Polymer lecture notes
- 1. 1 "I just want to say one word to you -- just one word -- 'plastics.'" Advice to Dustin Hoffman's character in The Graduate
- 2. 2 Polymers: Introduction • Polymer: High molecular weight molecule made up of a small repeat unit (monomer). – A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A • Monomer: Low molecular weight compound that can be connected together to give a poymer • Oligomer: Short polymer chain • Copolymer: polymer made up of 2 or more monomers – Random copolymer: A-B-B-A-A-B-A-B-A-B-B-B-A-A-B – Alternating copolymer: A-B-A-B-A-B-A-B-A-B-A-B-A-B – Block copolymer: A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B
- 3. 3 Types of Polymers • Polymer Classifications – Thermoset: cross-linked polymer that cannot be melted (tires, rubber bands) – Thermoplastic: Meltable plastic – Elastomers: Polymers that stretch and then return to their original form: often thermoset polymers – Thermoplastic elastomers: Elastic polymers that can be melted (soles of tennis shoes) • Polymer Families – Polyolefins: made from olefin (alkene) monomers – Polyesters, Amides, Urethanes, etc.: monomers linked by ester, amide, urethane or other functional groups – Natural Polymers: Polysaccharides, DNA, proteins
- 4. 4 Common Polyolefins Monomer Polymer Ethylene H3C CH3 n Repeat unitPolyethylene CH3 CH3 n CH3 CH3 CH3 CH3 CH3 CH3CH3 Propylene Polypropylene Ph CH3 n Ph Ph Ph Ph Ph PhPh Styrene Polystyrene Cl CH3 n Cl Cl Cl Cl Cl ClCl Vinyl Chloride Poly(vinyl chloride) F2C CF2 Tetrafluoroethylene F3C F2 C C F2 F2 C C F2 F2 C C F2 F2 C C F2 F2 C C F2 F2 C C F2 CF3 n Poly(tetrafluoroethylene): Teflon
- 5. 5 Polyesters, Amides, and Urethanes Monomer Polymer CO2HHO2C HO OH O O HO O H2 C H2 C O n Terephthalic acid Ethylene glycol Poly(ethylene terephthalate H Ester HO OH O O 4 H2N NH24 Adipic Acid 1,6-Diaminohexane Nylon 6,6 HO N H N H H O O 4 4 n CO2HHO2C Terephthalic acid NH2H2N 1,4-Diamino benzene Kevlar O HO O H N H N H n Amide HO OH Ethylene glycol H2 COCN NCO 4,4-diisocyantophenylmethane Spandex H2 C H N H N O HO O O H2 C H2 C O H n Urethane linkage
- 6. 6 Natural Polymers Monomer Polymer Isoprene n Polyisoprene: Natural rubber O H HO H HO H H OHH OH OH Poly(ß-D-glycoside): cellulose O H O H HO H H OHH OH OH H n ß-D-glucose H3N O O R Polyamino acid: protein H3N O H N R1 O H N Rn+1 O OH Rn+2n Amino Acid Base O OH OP O O O oligonucleic acid DNA Nucleotide Base = C, G, T, A Base O O OP O O O DNA DNA
- 7. 7 What Makes Polymers Unique? • Really big molecules (macromolecules) like polymers have very different properties than small molecules – Chain entanglement: Long polymer chains get entangled with each other. • When the polymer is melted, the chains can flow past each other. • Below the melting point, the chains can move, but only slowly. Thus the plastic is flexible, but cannot be easily stretched. • Below the glass transition point, the chains become locked and the polymer is rigid
- 8. 8 Physical Properties Stretch Linear Polymer The chains can be stretched, which causes them to flow past each other. When released, the polymer will not return to its original form. Stretch Cross-Linked Polymer The cross-links hold the chains together. When released, the polymer will return to it's original form. Relax
- 9. 9 Polymer Synthesis • There are two major classes of polymer formation mechanisms – Addition polymerization: The polymer grows by sequential addition of monomers to a reactive site • Chain growth is linear • Maximum molecular weight is obtained early in the reaction – Step-Growth polymerization: Monomers react together to make small oligomers. Small oligomers make bigger ones, and big oligomers react to give polymers. • Chain growth is exponential • Maximum molecular weight is obtained late in the reaction
- 13. 13 Addition Polymerization Propagation nA In A A A A n A* A A A A A m In A A A A n A *A A A A A m Combination *A A A A A m In A A A A n A B A A A A m Disproportionation Termination Reactive site is consumed A In A A A A n A A* Chain Transfer New reactive site is produced MW ∝ kpropagation kter mination MW % conversion 0 100 In* A Initiation In A A A A*
