organic chemistry chapter 22 polymer chemistry

Richard F. Daley and Sally J. Daley
www.ochem4free.com

Organic

Chemistry

Chapter 22
Polymer Chemistry

22.1 Structural Characteristics of Polymers 1138
22.2 Polymer Nomenclature 1141
22.3 Types of Polymerization Reactions 1144
22.4 Chain-Growth Polymerization 1146
Synthesis of Poly(vinyl acetate) 1155
Sidebar - Natural Rubber 1155
22.5 Controlling Stereochemistry in Vinyl Polymers 1157
22.6 Nonvinyl Chain-Growth Polymerization 1160
22.7 Step-Growth Polymerization 1163
Synthesis of Poly(ethylene terephthalate) 1165
Sidebar - Plastic Recycling 1167
22.8 Copolymers 1169
Sidebar - Plasticizers 1172
22.9 Cross-Linked Polymers 1173
Key Ideas from Chapter 22 1178

Organic Chemistry - Ch 22 1134 Daley & Daley























Copyright 1996-2001 by Richard F. Daley & Sally J. Daley
All Rights Reserved.

No part of this publication may be reproduced, stored in a retrieval system, or
transmitted in any form or by any means, electronic, mechanical, photocopying,
recording, or otherwise, without the prior written permission of the copyright

holder.
www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1135 Daley & Daley

Chapter 22

Polymer Chemistry




Chapter Outline

22.1 Structural Characteristics of Polymers
A look at the various types of polymers
22.2 Polymer Nomenclature
An introduction to naming polymers
22.3 Types of Polymerization Reactions
Categories of polymer forming reactions
22.4 Chain-Growth Polymerization
The mechanism for the formation of vinyl chain-growth polymers
22.5 Controlling Stereochemistry in Vinyl Polymers
The Ziegler-Natta polymerization catalyst for vinyl polymers
22.6 Nonvinyl Chain-Growth Polymerization
Mechanisms for chain-growth polymer formation for nonvinyl
polymers
22.7 Step-Growth Polymerization
The mechanisms for the formation of some representative step-
growth polymers
22.8 Copolymers

An examination of the various types of copolymers
22.9 Cross-Linked Polymers
Types of cross-linking that occur in polymers

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1136 Daley & Daley

Objectives

✔ Recognize the various types of polymer structures
✔ Know how to name both source-based and nonvinyl polymers
✔ Recognize the monomers that produce polymers
✔ Write the mechanisms for cationic, anionic, and radical
polymerization of vinyl monomers
✔ Know the stereochemical types of vinyl polymers
✔ Write the mechanism for representative non-vinyl chain-growth
polymerizations
✔ Recognize the similarity of the step-growth polymerization
reactions to those studied in earlier chapters
✔ Know the types of copolymers
✔ Recognize how cross-linking occurs in epoxy polymers



Observation is a passive science, experimentation
an active science.
— Claude Bernard


A

poly
of sm
monomers. Polyme
A polymer is a molecule
made up of many
smaller units called
monomers.
mer is a large molecule that consists of a number
aller repeating units made from molecules called
rs are formed by some repetitive reaction that
adds these monomer units one-by-one to the growing chain of the
polymer. The process of converting the monomer units to a polymer is
called polymerization.

A reaction forming a
polymer is a
polymerization
reaction.
Polymers, sometimes called macromolecules, affect your life in
many ways. For example, plastics are synthetic polymers, and they
are all around you. Industry makes plastics into such things as fibers,
structural materials, and protective films. Except for fuels, more
plastics are manufactured in the world than any other organic
material. One-third of all industrial chemists work in the polymer
industry.
Another way that polymers affect your life is in the natural
chemistry of the life processes. Proteins and enzymes are polyamide
polymers. Proteins are an important part of the structure of all

animals, and enzymes catalyze the chemical processes that make
www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1137 Daley & Daley
those bodies function. Cellulose and starches are polymers of
individual sugar molecules. Cellulose is the structural material of
plants, and starches are the energy storage medium for plants. Both
RNA and DNA are polymers of individual nucleic acids. These two
classes of molecules control the genetic make-up of your body. This
chapter focuses primarily on synthetic polymers. Chapters 24 and 25
cover some types of natural macromolecules.
Generally, the size and stereochemistry of the polymer
molecule determine the properties of that molecule. This chapter
examines how those features determine a polymer's physical
properties. This chapter also discusses polymer synthesis.

