10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 17: Stereoregular Polymerizations Final comments: copolymerization Recall azeotrope: occurs r1, r2 < 1 ⇒ 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 1.0 0 F1 1.00F11.0 0.51.00.500f1f1For 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: []M1 − = []o πM Mf1dM =ln []M = df1∫M [] ∫(1 − fMo F 1 )M ()of1 o []M ⎡ ⎤α⎡ f ⎤β()− ⎤γ π=1 −M =1 −⎢ff1 ⎥ ⎢() 2 ⎥⎢⎡f1 o δ)⎥ 1.01.0[] () f (f −δo⎣ 1 o ⎦ ⎣ 2 o ⎦⎣ 1 ⎦ where α= r2 1 − r2 r1 molmolβ= frafr cac1 − r1 γ= 1 − r1r2 (1 − r1 )(1 − r2 ) δ= 1 − r2 (2 − r1 − r2 ) 00f2 avg <F2> F2 avg <F1> F1 f1 f2avg <F2>F2avg <F1>F1f10 1.001.0ππFor 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.Stereochemistry of polymer chains R R R R isotactic (same side) RR RR syndiotactic (alternating) RR RR R atactic (no sequence, random) Dyad tacticity fraction m + r = 1.0 isodyad syndio (m = meso) (r = racemic) Triad tacticity mm + rr + mr =1 use percentages = 100% (use NMR to determine iso syndio hetero percentage) (mm) (rr) (mr) % %% CH2=CHR In “normal” solution/bulk free radical polymerization RH RR HH kr C+ H2C CHR RR R R H R C km 10.569, Synthesis of Polymers, Fall 2006 Lecture 17 Prof. Paula Hammond 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 Tp =100oC Tp =-78oCC C r= 0.73 r= 0.87 CH3 e.g. PVC H H2 Tp =60oC Tp =-38oCC C r=0.5 r=0.6 Cl Coordination Polymerization Coordinated catalyst: usually metal/metal halide or oxide complex: - promote stereospecific + controlled addition of monomer Ziegler and Natta Nobel Prize, 1963 (Germany) (Italy) e.g. 10.569, Synthesis of Polymers, Fall 2006 Lecture 17 Prof. Paula Hammond 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 α-olefins have H2C CH very high or H2C C transfer rates to CH3H2C monomer nH propylene (propane) Initiator: Group I-III metal (e.g. Al) + transition metal (e.g. Ti) G+ RHC CH2 HC CH2G R group escort for metal insertion of new monomer Ziegler-Natta Catalyst Usually TiCl4 or TiCl3 + AlR3 + AlR2Cl R=CH2CH3 or (CH2)nCH3 alkyl tail CH2 CH2 Cl Cl ClCl CHCH3H2C CHCH3 π completion TiTi CH2 ClCl Cl Cl CH2 CHCH3 CHCH3 CH2ClCl Cl Cl H2C CHCH3Ti CH3 Ti Cl Cl ClCl “Escort” process guides stereochemistry many of Z-N catalysts (crys) ⇒ isotactic addition (soluble) ⇒ syndiotactic H2C 10.569, Synthesis of Polymers, Fall 2006 Lecture 17 Prof. Paula Hammond 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 H2H2CH2=CH2 →C C n regular structure Traditional free radical polymerization Tubular Reactors: 2-6 cm in diameter 0.5 – 1.5 km in length heat toheat toCH2=CH2CH2=CH2peak T T > Tcritpeak T T > Tcrit∼ 300oC P > 200 MPa∼ 300oC P > 200 MPagas add initiator O2, H2O2, CTA 175oC π = 20-30% precipitate T = 25oC gasadd initiatorO2, H2O2, CTA175oCπ = 20-30%precipitateT = 25oCsupercriticalsupercriticalfluifl duid10.569, Synthesis of Polymers, Fall 2006 Lecture 17 Prof. Paula Hammond 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 Lecture 17 Prof. Paula Hammond 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,