mylar & Saran) as well as fibers.
Step-growth polymerization is also used for preparing a class of
adhesives and amorphous solids called epoxy resins. Here the covalent
bonding occurs by an SN2 reaction between a nucleophile,
usually an amine, and a terminal epoxide. In the following example, the
same bisphenol A intermediate used as a monomer for Lexan serves as a
difunctional scaffold to which the epoxide rings are attached. Bisphenol
A is prepared by the acid-catalyzed condensation of acetone with
phenol.
2.Thermosetting vs. Thermoplastic Polymers
Most of the polymers described above are classified as thermoplastic. This reflects the fact that above Tg
they may be shaped or pressed into molds, spun or cast from melts or
dissolved in suitable solvents for later fashioning. Because of their
high melting point and poor solubility in most solvents, Kevlar and
Nomex proved to be a challenge, but this was eventually solved.
Another group of polymers, characterized by a high degree of
cross-linking, resist deformation and solution once their final
morphology is achieved. Such polymers are usually prepared in molds that
yield the desired object. Because these polymers, once formed, cannot
be reshaped by heating, they are called thermosets .Partial
formulas for four of these will be shown below by clicking the
appropriate button. The initial display is of Bakelite, one of the first
completely synthetic plastics to see commercial use (circa 1910).
A natural resinous polymer called lignin has a cross-linked structure
similar to bakelite. Lignin is the amorphous matrix in which the
cellulose fibers of wood are oriented. Wood is a natural composite
material, nature's equivalent of fiberglass and carbon fiber composites.
A partial structure for lignin is shown here
The Age of Plastics
Historically, many eras were characterized by the materials that were
then important to human society (e.g. stone age, bronze age and iron
age). The 20th century has acquired several labels of this sort,
including the nuclear age and the oil age; however, the best name is likely the plastic age.
During this period no technological advancement, other than the
delivery of electrical power to every home, has impacted our lives more
than the widespread use of synthetic plastics in our clothes, dishes,
construction materials, automobiles, packaging, and toys, to name a few.
The development of materials that we now call plastics began with rayon
in 1891, continuing with Bakelite in 1907, polyethylene in 1933, Nylon
and Teflon in 1938, polypropylene in 1954, Kevlar in 1965, and is
continuing.
The many types of polymers that we lump together as plastics are,
in general, inexpensive, light weight, strong, durable and, when
desired, flexible. Plastics may be processed by extrusion,
injection-moulding, vacuum-forming, and compression, emerging as fibers,
thin sheets or objects of a specific shape. They may be colored as
desired and reinforced by glass or carbon fibers, and some may be
expanded into low density foams. Many modern adhesives involve the
formation of a plastic bonding substance. Plastics have replaced an
increasing number of natural substances. In the manufacture of piano
keys and billiard balls plastics have replaced ivory, assisting the
survival of the elephant. It is noteworthy that a synthetic fiber
manufacturing facility occupies a much smaller area of ground than would
be needed to produce an equal quantity of natural fibers, such as
cotton, wool or silk. With all these advantages it is not surprising
that much of what you see around you is plastic. Indeed, the low cost,
light weight, strength and design adaptability of plastics to meet a
variety of applications have resulted in strong year after year growth
in their production and use, which is likely to continue. Indeed, many
plastics are employed in disposable products meant only for a single
use.
The Law of Unintended Consequences
Successful
solutions to technological projects are often achieved by focusing on a
limited set of variables that are directly linked to a desired outcome.
However, nature often has a way of rewarding such success by exposing
unexpected problems generated "outside the box" of the defined project.