22.1 Structural Characteristics of Polymers

The composition of polymers is a sequence of repeating
monomer units that are covalently bonded together. The reactions
that connect these repeating units can involve any of the functional
groups discussed previously. The functional group on the repeating
unit provides the reactive site for the connecting reaction.
The repeating units of polymers have a variety of possible
structures. When all the repeating units in a particular polymer have
the same structure, that polymer is called a homopolymer.
In a homopolymer, all
the repeating units are
identical.

n A AAAAAAAA A

n
or


An example of a homopolymer is polyvinyl chloride.

or
n
CH
2
CH
Cl
CH
2
CH
Cl
CH
2
CHCH
2
CHCH
2
CHCH
2
CHCH
2
CH
Cl Cl Cl Cl Cl
Polyvinyl chloride

When different repeating units make up the polymer chain, the
polymer is called a copolymer. There are three types of copolymers:
1) alternating copolymers, 2) block copolymers, and 3) random
copolymers. If you designate the repeating units as A and B, the
following illustration shows representations of these three types of
copolymers.
A copolymer contains
more than one type of
monomer unit.

Alternating copolymer
ABABABAB
n A + n B

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1138 Daley & Daley
m A + n B AAAAABBB
Block copolymer

Random copolymer
ABBBAABB
m A + n B


An example of the alternating copolymer is Nylon 66.

n H
2
N(CH

2
)
6
NH
2
n ClC(CH
2
)
4
CCl
OO
NH(CH
2
)
6
NHC(CH
2
)
4
C
OO
+
Nylon 66
n


All the above examples are linear polymers. Some polymers
contain additional covalent links to repeating units at various
locations on the backbone. Such polymers are called branched
polymers. Linear or branched polymer chains can be connected by

some additional covalent links. These polymers are called cross-
linked polymers.
A linear polymer is a
molecule with a series
of connected repeating
units with no
branching.

A branched polymer
has bonds branching
from the backbone of a
linear polymer.

Cross-linked polymers
are linear polymer
molecules joined by a
branching connection.

Polymer End Groups
The structural drawings of all the polymers discussed in this chapter show a bond
extending out of the brackets that enclose the repeating unit. The functional groups
on the ends of the polymer chains are left unspecified because the end groups are an
insignificant portion of the total chain. These groups have very little effect on the
physical properties of the polymer. In a given sample of polymer, a variety of end
groups may be present depending on how the polymer was synthesized.

The physical properties of a specific polymer are the result of
two molecular characteristics: 1) the length of the molecule and 2) the
functional group associated with the repeating units. To determine the
length of a polymer chain, chemists use the molecular weight of the

polymer. Different polymers of the same chain length have similar
physical properties regardless of the functional group present unless
the functional group can hydrogen bond or disrupt the intermolecular
van der Waals forces. These two interactions are more important in
determining the physical properties of the polymer than is the
molecular weight.
The physical properties of interest to a consumer are those that
show how well the polymer performs in response to various stresses.
These responses include compressive, flexural, and tensile
strength, as well as impact resistance. Compressive strength is a
measure of how much compression a sample can tolerate before it
Compressive, flexural,
and tensile strength, as
well as impact
resistance are
measures of the
mechanical strength of
a polymer.
www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1139 Daley & Daley
fails. Flexural strength is a measure of resistance to breaking or
snapping when the sample is bent. Tensile strength is a measure of
resistance to stretching. Impact resistance is a measure of how well a
sample resists damage from a sudden impact.
Some polymers have the characteristics of a crystalline solid.
Crystalline polymers have chains that tend to orient themselves in
a regular way, similar to the way the molecules in a crystalline solid
orient themselves. The chains are held together in this regular
orientation by hydrogen bonds or dipole alignments. These polymers
generally have characteristic melting points, are strong, and

nonelastic. Linear polyethylene is an example of such a crystalline
polymer.
The molecules of a
crystalline polymer line
up in a regular way
similar to the smaller
molecules in a crystal.

