Lecture 16: Ziegler-Natta, Stereochemistry of Polymers

10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 16: Ziegler-Natta, Stereochemistry of Polymers “Precipitation Polymerization” polymer: -semicrystalline -semicrystalline polymer not soluble in monomer ⇒ crystalline regions insoluble ⇒ amorphous regions remain soluble Polymerization in bulk monomer As # of high MW chains ↑, precipitation occurs also: occur in polymer chains with enough irregularity to form short chains -some active sites remain accessible -some via v ia diffusion through pores M polymer flakes, polymer particles, pa rticles, etc. are porous porous M M monomer can still diffuse to active sites Kinetics - ill-defined and complex - similar to emulsion polymerization - can have red light/green light effect with free radicals ⇒ gain advantages → more temp/heat control → low η (can dilute slurry) → no surfactant Common Monomers Tm,crys Vinyl chloride Vinyl fluoride Vinylidene fluoride Acrylonitrile Tetrafluoroethylene (Teflon) 140 – 200oC 200 – 230oC 200oC 317oC 327oC Dispersion Polymerization - monomer organic solvent (good for monomer, bad for polymer) initiator particle stabilizer: repel sticky polymers, avoid coalescence As polymerization occurs, form large solid/semisolid particles of polymer Random copolymers Incorporating 2 or more different monomer units in chain growth process (radical, cationic, or anionic polymerizations) Consider 2 different monomers: 1 and 2 M1* + M1 M1* + M2 M2* + M1 M2* + M2 k11 M1* k12 M2* k21 M1* k22 M2* d [M 1 ] = k11 M 1* [M 1 ] + k 21 M 2* [M 1 ] dt d [M 2 ] − = k12 M 1* [M 2 ] + k 22 M 2* [M 2 ] dt − [ ] [ ] [ ] [ ] The ratio of rates of monomers entering polymer chains [ ] [ ] [ ] [ ] d [M 1 ] k11 M 1* [M 1 ] + k 21 M 2* [M 1 ] (relative rates) = d [M 2 ] k12 M 1* [M 2 ] + k 22 M 2* [M 2 ] Assume steady state concentration of both [M1*] and [M2*] ⇒ Rate of M2* → M1* = rate of M1* → M2* [ ] [ ] k 21 M 2* [M 1 ] = k12 M 1* [M 2 ] Simplify and combine with d [M 1 ] : d [M 2 ] d [M 1 ] [M 1 ](r1 [M 1 ] + [M 2 ]) = d [M 2 ] [M 2 ]([M 1 ] + r2 [M 2 ]) where r1 ≡ k11 k and r2 ≡ 22 k12 k 21 (reactivity rates) reactivity of M1* with M1 versus M1* with M2 reactivity of M2* with M2 versus M2* with M1 Fraction of each monomer: f1 = [M 1 ] [M 1 ] + [M 2 ] f2 = [M 2 ] [M 1 ] + [M 2 ] ⇒ expressions for monomer composition Define: F1 = 1 − F2 ≡ d [M 1 ] d [M 1 ] + d [M 2 ] instantaneous polymer composition Combine expressions and definitions: F1 = r1 f 12 + f 1 f 2 r1 f 12 + 2 f 1 f 2 + r2 f 22 copolymer composition equation r1 r2 Special Cases: 1. “Ideal” copolymerization: r1 ⋅ r2 = 1.0 k 22 k12 = k 21 k11 1 r2 = r1 ⇒ probability of M1* or M2* react with M1 vs M2 is equal 1.0 r1= 1 10 0 F1 r1= 2 0 1.0 r1= F1 line 0.8 r1= 1 10 f1 0.2 0.2 special case: r1 = r2 = 1.0 ⇒ f1 = F1 Bernoullian (random) arrangement of monomers large r1 ⇒ monomers want to react with M1 much more than M2 0 r1= 0.5 Simplified expression for ideal copolymerizations: F1 = r1 f 1 r1 f 1 + f 2 (r1⋅r2 = 1.0) 2. r1 = r2 = 0 neither M1 nor M2 react with themselves M1 → M2* M2 → M1* Perfectly alternating composition: M1M2M1M2… (not random at all) Regardless of f1: F1 = 0.5 2 extremes: • perfect Bernoullian (random) case: r1 = r2 = 1 r1r2=1 • perfect alternating case: r1 = r2 = 0 r1r2=0 As r1r2 product goes from 0 → 1.0, move from random to alternating sequencing: f1 = F 1 1.0 let r2= 0.5 F1 r1 r2= 0 0.5 r1= 2.0 (r2 = 0.5) r1= 0.5 (r2 = 0.5) 0 1.0 r1= 0.1 (r2 = 0.5) 0.5 0.8 0.2 0 f1 If r1 < 1.0 and r2 < 1.0 Then induce inflection ⇒ form an azeotrope: 1.0 at the azeotrope: F 1 = f1 If you can maintain f1 → copolymer comp will not change throughout polymer F1 f1 = F 1 0 1.0 1.0 0 f1 Find azeotrope condition: f1 = (1 − r2 ) (2 − r1 − r2 ) azeotrope exists at this monomer composition - Block polymer: If r1 > 1, r2 > 1 M1M1M1M1M2M2M2M2 - Consecutive homopolymer if r1 >> r2 M1 homopolymerizes Then M2 homopolymerizes r1 >> 1 r2 << 1