In the case of plastics, their advantageous durability and relative
low cost have resulted in serious environmental pollution as used items
and wrappings are casually discarded and replaced in a never ending
cycle. We see this every day on the streets and fields of our
neighborhoods, but the problem is far more dire. Charles Moore, an
American oceanographer, in 1997 discovered an enormous stew of trash,
estimated at nearly 100 million tons, floating in the Pacific Ocean
between San Francisco and Hawaii. Named the "Great Pacific Garbage
Patch", this stew of trash is composed largely (80%) of bits and pieces
of plastic that outweigh the plankton 6 : 1, in a region over twice the
size of Texas. Although some of this flotsam originates from ships at
sea, at least 80% comes from land generated trash. The information
provided here, and the illustration on the left, come from an article by
Susan Casey in
BestLife
Clock-wise circulation of currents driven by the global wind system and
constrained by surrounding continents form a vortex or gyre comparable
to a large whirlpool. Each major ocean basin has a large gyre in the
subtropical region, centered around 30º north and south latitude. The
North Atlantic Subtropical Gyre is known as the Sargasso Sea. The
larger North Pacific Subtropical Gyre, referred to as the doldrums, is
the convergence zone where plastic and other waste mixes together.
There are similar areas in the South Pacific, the North and South
Atlantic, and the Indian Ocean.
Aside from its disgusting aesthetic presence, the garbage patch is
representative of serious environmental and health problems. No one
knows how long it will take for some of these plastics to biodegrade, or
return to their component molecules. Persistent objects such as
six-pack rings and discarded nets trap sea animals. Smaller plastic
scraps are mistaken for food by sea birds; and are often found
undigested in the gut of dead birds. Nurdles, lentil-size pellets of
plastic, found in abundance where plastics are manufactured and
distributed, are dispersed by wind throughout the biosphere. They're
light enough to blow around like dust and to wash into harbors, storm
drains, and creeks. Escaped nurdles and other plastic litter migrate to
the ocean gyre largely from land. At places as remote as Rarotonga, in
the Cook Islands they're commonly found mixed with beach sand. Once in
the ocean, nurdles may absorb up to a million times the level of any
organic pollutants found in surrounding waters. Nurdles in the sea are
easily mistaken for fish eggs by creatures that would very much like to
have such a snack. Once inside the body of a bigeye tuna or a king
salmon, they become part of our food chain.
Recycling and Disposal
Most plastics crumble into ever-tinier fragments as they are exposed
to sunlight and the elements. Except for the small amount that's been
incinerated–and it's a very small amount–every bit of plastic ever made
still exists, unless the material's molecular structure is designed to
favor biodegradation. Unfortunately, cleaning up the garbage patch is
not a realistic option, and unless we change our disposal and recycling
habits, it will undoubtedly get bigger. One sensible solution would
require manufacturers to use natural biodegradable packaging materials
whenever possible, and consumers to conscientiously dispose of their
plastic waste. Thus, instead of consigning all plastic trash to a land
fill, some of it may provide energy by direct combustion, and some
converted for reuse as a substitute for virgin plastics. The latter is
particularly attractive since a majority of plastics are made from
petroleum, a diminishing resource with a volatile price.
The energy potential of plastic waste is relatively significant, ranging
from 10.2 to 30.7MJkgÃ1, suggesting application as an energy source
and temperature stabilizer in municipal incinerators, thermal power
plants and cement kilns. The use of plastic waste as a fuel source
would be an effective means of reducing landfill requirements while
recovering energy. This, however, depends on using appropriate
materials. Inadequate control of combustion, especially for plastics
containing chlorine, fluorine and bromine, constitutes a risk of
emitting toxic pollutants.
Whether used as fuels or a source of recycled plastic, plastic waste
must be separated into different categories. To this end, an
identification coding system was developed by the Society of the
Plastics Industry (SPI) in 1988, and is used internationally. This code,
shown on the right, is a set of symbols placed on plastics to identify
the polymer type, for the purpose of allowing efficient separation of
different polymer types for recycling. The abbreviations of the code
are explained in the following table.
| PETE | HDPE | V | LDPE |
|---|
polyethylene
terephthalate | high density
polyethylene | polyvinyl chloride | low density
polyethylene |
| |
| PP | PS | OTHER |
|---|
| polypropylene | polystyrene | polyesters, acrylics
polyamides, teflon etc. |
| |  |
Despite use of the recycling symbol in the coding of plastics, there
is consumer confusion about which plastics are readily recyclable. In
most communities throughout the United States, PETE and HDPE are the
only plastics collected in municipal recycling programs. However, some
regions are expanding the range of plastics collected as markets become
available. (Los Angeles, for example, recycles all clean plastics
numbered 1 through 7) In theory, most plastics are recyclable and some
types can be used in combination with others. In many instances,
however, there is an incompatibility between different types that
necessitates their effective separation. Since the plastics utilized in a
given manufacturing sector (e.g. electronics, automotive, etc.) is
generally limited to a few types, effective recycling is often best
achieved with targeted waste streams.