Orientation of the chains of a crystalline polymer


Amorphous polymers are similar to glassy solids.
Amorphous polymers do not have a characteristic melting point.
Instead, they often make an indistinct transition from the glassy solid
to a viscous liquid called the glass transition temperature. These
polymers do not have a regular orientation in the solid state.
Amorphous polymers are generally not particularly strong and tend to
be quite elastic. Rubber is an example of an amorphous polymer.
The molecules in an
amorphous polymer do
not have any preferred
alignment.

A glassy solid is a solid
that is often hard and
brittle.


Orientation of the chains of an amorphous polymer

The glass transition
temperature is the
temperature at which a
polymer transforms
from a glass to a
viscous liquid

Many polymers are neither completely crystalline nor
completely amorphous. Segments of the chains lie parallel to each
other in regions called crystallites. Other segments of the chains are
not ordered. Such polymers are called semicrystalline polymers.
These polymers have much of the strength of a crystalline polymer
along with much of the flexibility of an amorphous polymer. Nylon is
an example of a semicrystalline polymer.
Ordered regions among
the amorphous regions
in a semicrystalline
polymer are called
crystallites.

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1140 Daley & Daley
Semicrystalline
polymers are polymers
with regions of
disorder and
crystallites.
Orientation of the chains of a semicrystalline polymer
Crystalline regions
(Crystallites)

Amorphous region


Crystalline polymers generally are opaque, but amorphous
polymers generally are transparent. Thus, increasing the number of
crystallites in a polymer normally reduces the transparency of the
polymer. An example is polystyrene. Amorphous polystyrene is found
in a number of transparent, brittle "plastic" items, such as drinking
cups.

22.2 Polymer Nomenclature

The IUPAC has proposed some logical rules for naming
polymers, but polymer chemists seldom use them because many
polymers are so branched and cross-linked that their names are very
complex. Thus, this section is only an introduction to the
fundamentals of naming polymers. Many polymer chemists use source
based naming. They name the monomer then add the poly- prefix. A
complication with this method is that chemists use the common names
of the monomers more often than their IUPAC names. For example,
the common name for ethenylbenzene is styrene, so chemists call the
polymer of styrene polystyrene. In addition to using common names,
chemists refer to many polymers by their trade names. They use the
trade name Teflon
®
more frequently than the IUPAC name of
polytetrafluroethylene.
Vinyl polymers are among the easiest polymers to name when
following the common name method. Simply use the monomer's
common name with the prefix poly If the monomer's name includes

more than one word, or if a letter or number precedes the name,
enclose the monomer's name in parentheses. Thus, the polymer of 1-
pentene is poly(1-pentene). Table 22.1 lists a few common names for
vinyl polymers.

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1141 Daley & Daley
Monomer Monomer Name Polymer Polymer Name
CH
2
CH
2

Ethylene CH
2
CH
2
n

Polyethylene
CH
2
CHCH
3

Propylene
n
CH
2
CH

CH
3

Polypropylene
CH
2
CHCN

Acrylonitrile
n
CH
2
CH
CN

Polyacrylonitrile
CH
2
C
CH
3
COOCH
3


Methyl methacrylate
CH
2
C
CH

3
COOCH
3
n



Poly(methyl methacrylate)
CH
2
CH
O
OCCH
3

Vinyl acetate
n
CH
2
CH
OCCH
3
O

Poly(vinyl acetate)
CH
2
CHCl

Vinyl chloride

n
CH
2
CH
Cl

Poly(vinyl chloride)
CH
2
CH

Styrene
n
CH
2
CH

Polystyrene
CF
2
CF
2

Tetrafluoroethylene
n
CF
2
CF
2

Polytetrafluoroethylene

Table 22.1. Representative names for some vinyl polymers.

Nonvinyl polymers generally have some atom other than
carbon as a part of the backbone of the polymer. As with the vinyl
polymers, the nomenclature of these polymers is often source based or
based on their trade names. Nylon is an example of a family of
compounds that chemists call by their trade names. To name them,
use the word Nylon followed by a number for the number of carbons in
the monomer(s) for that Nylon. Nylon 6 is a polymer made from
monomers that consist of a single cyclic amide with six carbons. The
monomers in Nylon 68 are diamines with six carbons and dicarboxylic
acids with eight carbons.

