CHEMISTRY FORM 6

ORGANIC CHEMISTRY

CHAPTER 9 :

POLYMER

9.0 Introduction

� Homopolymers – polymers that are made from the same type of repeating unit.

� Copolymers – polymers that are made from 2 or more types of monomer

Polymers

Natural polymer Synthetic polymer

Polymers that are obtained naturally

from animals or plants

Polymers which are synthesised

chemically by human.

Example : proteins, natural rubber,

starch, cellulose, cotton, wool, starch

Example : polyethene, polypropene,

Teflon, polyvinylchloride.

Linear copolymer Branched-chain copolymer Cross-linked copolymer

• Polymers which are

arranged in a straight

line.

• There are 3 linear

copolymers

• Polymers which contain

side chain of polymer in

the parent chain.

• Polymers which are

joined together by

adding alien substance

in between them.

9.2 Polymerisation

� Polymerisation – process where monomer are joined together to form long chain of polymer

� There are generally 2 type of polymerisation take place

� condensation polymerisation • additional polymerisation

� Following table compare and contrast between condensation polymerisation and additional polymerisation

Additional polymerisation Condensation polymerisation

Formed when unsaturated organic

molecules joined together using π-

electrons to form covalent bond of long

polymeric chain.

Formed when 2 molecules, each with 2

same functioning group (may be different)

at the end of the molecule, joined up via

condensation reaction

No side product is formedSmall molecule is form as side product

(H2O,HCl)

Empirical formula of the monomer is the

same as the empirical formula of polymer

formed

Empirical formula of the monomer is

different from the empirical formula of

polymer formed

9.2 Polymerisation

� Polymerisation – process where monomer are joined together to form long chain of polymer

� Type of polymerisation

A) Condensation polymerisation

� Polymerisation which will eliminate small molecules such as water, ammonia, methanol or HCl

� Polymerisation must have 2 different functioning groups at its end of each monomer

� Condensation polymerisation of polyamide :

� Formation of polyamide is done by reacting dicarboxylic acid with diamine

� Example : Formation of nylon-6,6 (a type of polyamide)

� Nylon is a common polyamide use in industrial as synthetic fiber. The term 6,6 indicating the 6 C in dioic acid and 6 C in diamine

� Nylons have peptide linkage as functional groups which are also found in polypeptide and proteins

� High tensile strength & high melting point (2650C) of nylon are due to hydrogen bond between peptide

Nylon 6,10

� Nylon 6,10 is formed by condensation of hexan-1,6-diamine with decan-1,10-dioic acid.

� Similar to Nylon 6,6 water is produce as side product.

� Nylon 6,10 is used to make synthetic bristles

A2) Condensation Polymerisation – Formation of Polyester.

� For the reaction of polymerisation, reacting dicarboxylic acid and dihydric alcohol are used as the starting material to form polyester.

dicarboxylic acid dihydric alcohol

polyester

� Example of polyester : Terylene

� Terylene is also known as PET (PolyEthene Terephthalate)

� In industrial, a more reactive chemical (dimethyl benzene-1,4-dicarboxylate) is used. Methanol is produced as side product of the reaction.

9.3 Polymerisation

� Polymerisation – process where monomer are joined together to form long chain of polymer

� Type of polymerisation

A) Addition polymerisation

� Addition polymerisation are formed when monomers with double bond, are joined by using covalent bond to form large molecule (polymer)

� Homopolymers – polymers that are made from the same type of monomer.

� Example of polymers which undergoes additional polymerisation.

Monomer Polymer Description

Ethene Polyethene

(PE)

• Low density polyethene (LDPE)

• Condition : 200oC and 1200 atm under oxygen.

• Contain branched chains which decrease the

density (less pack). This cause LDPE to have low

density and soften in boiling water and is easily

deformed.

• High density polyethene (HDPE)

• Condition : 60oC and 1 atm + Ziegler-Natta catalyst.

• Produce fewer branches which allow the polymer to

pack closer to each other. As a result, HDPE has

higher melting point, density, tensile strength and

harder than LDPE.

