chemistry. polymers session session objectives 1.introduction 2.classification of polymers 3.general...
TRANSCRIPT
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