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1142 Daley & Daley
Nylon 68
From an 8 carbon carboxylic acidFrom a 6 carbon amine
n
NHCH
2
CH
2
CH
2
CH
2
CH
2

CH
2
NHCCH
2
CH
2
CH
2
CH
2
CH
2
CH
2
C
OO


Table 22.2 lists a few representative nonvinyl polymer names.

Monomer Monomer Name Polymer Polymer Name
O
CH
2
CH
2


Ethylene oxide

n
CH
2
CH
2
O


Poly(ethylene oxide)
O
O



Propiolactone
n
OCH
2
CH
2
C
O



Polypropiolactone
HOCH
2
CH
2

OH
+
COHHOC
OO

Ethylene glycol
+
Terephthalic acid

COCH
2
CH
2
OC
OO
n



Poly(ethylene terephthalate)
NH
O




Caprolactam

n
NH(CH

2
)
5
C
O




Polycaprolactam
Nylon 6
H
2
N(CH
2
)
6
NH
2


+

HOOC(CH
2
)
6
COOH
Hexamethylenedi-
amine

+
Sebacic acid
NH(CH
2
)
6
NHC(CH
2
)
6
C
OO
n

Poly(hexamethylene
sebacate)
Nylon 68

Table 22.2. Representative names for some nonvinyl polymers.

Exercise 22.1

Name the following polymers and draw structural formulas for the
monomers that form each of the polymers.

a)

n
CH
2

CH
CH
2
CH
3

b)

n
OCH
2
CH
2
CH
2
C
O

c)

CH
2
CH
OH
n

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1143 Daley & Daley
d)

n
e)

CHCH
CH
2
CH
3
CH
3
n

f)

NH(CH
2
)
6
NHC(CH
2
)
4
C
OO
n


Sample solution
b) Polybutyrolactone

O
O


22.3 Types of Polymerization Reactions

Chemists classify polymerization reactions in terms of their
reaction mechanisms. There are two types of polymerization reaction
mechanisms: 1) chain-growth polymerizations and 2) step-
growth polymerizations.
Chain-growth
polymerization adds
monomer units with
the same functional
group to the chain.

In step-growth
polymerization, one
functional group at the
end of the chain
requires a different
functional group to
react and lengthen the
chain.
A chain-growth polymerization begins when an initiator reacts
with a monomer molecule to create a reactive site. This reactive site
then reacts with another monomer molecule joining the two, as well as
creating a new reactive site. Monomer molecules continue adding to
the reactive site and forming a new reactive site as long as monomer
molecules are available or until some termination reaction occurs. A

polymer that forms via a chain-growth polymerization usually forms
from one monomer or group of monomers with the same reactive
functional group. The formation of polyethylene from ethylene in the
presence of an initiator is an example of a chain-growth
polymerization.

Initiator
CH
2
CH
2
n
CH
2
CH
2
Polyethylene


The mechanism for a chain-growth polymerization is often a
radical reaction. As you read through the description of the chain-
growth polymerization above, you may have recognized the initiation,
propagation, and termination steps from the radical reaction that you
studied in Chapter 21. However, not all initiators generate radicals in
the initiation and propagation steps; instead, many generate cationic
or anionic intermediates. The important consideration is not whether
the reaction proceeds as a radical reaction or generates a cationic or
www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1144 Daley & Daley
anionic intermediate, but that the initiator makes the chain reaction

possible.
A step-growth polymerization begins with a mixture of
monomers that contain different functional groups. Although the
mixture can contain numerous different functional groups, for
simplicity look at how the reaction proceeds with a mixture of
monomers that contains only two different functional groups. Each
functional group type can react with the other type in the mixture but
not with itself. The chain begins when one monomer joins with
another monomer containing the other functional group type. The first
monomer then reacts with the end of this two unit chain. Unlike the
chain-growth polymerization, a step-growth polymerization usually
does not have a radical, cation, or anion at the end of the growing
chain. Instead, the chain grows by the reaction of the functional group.
The formation of Nylon 66 is an example of a step-growth
polymerization.

