organic chemistry chapter 22 polymer chemistry

Chapter 22 Polymer ChemistryChapter Outline 22.1 Structural Characteristics of Polymers A look at the various types of polymers 22.2 Polymer Nomenclature An introduction to naming po

Richard F Daley and Sally J Daley

www.ochem4free.com

Organic

Chapter 22 Polymer Chemistry

22.1 Structural Characteristics of Polymers 1138

Synthesis of Poly(ethylene terephthalate) 1165 Sidebar - Plastic Recycling 1167

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

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

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

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

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

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

CH2CHCH2CHCH2CHCH2CHCH2CH

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

m A + n B A A A A A B B B Block copolymer

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

has bonds branching

from the backbone of a

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

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

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

an example of a semicrystalline polymer

Ordered regions among

the amorphous regions

in a semicrystalline

polymer are called

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

Monomer Monomer Name Polymer Polymer Name

Poly(vinyl acetate)

CH2 CHCl Vinyl chloride

n

CH2CH Cl

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

Table 22.2 lists a few representative nonvinyl polymer names

O O

Ethylene glycol + Terephthalic acid CH2CH2OC CO

H2N(CH2)6NH2

+

HOOC(CH2)6COOH

amine

+ Sebacic acid

NH(CH2)6NHC(CH2)6C

O O

n

Poly(hexamethylene sebacate)

n

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

or anionic intermediates The important consideration is not whether the reaction proceeds as a radical reaction or generates a cationic or

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

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

Propagation step

•

O O

Termination step

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

+H

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

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

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

HH

In the propagation step, the carbocation reacts with a molecule of styrene to form a new carbocation This step repeats itself until the

reaction either runs out of reactants or until the polymers react in a termination step

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

H R

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

Propagation step

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

x x

x x

Predict the order of reactivity of styrene, 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

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%)

CH2

CH3COCHO

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 80oC Once the exothermic polymerization reaction has begun, maintain the reaction at 80oC 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 4oC Filter the polymer beads from the solution and wash repeatedly with water Dry the beads at 30oC 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

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