10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 17: Stereoregular Polymerizations Final comments: copolymerization Recall azeotrope: occurs r1, r2 < 1 1.0 ⇒ f1 = F1 monomer comp = final copolymer comp - if you have no perturbations, can get same composition throughout full range of π - if there is perturbation → move further and further away from f1 = F1 F1 0 1.0 0 0.5 f1 For all copolymerizations in which - azeotrope comp - perfectly alternating system (f1 = F1) We have unchanged polymer composition BUT for all other cases (most cases), monomer compositions are continually changing. - must create different expressions and integrate to get polymer composition averaged over conversion Skeist: 1− M ∫ Mo [M ] [M ]o =π [M ] = dM = ln [M ]o M [M ] π = 1− [M ]o f1 df 1 ∫ (F − f ) ( ) f1 o 1 1 α β ⎡ f ⎤ ⎡ f ⎤ ⎡( f ) −δ ⎤ = 1− ⎢ 1 ⎥ ⎢ 2 ⎥ ⎢ 1 o ⎥ ⎣ ( f 1 )o ⎦ ⎣ ( f 2 )o ⎦ ⎣ ( f 1 − δ ) ⎦ where α = γ 1.0 f1 r2 1 − r2 r β= 1 1 − r1 avg mol frac fra c avg 1 − r1r2 γ = (1 − r1 )(1 − r2 ) δ= f2 1 − r2 (2 − r1 − r2 ) F1 0 0 F2 π For r2 > r1 (M2 consumed more rapidly) Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. 1.0 1.0 Stereochemistry of polymer chains R R R R isotactic (same side) RR RR syndiotactic (alternating) RR R R R atactic (no sequence, random) Dyad tacticity fraction m + r = 1.0 isodyad (m = meso) syndio (r = racemic) Triad tacticity iso (mm) % mm + rr + mr =1 use percentages = 100% (use NMR to determine percentage) hetero (mr) % syndio (rr) % CH2=CHR In “normal” solution/bulk free radical polymerization RR H R H + R H2C CHR kr H C R R km R H C R 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 17 Page 2 of 6 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. kr ⇒ determine tacticity km kr ∼ 1.0 ⇒ atactic km kr << 1.0 ⇒ isotactic km kr >> 1.0 ⇒ syndiotactic km Sterics prefer syndiotactic addition. Ea slightly higher for iso addition As T↑ iso fractions ↑ T↓ syndio fractions ↑ e.g. PMMA: CH3 H2 C C T p = 100oC r = 0.73 Tp = -78oC r = 0.87 Tp = 60oC r = 0.5 Tp = -38oC r = 0.6 CH3 e.g. PVC H H2 C C Cl Coordination Polymerization Coordinated catalyst: usually metal/metal halide or oxide complex: - promote stereospecific + controlled addition of monomer Ziegler and Natta (Germany) (Italy) Nobel Prize, 1963 e.g. 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 17 Page 3 of 6 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. CH3 CH H2C H2C n or CH3 H2C α-olefins have very high transfer rates to monomer C H propylene (propane) Initiator: Group I-III metal (e.g. Al) + transition metal (e.g. Ti) + G RHC H C CH2 H2 C G R group metal escort for insertion of new monomer Ziegler-Natta Catalyst Usually TiCl4 or TiCl3 + AlR3 + AlR2Cl R=CH2CH3 or (CH2)nCH3 alkyl tail Cl CH2 Cl Ti Cl H2C CHCH3 Cl CH2 Cl CHCH3 Ti CH2 Cl Cl Cl π completion CH2 Cl H2C Cl Ti Cl CHCH3 CHCH3 Cl CH3 Cl CH2 Cl Ti Cl H2C CHCH3 Cl “Escort” process guides stereochemistry many of Z-N catalysts (crys) ⇒ isotactic addition (soluble) ⇒ syndiotactic 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 17 Page 4 of 6 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. Consider polypropylene CH3 H2 C C n CH3 Atactic PP: Tg ∼ -70oC fully noncrystalline liquid-like, paste Isotactic PP: Tm = 170oC highly crystalline amorphous region: Tg ∼ -50oC - -60oC insoluble Syndiotactic PP: very crystalline Tm ∼ 130oC CH2=CH2 → H2 H2 C C n regular structure Traditional free radical polymerization Tubular Reactors: 2-6 cm in diameter 0.5 – 1.5 km in length CH2=CH2 heat to gas 175oC peak T 300oC ∼ T > Tcrit P > 200 MPa supercritical fluid flui d add initiator O2, H2O2, CTA π = 20-30% precipitate T = 25oC 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 17 Page 5 of 6 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date. Chain transfer to monomer and polymer: + + Branches hinder crystallinity Low density PE (LDPE) ρ = 0.9 g/cm3 Tg = -120oC Tm = 105oC Highly branched, Irregular length Z-N catalyst: reduce transfer minimal branching High density PE (HDPE) % crystallinity: 80-90% ρ = 0.94 – 0.96 Tm > 130oC 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 17 Page 6 of 6 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.