The plastic trash from most households, even with some user
separation, is a mixture of unidentified pieces. Recycling of such
mixtures is a challenging problem. A float/sink process has proven
useful as a first step. When placed in a medium of intermediate
density, particles of different densities separate-lower density
particles float while those of higher density sink. Various separation
media have been used, including water or water solutions of known
density (alcohol, NaCl, CaCl2 or ZnCl2).
As shown in the following table, the densities of common plastics differ
sufficiently to permit them to be discriminated in this fashion. The
cylindroconical cyclone device, shown on the right, provides a
continuous feed procedure in which the material to be separated is
pumped into the vessel at the same time as the separating media. Some
polymers, such as polystyrene and polyurethane, are commonly formed into
foamed solids that have a much lower density than the solid material.
Densities of Typical Plastics
| PE & PP | ABS & SAN
& nylons | PMM & acrylics
& polycarbonates | PETE & PVC
& Bakelite |
|---|
| 0.90-0.99 | 1.05-1.09 | 1.10-1.25 | 1.3-1.6 |
|
| PE = polyethylene & PP = polypropylene
ABS = acrylonitrile-butadiene-styrene copolymer
SAN = acrylonitrile-styrene copolymer
PMM = polymethyl methacrylate
PETE = polyethylene terephthalate
PVC = polyvinylchloride (rigid) |
| |  |
One serious problem in recycling is posed by the many additives found
in plastic waste. These include pigments for coloring, solid fibers in
composites, stabilizers and plasticizers. In the case of PETE (or
PET), which is commonly used for bottles, some waste may be mechanically
and thermally treated to produce low grade packaging materials and
fibers. To increase the value of recovered PETE it may be depolymerized
by superheated methanol into dimethyl terephthalate and ethylene
glycol. These chemicals are then purified and used to make virgin PETE.
Hydrocarbon polymers such as polyethylene and polypropylene may be
melted and extruded into pellets for reuse. However, the presence of
dyes or pigments limits the value of this product.
Biodegradable Polymers
Plastics derived from natural materials, such as cellulose, starch
and hydroxycarboxylic acids are more easily decomposed when exposed to
oxygen, water, soil organisms and sunlight than are most petroleum based
polymers. The glycoside linkages in polysaccharides and the ester
groups in polyesters represent points of attack by the enzymes of
microorganisms that facilitate their decomposition. Such biodegradable
materials can be composted, broken down and returned to the earth as
useful nutrients. However, it is important to recognize that proper
composting is necessary. Placing such materials in a landfill results
in a slower anaerobic decomposition, which produces methane, a
greenhouse gas.
Derivatives of cellulose, such as
cellulose acetate,
have long served for the manufacture of films and fibers. The most
useful acetate material is the diacetate, in which two thirds of the
cellulose hydroxyl groups have been esterified. Acetate fibers loose
strength when wet, and acetate clothing must be dry cleaned. The other
major polysaccharide, starch, is less robust than cellulose, but in
pelletized form it is now replacing polystyrene as a packing material.
The two natural polyesters that are finding increasing use as
replacements for petroleum based plastics are polylactide (PLA) and
polyhydroxyalkanoates (PHA), the latter most commonly as copolymers with
polyhydroxybutyrate (PHB). Structures for the these polymers and their
monomer precursors are shown below.
 | |  |
|---|
PLA is actually a polymer of lactic acid, but the dimeric lactide is
used as the precursor to avoid the water that would be formed in a
direct poly-esterification. Bacterial fermentation is used to produce
lactic acid from corn starch or cane sugar. After dimerization to the
lactide, ring-opening polymerization of the purified lactide is effected
using stannous compounds as catalysts. PLA can be processed like most
thermoplastics into fibers and films. In situations that require a high
level of impact strength, the toughness of PLA in its pristine state is
often insufficient. Blends of PLA with polymers such as ABS have good
form-stability and visual transparency, making them useful for low-end
packaging applications. PLA materials are currently used in a number of
biomedical applications, such as sutures, stents, dialysis media and
drug delivery devices. However, one of the drawbacks of polylactides for
biomedical applications is their brittleness.