Propene Polypropene

(PP)

• PP Condition : 60oC and 1 atm + Ziegler-Natta

catalyst.

• The presence of methyl group increase the strength

& hardness of PP. Hence PP has high melting point

(1760C) and relatively high density

Phenylethen

ePolystyrene

(PS)

• In laboratory, PS is prepared by adding phenylethene and

benzoyl peroxide (as catalyst) in a test tube. Test tube is

then placed in a beaker containing boiling water (water

bath)

• The polymer formed looks like glass.

Chloroethen

ePolyvinylchlorid

e (PVC)

• PVC is a hard polymer due to the polar C–Cl. This give

rise to permanent dipole–dipole forces which are stronger.

• A plasticiser is an additive added to PVC to make it more

flexible and softer. It formed between the chain enable

them to slide over each other easily.

Tetrafluoroet

heneTeflon

• Teflon has a melting point (327 °C) that is unusually high for an addition polymer. The reaction is highly exothermic

as water helps to dissipate the heat that is produced.

• Teflon is highly resistant to chemical attack and has a low

coefficient of friction. Teflon is used in greaseless

bearings, in liners for pots & pans, and many special

situations that require a substance that is resistant to

corrosive chemicals

9.3.1 Effect of Ziegler-Natta catalysts on the stereochemistry of polymerisation

� Karl Ziegler (a German chemist) and Giulio Natta (an Italian chemist) announced independently in 1953 the discovery of catalysts that permit stereochemical control of polymerization reactions called as Ziegler–Natta catalyst.

� The Ziegler-Natta catalysts are prepared from transition metal halides and a reducing agent ⇒⇒⇒⇒ the catalysts used are prepared from titanium tetrachloride (TiCl4) and trialkylaluminum (R3Al).

� Ziegler-Natta catalysts are generally employed as suspended solids ⇒polymerization probably occurs at metal atoms on the surfaces of the particles.

1) The mechanism for the polymerization is an ionic mechanism.

2) There is evidence that polymerization occurs through an insertion of the alkene monomer between the metal and the growing polymer chain.

� The polymer formed using Ziegler-Natta catalyst may exist in 3 configurations, depending on the condition of the reaction used

1. Atactic Polymers

� The stereochemistry at the stereocenters is random, the polymer is said to be atactic (a = without + Greek: taktikos, order).

� In atactic polypropylene the methyl groups are randomly disposed on either side of the stretched carbon chain ⇒ (R-S) designations along the chain is random.

� Polypropylene produced by radical polymerization at high pressure is atactic.

� Atactic polymer is noncrystalline ⇒ it has a low softening point and has poor mechanical properties

H

H

H

CH 3

H

H

CH 3

H

H

H

H

CH 3

H

H

CH 3

H

H

H

H

CH 3

H

H

H

CH 3

2. Syndiotactic Polymers

� Figure 11.2 Syndiotactic polypropylene.

� The stereochemistry at the stereocenters alternates regularly from one side of the stretched chain to the other is said to be syndiotactic(syndio: two together) ⇒ (R-S) designations along the chain would alternate (R), (S), (R), (S), (R), (S) and so on.

H

H

H

CH 3

H

H

H

CH 3

H

H

CH 3

H

H

H

H

CH 3

H

H

CH 3

H

H

H

H

CH3

3. Isotactic Polymers

Figure 11.3 Isotactic polypropylene.

� The stereochemistry at the stereocenters is all on one side of the stretched chain is said to be isotactic.

� The configuration of the stereocenters are either all (R) or all (S) depending on which end of the chain is assigned higher preference.

H

H

H

CH 3

H

H

CH 3

H

H

H

CH 3

H

H

H

CH 3

H

H

H

CH 3

H

H

H

CH 3

H

7.3 Coordination polymerisation – by Ziegler-Natta catalyst

7.4 Addition polymerisation Mechanism

� There are 3 types of addition polymerisation mechanism where

A) Free radical polymerisation B) Cationic polymerisation C) Anionic polymerisation

A) Free radical polymerisation

� Using an initiator as the radical source, the polymerisation begin with breaking the covalent bond in peroxide from the organic peroxide compound (example : benzoyl peroxide)

� Step 1 : Initiation

Step 2 : Propagation

Step 3 : Termination

B) Cationic polymerisation

� Using Bronsted–Lowry acid such as sulphuric acid and chloric (VII) acid (HClO4) and Lewis acid such as boron trifluoride, BF3 or aluminium trichloride, AlCl3 as catalyst (initiator) by donating proton.

