CHEMISTRY FORM 6 SEM 3 09.pdf

CHEMISTRY FORM 6 ORGANIC CHEMISTRY CHAPTER 9 : POLYMER

9.0

Introduction 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.


Homopolymers – polymers that are made from the same type of

repeating unit.
Copolymers – polymers that are made from 2 or more types of monomer

Linear copolymer • Polymers which are arranged in a straight line. • There are 3 linear copolymers

Branched-chain copolymer

Cross-linked copolymer

• 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 Formed when unsaturated organic molecules joined together using πelectrons to form covalent bond of long polymeric chain.

Condensation polymerisation Formed when 2 molecules, each with 2 same functioning group (may be different) at the end of the molecule, joined up via condensation reaction

Small molecule is form as side product (H2O,HCl) Empirical formula of the monomer is the Empirical formula of the monomer is same as the empirical formula of polymer different from the empirical formula of formed polymer formed No side product is 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-1,6diamine with decan-1,10dioic 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

Ethene

Polyethene (PE)

Propene

Description • 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.

• 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 Polypropene (1760C) and relatively high density (PP)

•

Phenylethen e

Polystyrene (PS)

• • •

Chloroethen e

Polyvinylchlorid e (PVC) •

• Tetrafluoroet hene

Teflon

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. 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. 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 H H H H H H H H H CH 3 CH 3 CH

H







H

H

H

H

3

H

CH 3 CH 3 H CH 3 H H 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

2. Syndiotactic Polymers

H

H H

H

H CH 3

H CH 3

H H

H H

H CH 3

H CH 3

H H

H H

CH 3 H

CH 3

H


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.

3.

Isotactic Polymers

H

H H

H

H H

H CH 3

H H

H CH 3

H H

H CH 3

H H

H CH 3

H H

CH 3

CH 3

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.

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

Step 3 :

:

Propagation

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 2methylpropenoate (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 advantage

because these reaction are far less affected by the presence of impurities than free radical reactions.

7.5

Classification of Polymer Fibers

Polymers that can be drawn out as threads and then spun and woven into fabrics.

Plastics

Resins

Elastomers

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


Plastic – Solid polymers which are capable of being remoulded because of

heating, these polymers soften
Plastic can be classified into two main categories

Thermoplastic 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 plastics Thermosetting are hard and cannot be remelted They are made from cross-linked polymer Example : bakelite ; epoxy & ureamethanal 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

- Natural rubber are easily oxidise by air (O2) and even ozone (O3). Ozone causes rubber to harden and crack, decreasing the life of Resistance tyres. to - This is due to double bond in the rubber, thus can be react easily oxidation by oxygen and ozone. - This can be prevent by adding sulphur Effect of heat

Effect of solvent

- Rubber is not a very stable compound. - At low temperature, rubber is hard and brittle - At high temperature, rubber become soft and sticky - 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 polyglycolic acid (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) also react with halogens/in U.V. light

Additional polymerisation Condensation polymerisation Hydrogen bonding

Not possible in muscle (1) muscle is internal and no halogens (1)

CARBOXYLIC ACID OR ACYL CHLORIDE

Ester bond

Dilute HCl under reflux