Polymer Important Information
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.
1
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 Mw is necessarily skewed to higher values, and
is always greater than Mn. As the weight dispersion of molecules in a sample narrows, M w
approaches Mn, and in the unlikely case that all the polymer molecules have identical weights (a
pure mono-disperse sample), the ratio Mw / Mn becomes unity.
The influence of different mass distributions on Mn and Mw may be examined with the aid
2
of a simple mass calculator.
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.
Polymers & Its Application Area:Name(s)
Formula
Monomer
Properties
Application Area
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
–[CH2-CH(CH3)]n–
propylene
CH2=CHCH3
atactic: soft,
elastic solid
isotactic: hard,
strong solid
Similar to LDPE
carpet, upholstery
Transparent ,
solid
Bottles for soft
Drinks and other
Beverages.
vinyl chloride
CH2=CHCl
strong rigid solid
Pipes, siding,
flooring
Poly(vinylidene
–(CH2-CCl2)n–
chloride)
(Saran A)
vinylidene chloride
CH2=CCl2
dense, highmelting 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)
–(CH2-CHCN)n–
acrylonitrile
CH2=CHCN
high-melting
solid
soluble in
organic solvents
Rugs, blankets
clothing
Polypropylene
(PP) different
grades
ethylene
CH2=CH2 &
Terephthlic acid
Poly(ethylene
terephthlate)
(PET)
n
Poly(vinyl
chloride)
(PVC)
Polyacrylonitri
le
(PAN, Orlon,
Acrilan)
–(CH2-CHCl)n–
3
Polytetrafluoro
–(CF2-CF2)n–
ethylene
(PTFE, Teflon)
tetrafluoroethylene
CF2=CF2
Poly(methyl
methacrylate)
(PMMA,
Lucite,
Plexiglas)
–[CH2C(CH3)CO2CH3]n–
methyl methacrylate
hard, transparent
CH2=C(CH3)CO2CH3 solid
Lighting covers,
signs skylights
Poly(vinyl
acetate)
(PVAc)
–(CH2CHOCOCH3)n–
vinyl acetate
CH2=CHOCOCH3
soft, sticky solid
Latex paints,
adhesives
cisPolyisoprene
natural rubber
isoprene
–[CH2-CH=C(CH3)CH2=CHCH2]n–
C(CH3)=CH2
soft, sticky solid
Requires
vulcanization
for practical use
tough, rubbery
solid
Synthetic rubber
oil resistant
Tough, hard
engineering
plastic
Fibers, molded
objects
Polychloropren
–[CH2e (cis + trans)
CH=CCl-CH2]n–
(Neoprene)
Polyamides(Ny
lon)
Polyesters
(Dacron, Mylar,
Fortrel)
Polyesters
(Glyptal resin)
Polyesters
(Casting resin)
chloroprene
CH2=CH-CCl=CH2
resistant, smooth
solid
Non-stick surfaces
electrical insulation
Linear polyesters,
fibers, recording
tape
Cross-linked
polyester, paints
Cross-linked with
styrene and
benzoylperoxide,
fiberglass boat
resin, casting resin
4
Phenolformaldehyde
(Bakelite)
Mixed with fillers,
Molded electrical
cases, adhesives,
laminates, varnishes
Cellulose
acetate
(cellulose is a
polymer of
glucose)
Photographic film
Silicones
Water-repellent
coatings,
Temperatureresistant
Fluids and rubber
Polyurethanes
Foams, rigid and
Flexible, fibers
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.
5
• 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 C5H8 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 C6H12O6 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
6
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 crosslinked 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.
7
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)
_
_
_
80
90
95
_
105
_
110
100
10
110
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.
8
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.
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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.
10
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.
11
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
12
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).
13
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
14
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~.
15
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~.
Graft Copolymers
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.
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
16
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.
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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 chainchain 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.
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 70
HO-CH2CH2-OH
265
18
~[CO(CH2)4CO-NH(CH2)6NH]n~
~[CO(CH2)5NH]n~
polyester
meta
HO2C-C6H4-CO2H 50
HO-CH2CH2-OH
240
polycarbonate
Lexan
(HO-C6H4-)2C(CH3)2
(Bisphenol
X2C=O
(X = OCH3 or Cl)
150
267
polyamide
Nylon 66
HO2C-(CH2)4-CO2H
H2N-(CH2)6-NH2
45
265
53
223
polyamide
Kevlar
para
HO2C-C6H4-CO2H --para H2N-C6H4-NH2
500
polyamide
Nomex
meta
HO2C-C6H4-CO2H 273
meta H2N-C6H4-NH2
390
polyurethane
Spandex
HOCH2CH2OH
---
polyamide
Nylon
Perlon
A)
6
52
The difference in Tg and Tm 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 Tg and Tm 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.
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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.
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.
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
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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 lignan is shown here
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