Step 1 : Initiation step – formation of carbocation

Step 2 : Propagation step : reaction of carbocation

C) Anionic Polymerisation

� Anionic polymerisation occurs via carbanion intermediates. The initiator of anionic polymerisation is usually a nucleophile (Lewis Base) such as

√ Lithium amide (Li+NH2-) in liquid ammonia

√ Butyllithium (CH3CH2CH2CH2Li)

� A good monomer for anionic polymerisation should contain at least one electron withdrawing group to decrease the electron density of the C in C=C. Examples of monomer with strong electrophile

Chloroethene

(vinyl chloride)

Propenenitrile

(acrylonitrile)

Phenylethene

(styrene)

Methyl 2-

methylpropenoate

(methyl methacrylate)

� Example : anionic polymerisation mechanism of propenenitrile using butyllithium

� Step 1 : Initiation step – Formation of carbanion using butyllithium

� Step 2 : Propagation of monomer using carbanion

� Addition polymerisation by ionic mechanisms have the advantagebecause these reaction are far less affected by the presence of impurities than free radical reactions.

7.5 Classification of Polymer

� Plastic – Solid polymers which are capable of being remoulded because of heating, these polymers soften

� Plastic can be classified into two main categories

Fibers Plastics Resins Elastomers

Polymers that can

be drawn out as

threads and then

spun and woven

into fabrics.

Solid polymers

which are capable

of being remoulded

because of heating,

these polymers

soften

Solid or semi-solid

which are incapable

of being remoulded

because they do

not soften on

heating

Polymers that can

be stretch and the

revert to the original

shape and size

when released

Thermoplastic Thermosetting plastics

Thermoplastic can be moulded and

remoulded.

They are made from linear polymer

Example : polyethene ; polypropene ;

PVC

When heated, the distance between the

chain increase significantly and the

polymer soften and becomes more

flexible. On cooling, the process is

reversed.

Thermosetting are hard and cannot be

remelted

They are made from cross-linked

polymer

Example : bakelite ; epoxy & urea-

methanal resin

They are not softened easily because

the individual polymer chains are linked

by strong covalent bonds. They do not

decompose easily and cannot be

remoulded on cooling

9.7 Natural rubber

� Monomer of natural rubber is 2-methylbutan-1,3-diene with the structural formula :

� Unlike protein and starch, natural rubber are linked together by addition polymerisation.

� The equation can be written as :

� Properties of natural rubber

Properties Description

Elasticity

- Elasticity is the ability of substance to stretch when pulled and

return to original shape when forces are lifted

- Natural rubber has a low elasticity as it cannot revert when forces

are released

Resistance

to

oxidation

- Natural rubber are easily oxidise by air (O2) and even ozone (O3).

Ozone causes rubber to harden and crack, decreasing the life of

tyres.

- This is due to double bond in the rubber, thus can be react easily

by oxygen and ozone.

- This can be prevent by adding sulphur

Effect of

heat

- Rubber is not a very stable compound.

- At low temperature, rubber is hard and brittle

- At high temperature, rubber become soft and sticky

Effect of

solvent

- Rubber is water repellant. It is impermeable to water, as it does

not allow water to pass through.

- Since it is not easily dissolve in water, it easily dissolve in organic

solvent such as benzene, petrol and alcohol.

� The properties of the natural rubber can be improve by adding sulphur into the rubber via the 2 reaction below

� Heating natural rubber with sulphur to about 140oC using zinc as catalyst

� Mixing a solution of disulphur dichloride, S2Cl2, in methylbenzene with natural rubber

� Sulphur added to rubber will cross linked via disulphide linkage (-S-S-) between rubber polymeric chain and form vulcanised rubber.