NHCH
2
CH
2
CH
2
CH
2
CH
2
CH
2
NHCCH
2

CH
2
CH
2
CH
2
C
OO
H
2
NCH
2
CH
2
CH
2
CH
2
CH
2
CH
2
NH
2
+ HOCCH
2
CH
2
CH
2

CH
2
COH
OO
n
Nylon 66


The formation of Nylon 66 has two steps in the step-growth
mechanism. A monomer containing the diamine and another monomer
containing the dicarboxylic acid react joining the two. Either of the
following two reactions then occurs. The diamine reacts with the
carboxylic end of the two unit polymer, or the dicarboxylic acid reacts
with the amine end. Subsequent reactions occur with the proper
reagent reacting with the appropriate end of the growing chain.

Step 1
H
2
NCH
2
CH
2
CH
2
CH
2
CH
2
CH

2
NH
2
+ HOCCH
2
CH
2
CH
2
CH
2
COH
OO
H
2
NCH
2
CH
2
CH
2
CH
2
CH
2
CH
2
NHCCH
2
CH

2
CH
2
CH
2
COH
OO


Step 2
www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1145 Daley & Daley
H
2
NCH
2
CH
2
CH
2
CH
2
CH
2
CH
2
NHCCH
2
CH
2

CH
2
CH
2
COH
OO
H
2
NCH
2
CH
2
CH
2
CH
2
CH
2
CH
2
NH
2
H
2
NCH
2
CH
2
CH
2

CH
2
CH
2
CH
2
NHCCH
2
CH
2
CH
2
CH
2
CNHCH
2
CH
2
CH
2
CH
2
CH
2
CHNH
2
OO


OR

HOCCH
2
CH
2
CH
2
CH
2
COH
OO
H
2
NCH
2
CH
2
CH
2
CH
2
CH
2
CH
2
NHCCH
2
CH
2
CH

2
CH
2
COH
OO
HOCCH
2
CH
2
CH
2
CH
2
CNHCH
2
CH
2
CH
2
CH
2
CH
2
CH
2
NHCCH
2
CH
2
CH

2
CH
2
COH
OOOO


Another possible second step that the step-growth mechanism
can follow is the reaction of the amine end of one chain with the
carboxylic acid end of another chain. When this occurs, instead of all
the polymer molecules growing steadily at the same rate, some grow
much more rapidly. This way of joining results in a wide variety of
possible molecular weights, making it more difficult to predict the
characteristics of the polymer in comparison to a reaction mixture in
which each chain grows at a similar pace.

22.4 Chain-Growth Polymerization

Chain-growth polymers form from radicals, cations, or anions.
Because a wide variety of monomers lend themselves so readily to the
formation of radicals, most chain-growth polymerization reactions
proceed via radical intermediates. Some monomers do polymerize with
ionic initiators, but that number is far fewer than those that
polymerize with a radical initiator. Chain-growth polymerization
reactions usually form vinyl polymers. One such vinyl polymer is
polystyrene. Polystyrene polymerizes under radical, cationic, and
anionic initiation. This section examines all three.
The radical polymerization of styrene uses benzoyl peroxide as
the initiator. As with the radical reactions you studied in Chapter 21,
a radical polymerization follows three steps: 1) the initiation step, 2)

the propagation step, and 3) the termination step. In the first reaction
of the initiation step, the benzoyl peroxide undergoes a homolytic
cleavage to form two benzoyloxy radicals. In the second reaction of the
initiation step, each benzoyloxy radical reacts with a molecule of
styrene to form a benzylic radical.
www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1146 Daley & Daley

Initiation step
••
••
••
••
••
••
••
••
••
•
•
•
•
•
•
•
O
O
O
O
O

O

•
•
••
••
••
•
O
O
••
••
••
•
•
•
O
O


In the propagation step, the chain begins to grow as the
benzylic radical reacts with a molecule of styrene to add the styrene
and to form a new benzylic radical. This new benzylic radical then
reacts with another molecule of styrene adding the styrene and
forming another new radical. This process continues many times as
the polymer chain lengthens. Once started, the rate of reaction is
relatively high for a chain formation. Approximately 1500-1600
styrene monomer units add to the growing chain each second.

Propagation step

•
O
O
••
••
••
•
•
•
•
••
••
••
•
O
O


The propagation reaction continues until it reacts in a
termination reaction or until it uses up all the monomer molecules in
the reaction mixture. The most common termination reaction
combines two radicals from two growing chains to form a longer chain.
This reaction terminates both chain reactions because the reaction
forms no new radical.