- 14. 14 Types of Addition Polymerizations Ph Anionic C3H7 Li C4H9 Ph Li+ Ph n C4H9 Ph Ph Li+ n Ph Radical PhCO2• Ph n Ph Cationic Cl3Al OH2 H Ph HOAlCl3 Ph n H Ph Ph n HOAlCl3 PhCO2 Ph PhCO2 Ph Ph n
- 15. 15 Step-Growth Polymerization Stage 1 Consumption of monomer n n Stage 2 Combination of small fragments Stage 3 Reaction of oligomers to give high molecular weight polymer
- 16. 16 Step-Growth Polymerization • Because high polymer does not form until the end of the reaction, high molecular weight polymer is not obtained unless high conversion of monomer is achieved. Xn = 1 1− p Xn = Degree of polymerization p = mole fraction monomer conversion 1 10 100 1000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Mole Fraction Conversion (p) Degree of Polymerization
- 17. 17 Nylon-6,6 Cl Cl O O 4 H2N NH24 Adipoyl chloride 1,6-Diaminohexane Cl N H N H H O O 4 4 NaOH HO N H N H H O O 4 4 n 6 carbon diacid 6 carbon diamine Nylon-6,6 Diamine, NaOH, in H2O Adipoyl chloride in hexane Nylon 6,6
- 18. 18 Nylon-6,6 Diamine, NaOH, in H2O Adipoyl chloride in hexane Nylon 6,6 Since the reactants are in different phases, they can only react at the phase boundary. Once a layer of polymer forms, no more reaction occurs. Removing the polymer allows more reaction to occur.
- 19. 19 Molecular Weight of Polymers Unlike small molecules, polymers are typically a mixture of differently sized molecules. Only an average molecular weight can be defined. • Measuring molecular weight • Size exclusion chromatography • Viscosity • Measurements of average molecular weight (M.W.) • Number average M.W. (Mn): Total weight of all chains divided by # of chains • Weight average M.W. (Mw): Weighted average. Always larger than Mn • Viscosity average M.W. (Mv): Average determined by viscosity measurements. Closer to Mw than M # of molecules Mn Mw increasing molecular weight Mv
- 20. 20 What the Weights Mean Mn: This gives you the true average weight Let's say you had the following polymer sample: 2 chains: 1,000,000 Dalton 2,000,000 5 chains: 700,000 Dalton 3,500,000 10 chains: 400,000 Dalton 4,000,000 4 chains: 100,000 Dalton 400,000 2 chains: 50,000 Dalton 100,000 10,000,000 10,000,000/23 = 435,000 Dalton 1 Dalton = 1 g/mole
- 21. 21 Weight Average Molecular Weight Mw: Since most of the polymer mass is in the heavier fractions, this gives the average molecular weight of the most abundant polymer fraction by mass. 2,000,000 10,000,000 = 0.20×1,000,000 = 200,000 3,500,000 10,000,000 = 0.35× 700,000 = 245,000 4,000,000 10,000,000 = 0.40×400,000 =160,000 400,000 10,000,000 = 0.04 ×100,000 = 4,000 100,000 10,000,000 = 0.01× 50,000 = 500 Total = 609,500
- 22. 22 Polymer Microstructure Polyolefins with side chains have stereocenters on every other carbon CH3 n CH3 CH3 CH3 CH3 CH3 CH3CH3 With so many stereocenters, the stereochemistry can be complex. There are three main stereochemical classifications for polymers. Atactic: random orientation Isotactic: All stereocenters have same orientation Syndiotactic: Alternating stereochemistry
- 23. 23 How to Determine Microstructure? 13 C NMR is a very powerful way to determine the microstructure of a polymer. 13 C NMR shift is sensitive to the two stereocenters on either side on sptectrometers > 300 MHz. This is called pentad resolution. r mm rmr mmrm pentad m = meso (same orientation) r = racemic (opposite orientation) 12 1 2 13 C NMR spectrum of CH3 region of atactic polypropylene
- 24. 24 Why is this important? • Tacticity affects the physical properties – Atactic polymers will generally be amorphous, soft, flexible materials – Isotactic and syndiotactic polymers will be more crystalline, thus harder and less flexible • Polypropylene (PP) is a good example – Atactic PP is a low melting, gooey material – Isoatactic PP is high melting (176º), crystalline, tough material that is industrially useful – Syndiotactic PP has similar properties, but is very clear. It is harder to synthesize