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

Introduction 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 copolymers Addition and condensation polymers Based on molecular forces Elastomers Fibres Thermoplastics Thermosetting polymers Based on source of availability Naturally occuring polymers Semisynthetic polymers Synthetic 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, CN, or NO 2. Initiator: Grignard or organolithium reagent. Anionic intermediate usually gives isotactic or syndiotactic polymers.

Plexiglass: anionic polymerization 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.

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 TiCl 4 mixed with solution of (CH 3 CH 2 ) 3 Al 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.

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

Melamine-formaldehyde resins Melamine Three active sites Formaldehyde Plus

Melamine Formaldehyde Water

Polyurethanes 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( ) 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--hydroxyvalerate Copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid.

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