Termination step

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1147 Daley & Daley

•
m
n
•
m
n


Another reaction that occurs in radical chain-growth
polymerization is a chain-transfer reaction. In a chain-transfer
reaction, the end of a growing chain abstracts a hydrogen from the
benzylic position of another chain. This abstraction creates a new
radical in the middle of the chain.
In a chain-transfer
reaction, one growing
polymer chain
abstracts a hydrogen
atom from another
polymer chain.

n
•
n
•
x
y
H
y
x
+

H


This new radical site serves as a reaction site for additional monomer
molecules to branch off the main polymer chain.

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1148 Daley & Daley
x
y
•
y
x
•


Exercise 22.2

A polymerization reaction involving 1,3-butadiene and a radical
initiator forms two different repeating polymer units. Account for the
formation of these two units.

CH
2
CH CH CH
2
CH
2
CH
CH CH

2


Cationic polymerization is similar to radical polymerization in
that both react with the initiator to form a reactive site on the same
carbon of the styrene. To run cationic polymerization reactions,
chemists use strong Brønsted-Lowry acids, as well as Lewis acids.
When they use a Lewis acid, they must also use some hydrogen halide
and water. The requirement for the hydrogen halide and water
suggests an involvement of a proton acid in the reaction.
Again using the polymerization of styrene as an example, look
at the mechanism of a cationic polymerization. The initiation step in a
cationic polymerization of styrene adds a proton to the double bond of
the styrene to form a carbocation.

Initiation step

H
H


In the propagation step, the carbocation reacts with a molecule of
styrene to form a new carbocation. This step repeats itself until the
www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1149 Daley & Daley
reaction either runs out of reactants or until the polymers react in a
termination step.

Propagation step

n
H
Repeat
H
H


The reaction can terminate by losing a proton, by reacting with a
nucleophile, or by the carbocation removing a hydride from another
polymer molecule. All three of these steps are typical carbocation
termination reactions.

Termination step

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1150 Daley & Daley
•
•
-H
Nu
HR
H
H
n
H
Nu
n
H
n
H

n


The removal of the hydride from another polymer chain can lead to
branching of the polymer chain similar to the branching in the radical
reaction.

Exercise 22.3

What product would you expect in a cationic polymerization reaction
of styrene if you added 1% 1,4-divinylbenzene to the reaction mixture?
What difference would you expect this small amount of added material
to make in the properties of the polymer?

The third type of chain-growth polymerization reaction
presented in this section is an anionic initiation. Alkali metals or
organometallic compounds catalyze some polymerization reactions.
The reactive species in these reactions is a carbanion. Thus, the
reaction is called an anionic polymerization reaction. Styrene is one of
those compounds that polymerizes with an anionic initiator. To run an
anionic polymerization with styrene, the chemist usually prepares the
initiator before adding the styrene to the reaction mixture.
Anionic initiation
requires a strong base
to initiate the polymer
chain growth.
A common initiator in an anionic polymerization is sodium
naphthalide—the radical anion of naphthalene. Chemists prepare
sodium naphthalide by reacting sodium with naphthalene in THF
solvent.

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1151 Daley & Daley

Na
•
Na
THF


After preparing the sodium naphthalide, they add the styrene. The
two rapidly react, transferring the radical electron in the sodium
naphthalide to the styrene to form a radical anion of styrene.

••
•


The radical anion is a very reactive species, so it immediately
combines with another radical anion to form a dianion.

Initiation step
•
•
•
•
•
••
••
•

This dianion, usually called distyryl sodium, is the starting point for
the polymer formation. As the polymer forms, the anion reacts with
additional molecules of styrene at each end of the distyryl sodium to
form a polymeric dianion.

Propagation step

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1152 Daley & Daley
•
•
•
•
•
•
•
•
••
Repeat
xx
••


Each polymer chain grows at a remarkably similar rate. Thus,
the process produces the polymeric dianions within a narrow range of
molecular weights. Their actual molecular weight depends on the ratio
of the amount of initiator and of styrene present in the reaction
mixture, as the amount of initiator determines the actual number of
polymers that form. The reaction has no important termination

reactions. The dianion is relatively stable until a source of protons is
added to the solution.
www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1153 Daley & Daley

••
xx
xx
HCl
CH
3
OH
••
H
H


Exercise 22.4

Predict the order of reactivity of styrene, p-chlorostyrene, and p-
methoxystyrene in radical, cationic, and anionic polymerization
reactions.