Lactic acid has a chiral center, the (S)(+)-enantiomer being the
abundant natural form (L-lactic acid). Due to the chiral nature of
lactic acid, several distinct forms of polylactide exist.
Poly-L-lactide (PLLA) is the product resulting from polymerization of
(S,S)-lactide. PLLA has a crystallinity of around 37%, a glass
transition temperature between 50-80 ºC and a melting temperature
between 173-178 ºC. The melting temperature of PLLA can be increased
40-50 ºC and its heat deformation temperature can be increased from
approximately 60 ºC to up to 190 ºC by physically blending the polymer
with PDLA (poly-D-lactide). PDLA and PLLA form a highly regular
stereocomplex with increased crystallinity.
PHA (polyhydroxyalkanoates) are synthesized by microorganisms such as Alcaligenes eutrophus,
grown in a suitable medium and fed appropriate nutrients so that it
multiplies rapidly. Once the population has increased, the nutrient
composition is changed, forcing the micro-organism to synthesize PHA.
Harvested amounts of PHA from the organism can be as high as 80% of the
organism's dry weight. The simplest and most commonly occurring form of
PHA is poly (R-3-hydroxybutyrate), PHB or P(3HB)). Pure PHB, consisting
of 1000 to 30000 hydroxy acid units, is relatively brittle and stiff.
Depending upon the microorganism, many of which are genetically
engineered for this purpose, and the cultivation conditions, homo- or
copolyesters with different hydroxyalkanic acids may be generated. Such
copolymers may have improved physical properties compared with homo
P(3HB). Presently, these PHAs cost about twice as much as
petroleum-based plastics. An engineered switch-grass that grows PHA
inside its leaves and stems has also been created, offering the
possibility of avoiding some of the costs associated with large scale
bacterial fermentation.
In contrast to P(3HB), the polymer of 4-hydroxybutyrate, P(4HB), is
elastic and flexible with a higher tensile strength. Copolymers of
P(3HB) and P(4HB) are synthesized by Comamonas acidovarans. The
molecular weigh remains roughly the same (400,000-700,000 Da), but
thermal properties correlate with the ratio of these monomer units. The
mp decreases from 179 to 130 (or lower) with an increase in 4HB, and as
4HB increases from 0% to 100% the Tg decreases from 4 to -46.
4-Hydroxybutyrate (4HB) is produced from 1,4-butanediol by
microorganisms such as Aeromonas hydrophila, Escherichia coli , or Pseudomonas putida.
Fermentation broth containing 4HB has then been used for the
production of the homopolymer P(4HB), as well as copolymers with P(3HB),
[P(3HB-4HB)]. The following table lists some of the properties of these
homo-polymers and co-polymers.
Properties of Some Polymers
| Polymer | Tm ºC | Tg ºC | %Crystallinity | Tensile
Strength |
|---|
| P(3HB) | 179 | 4 | 70 | 40 MPa |
| P(4HB) | 53 | -47 | 53 | 100 |
co-Polymer
3HB-20%3HV | 145 | -1 | 50 | 32 |
co-Polymer
3HB-7%3HD | 133 | -8 | > 50 | 17 |
| isotactic-PP | 176 | 0 | > 50 | 40 |
| LDPE | 110 | -100 | < 50 | 10 |
|
|---|
| 3HV = 3-hydroxyvalerate, 3HD = 3-hydroxydecanoate |
It remains an open question whether it's more energy and cost
efficient to use biodegradable plastic or to recycle petroleum-based
plastic. There is little doubt, however, that biodegradable materials
lead to less environmental pollution when randomly discarded after use,
as is often the case.