� Disulphide linkage formed between rubber polymers will prevent the rubber chain to slipped from each other hence increase the elasticity of rubber.

� Furthermore, as disulphide linkage formed between rubber polymers make used of the π-electron in rubber, this will caused lesser C=C inside the chain, hence increase the resistant toward oxidation, and also toward heat.

� The vulcanized elastomer produced in greatest quantity is styrenebutadiene rubber (SBR). SBR is commercially prepared from styrene and butadiene via a free-radical polymerization process. It is called a copolymer, because it is made from two different monomers

styrene (butane-1,3-diene) styrenebutadiene rubber

� The tyre produced by vulcanising SBR produce the highest quality rubber, which is suitable to make high grade tyre for automobile vehicles

7.6 Problems arise in using polymers

� Polymers might bring a lot of conveniences in our daily life. However, at the same time, it causes some problems too.

� The main problem dealing polymers is the method of their disposal. Polymers, especially poly(alkanes) decompose very slowly in environment as they are non-biodegradable (cannot be decompose by bacteria) and are resistance to most chemicals.

� There are generally 3 options on disposal of polymers

1. Recycling polymers – By sorting them according to their type of polymers, they can be recycle accordingly. However, the disadvantage is the cost of recycling. The amount of energy used to collect and reprocess materials, can be greater than the amount of energy used to make new products from new materials.

2.Combustion of polymers – since poly(alkanes) are hydrocarbon, they are good fuels. Burning waste poly(alkanes) would both deal with problems of disposing and also reduce the amount of main hydrocarbon to use as fuels. However, the disadvantage of burning is the toxin fumes produced, which is harmful to human body and the pollution problems caused to the environment.

3. Pyrolysis – by burning poly(alkanes) under high temperature, it will be broken down into smaller useful molecules. It is similar to the cracking of alkanes, where a mixture of hydrocarbons is produced, containing alkane, alkene and arenes. Alkenes extracted can be recycle and make more polymers.

9.9 Recycling polymers

� There are many logistical problems that limit the effectiveness of polymer recycling, most significantly the collection and sorting of used polymer products. Different kinds of polymers must be recycled in different ways.

� For example, when recycling PET, a small amount of a different polymer present in the batch will interfere with the recycling process. As such, polymer recycling requires that polymer products be sorted by hand.

� To facilitate the sorting process, most polymer products are labeled with recycling codes that indicate their composition. These codes (1–7) indicate the type of polymer used and are arranged in order of ease with which the polymer can be recycled (1 being the easiest and 7 being the most difficult).

� Table below indicates the seven recycling codes, the polymers that correspond with each code, and several uses for the recycled products.

� In many cases, the recycled polymer can be contaminated with adhesives and other materials that may have survived the washing stage. Therefore, recycled polymers cannot be used for food packaging.

� Frequently, however, plastics are simply thrown away rather than recycled, and much work has therefore been carried out on developing biodegradable polymers, which can be broken down rapidly by soil microorganisms.

� Among the most common biodegradable polymers are polyglycolicacid (PGA), polylactic acid (PLA), and polyhydroxybutyrate (PHB). All are polyesters and are therefore susceptible to hydrolysis of their ester links.

Chlorine gas (in CCl4)

Pumice at 500oC

Additional polymerisation

PVC undergoes hydrolysis when exposed with concentrated sodium hydroxide [1]

Polyethene is stable against concentrated sodium hydroxide as it contain saturated hydrocarbon

[1] POLYPHENYLETHENE is stable against NaOH [1]

Additional polymerisation

The presence of C=C caused molecule to be less elastic [1]

alkane

Chemically inert since it contain only saturated hydrocarbon

Resistant to water since it is made of hydrophobic hydrocarbon

Alkanes react with oxygen (combustion) Not possible in muscle (1)

also react with halogens/in U.V. light muscle is internal and no halogens (1)

Additional polymerisation

Condensation polymerisation

Hydrogen bonding

CARBOXYLIC ACID OR ACYL CHLORIDE

Ester bond

Dilute HCl under reflux