Polystyrene has a number of properties that make it a valuable
industrial material. It is an amorphous polymer. When made in a
radical polymerization reaction, polystyrene can form with molecular
weights in excess of two million, although most commercial
polystyrene has molecular weights under a million. The glass
transition temperature for polystyrenes is above room temperature.
Polystyrene is also a good thermoplastic material. It can be melted

and remolded repeatedly, allowing ready recycling of the waste
materials from a molding or from a discarded molded object. A
thermoplastic material is different from a thermosetting material. A
thermosetting material cannot be melted and remolded.
A thermoplastic
material readily melts
to allow remolding into
desired shapes.
A thermosetting
material reacts when
heated, thus, it forms a
new polymer that does
not melt and remold
again.
www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1154 Daley & Daley

Uses of Polystyrene
Polystyrene is a plastic that has a number of different industrial and consumer uses.
One use involves molding it into cases for televisions and radios. In another process,
manufacturers mix a low boiling material with the polystyrene, then heat the
mixture. When the polymer softens, the low boiling compound vaporizes and produces
a foam. Because the foams do not conduct heat well, they work well when molded into
disposable cups for hot drinks and ice chests. Since the foam is quite rigid, it also
makes excellent insulation for construction.

Synthesis of Poly(vinyl acetate)

(92%)
CH

2
CH
3
COCH
O
O
CH
3
CO
n
(PhCO)
2
O


Fit a 500 mL reaction kettle with a reflux condenser, mechanical stirrer,
thermometer, and addition funnel. Place 80 mL of freshly distilled water in the kettle.
Dissolve 80 mg of poly(styrene-co-maleic anhydride) (m.w. about 2000 with 67%
styrene content) in the minimum quantity of water and exactly neutralize with 1M
sodium hydroxide solution. Add this solution to the water. Prepare a solution of 86g of
vinyl acetate and 150 mg of benzoyl peroxide. Add 10 mL of the vinyl acetate solution
to the water and warm to 80
o
C. Once the exothermic polymerization reaction has
begun, maintain the reaction at 80
o
C by either heating or cooling as required. Add the
remaining vinyl acetate solution during a two hour period. Continue heating the
reaction mixture for an additional 2 hours. While continuing to use the mechanical
stirrer, steam distill the reaction mixture until the distillate contains no more vinyl

acetate monomer. Cool, with agitation, to 4
o
C. Filter the polymer beads from the
solution and wash repeatedly with water. Dry the beads at 30
o
C under reduced
pressure. The final product has a molecular weight of about 1,000,000. Yield of dry
polymer is 79.1 g.

Discussion Questions

1. Write a mechanism for the formation of poly(vinyl acetate).
2. This process is called suspension polymerization. What is the purpose of the
sodium salt of the poly(styrene-co-maleic anhydride) polymer in the reaction
mixture?

[Sidebar]

Natural Rubber

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1155 Daley & Daley
Rubber is the most important and widely used natural
polymer. A variety of plants in the tropical regions of the world
produce rubber, but the major source of commercial rubber is the
Hevea brasiliensis tree originally found in Brazil. The Hevea
brasiliensis tree is now mostly grown in Southeast Asia. The Mayans
also obtained rubber from this tree. They called it caoutchouc, or “the
weeping tree.” Joseph Priestly, the noted 18th century chemist, coined
the name rubber when he found that caoutchouc rubbed out pencil

marks.
Hevea rubber is obtained by tapping the rubber tree and
collecting the viscous liquid, called latex, that flows out. Quantities of
latex are obtained by tapping each tree every other day. Raw latex
contains about 32 - 35% rubber and 5% other organic compounds such
as sugars, fats, and steroids.
Rubber is a polymer made up of 2-methyl-1,3-butadiene
(isoprene) repeating units.

