Chemistry

Polymers

Session

Session objectives

1. Introduction

2. Classification of polymers

3. General methods of polymerization

4. Natural rubber

5. Vulcanization of rubber

6. Synthetic rubber

7. Condensation polymerisation

8. Molecular mass of polymers

9. Biopolymers

10. Some commercially important polymers

IntroductionPolyvinylchloride (PVC)

• A polymer is a large molecule composed of many smaller repeating units.

• First synthetic polymers:

Polyvinyl chloride (PVC) in 1838

Polystyrene in 1839

• Now, 250 billion pounds produced annually, worldwide.

Classification

Based on mode of polymerization

Homopolymers and copolymersAddition and condensation polymers

Based on molecular forces

ElastomersFibresThermoplasticsThermosetting polymers

Based on source of availability

Naturally occuring polymersSemisynthetic polymersSynthetic polymers

Addition Polymers

• Three kinds of intermediates:

Free radicals

Carbocations

Carbanions

• Examples of addition polymers:

polypropylene plastics

Polystyrene foam insulation

poly(acrylonitrile) Orlon® fiber

poly(methyl -methacrylate) Plexiglas

Free Radical Polymerization

Free radical polymerization is nearly random, giving branched atactic polymers.

Plasticizers

• Nonvolatile liquid that dissolves, lowers the attraction between chains, and makes the polymer more flexible.

• Example: Dibutyl phthalate is added to poly(vinyl chloride) to make it less brittle. The plasticizer evaporates slowly, so “vinyl” becomes hard and inflexible over time…..The foggy film that forms on your windshield on a hot day.

Chain Branching

• Low-density polyethylene:

soft and flimsy

highly branched, amorphous structure

Cationic Polymerization

Alkene is treated with an acid.

Intermediate must be a stable carbocation.

Anionic Polymerization

Alkene must have an electron-withdrawing group like C=O, CN, or NO2.

Initiator: Grignard or organolithium reagent.

Anionic intermediate usually gives isotactic or syndiotactic polymers.

Plexiglass: anionic polymerization

Me CO2Me

Polymethylmethacrylate:

cat Nu–

Me

CO2Me

Nu

Me CO2Me

Me CO2Me

MeO2C Me

Windshields, plastic coatings, hard and soft contact lenses.

Copolymers

Two or more different monomers.

Saran: alternating molecules of vinyl choride and 1,1-dichloroethylene.

ABS plastic: acrylonitrile, butadiene, and styrene.

Natural Rubber

Soft and sticky, obtained from rubber tree.

Long chains can be stretched, but then return to original structure.

Chains slide past each other and can be pulled apart easily.

Structure is cis-1,4-polyisoprene.

isoprenen

polyisoprene

Vulcanization

Process was discovered accidentally by Goodyear when he dropped rubber and sulfur on a hot stove.

Sulfur produces cross-linking that strengthens the rubber.

Hardness can be controlled by varying the amount of sulfur.

Synthetic Rubber

With a Ziegler-Natta catalyst, a polymer of 1,3-butadiene can be produced, in which all the additions are 1,4 and the remaining double bonds are all cis.

It may also be vulcanized.

Ziegler-Natta Catalyst

Polymerization is completely stereospecific.

Either isotactic or syndiotactic, depending on catalyst.

Polymer is linear, not branched.

Example of catalyst: solution of TiCl4 mixed with solution of (CH3CH2)3Al and heated for an hour.

Stereochemistry

Isotactic and syndiotactic polymers are stronger and stiffer due to their regular packing arrangement.

Condensation Polymers

Polymer formed by ester or amide linkages between difunctional molecules.

Step growth: Monomers do not have to add one at a time. Small chains may condense into larger chains.

Common types:

Polyamides

Polyesters

Polycarbonates

Polyurethanes

Polyamides (Nylons)

Heating a diamine with a diacid produces a polyamide called Nylon.

Nylon 66 is from adipic acid and hexamethylene-diamine at 280°C.

Polyesters

The polyester from dimethyl terephthalate and ethylene glycol is called Dacron and Mylar to make fibers.

OHOH O

H +: O: OH OH

:OH

H+

OH OH2

+

OOHOHOH

OHOH

HO

HO OH

OH

OH

Phenol-formaldehyde resins which he called Bakelite.Thermoset resin.Replaced rubber for insulation in electrics.

Phenol formaldehyde polymer :Bakelite

Melamine-formaldehyde resins

Melamine

C

NC

N

CNN

N

N

H

H

HH

H

H

Three active sites

Formaldehyde

C

O

H H

Plus

Melamine Formaldehyde Water

Polyurethanes

n

NCO N C O

CH3

HO CH2CH2 OH+

CN

H O

O CH2CH2 O C

O

N

H

CH3

N

H

C

O

O CH2CH2

Esters of carbamic acid, R-NH-COOH.

Urethanes are prepared by reacting an alcohol with isocyanate.

Polyurethanes are prepared by reacting a diol with a diisocyanate.

Molecular mass of polymers

Expressed as number average molecular mass( ) or

weight average molecular mass( )

nM

wM

2i i

wi i

i

NMM

NM

i i

ni

i

NMM

N

The ratio of the weight and numbe average molecular masses is called Poly Disparity Index(PDI).

PDI

Unity for some natural polymers

Greater than one for synthetic polymers

Biopolymers

Biodegradable polymers

Poly-hydroxybutyrate-co--hydroxyvalerateCopolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid.

CH — CH— CH — COOH + 3 2 CH — CH — CH— CH — COO H3 2 2

OH OH

— O— CH— CH — CO—2

On

PHBV

R

Biopolymers

Poly(glycolic acid) and poly(lactic acid)

Biodegradable polymers such as sutures, Dextron; used in post operative stitches.

Nylon-2-nylon-6

Alternative of glycine and amino caproic acid and is biodegradable.

Some common polymers and their uses

Thank you