(Isoprene)
2-Methyl-1,3-butadiene


The polymer contains cis repeating units and has a molecular weight
ranging from 100,000 up to 1,000,000.

n
Rubber


A related polymer, called gutta percha, is found in trees of the
genus Dichopsis, which is native to Southeast Asia. Gutta percha has
a structure with trans double bonds and a much lower molecular
weight. A typical sample of gutta percha has a molecular weight of
about 7,000. Gutta percha is not widely used today, but has been used
in a variety of applications from golf ball covers to electrical
insulation.

n
Gutta percha

www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1156 Daley & Daley

The cis arrangement of the double bonds in rubber prevents
the rubber molecules from fitting into an ordered structure. Thus,
rubber is an amorphous polymer. Because of the random coiling of its
polymer chains, rubber stretches easily. When stretched, the rubber
molecules are forced into a higher energy state. When the tension is
released, rubber snaps back to its original random coiled state. On the
other hand, molecules of gutta percha pack close together so it is more
crystalline than rubber. In general, gutta percha is harder and less
flexible than rubber.
Raw rubber is affected by environmental factors such as light,
temperature, and oxygen. These factors make rubber unsuitable for a
number of applications. In 1839, Charles Goodyear devised a method
of reacting rubber with sulfur to form a more durable material. This
process, which he called vulcanization, forms sulfur bonds in the
molecule. Vulcanization forms both the cyclic structure shown on the
left and the more desirable cross-linked structure shown on the right.

S
Vulcanized rubber
S
S


Increasing the amount of sulfur makes the vulcanized polymer harder
and more durable. Adding 3 - 5% sulfur makes a product good for
rubber bands and inner tubes. Adding 20 - 30% sulfur makes a hard

rubber that was once widely used in ways that a hard synthetic plastic
is used today. The vulcanization process made early automobile tires
possible.

22.5 Controlling Stereochemistry in Vinyl Polymers

The substituents on a vinyl polymer arrange themselves in one
of three possible ways. Two of these possible arrangements are
stereoregular; the third arrangement is random. Which arrangement
the substituents take depends on the stereochemical relationship of
the substituents present on the polymer backbone. The arrangement
of the substituents on the backbone of the polymer is called the
tacticity of that polymer. The three possible arrangements of
substituents can be shown using polypropylene.
www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 22 1157 Daley & Daley
The tacticity of a
polymer is the
stereochemical
relationship among the
side groups on the
chain.

CH
3
CH
3
CH
3
CH

3
CH
3
HHHHH

Isotactic polypropylene

CH
3
CH
3
CH
3
HCH
3
HCH
3
HHH

Syndiotactic polypropylene

CH
3
CH
3
CH
3
HCH
3
HHHHCH

3

Atactic polypropylene

An isotactic polymer arranges all its substituents on the same
side of the polymer chain when you view the chain as a zig-zag
structure. A syndiotactic polymer has its substituents on alternating
sides of the zig-zag structure. The atactic polymer has its
substituents arranged randomly on the chain.
In the isotactic
arrangement, the
substituents are on the
same side of the carbon
backbone of the
polymer.

In the syndiotactic
arrangement, the
substituents are on
alternating sides.

In the atactic
arrangement, they are
on random sides of the
backbone.
The tacticity of a polymer strongly affects that polymer’s
properties. For example, syndiotactic and isotactic polypropylene are
more crystalline than atactic polypropylene. Before the 1950s,
chemists produced polymers that were largely atactic because they
could not control the tacticity of the polymer. In the early 1950s,

chemists developed new catalyst systems that selectively produced
stereoregular polymers. Having control over the stereochemistry
during polymerization had important consequences in the polymer
industry. For example, atactic polypropylene is a soft, low-melting
amorphous solid, but isotactic polypropylene is highly crystalline and
melts at 170
o
C.
Karl Zeigler of Germany developed a series of catalysts that
polymerize ethylene at low temperatures and low pressures to give a
polyethylene with a high molecular weight and very little branching.
This polyethylene is denser, tougher, and has a higher melting point
than the polyethylene produced by the earlier high temperature, high
pressure methods. Recognizing that Zeigler's catalysts were capable of
polymerizing 1-alkenes (called α olefins in the polymer industry) to
yield stereoregular polymers, Guilio Natta of Italy developed the
methodology to do it. In 1963, Zeigler and Natta jointly received the
Nobel Prize for the discovery and development of these catalyst
systems.
www.ochem4free.com 5 July 2005