10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 11: Radical Polymerization, Homogeneous Reaction Rate Kinetics How to determine MW in free radical polymerization Kinetic Chain Length ν = # of monomers added per effective free radical ν= rate of chain growth = rate of chain growth rate of chain initiation rate of chain termination Rp pkp []R Mν= = = Ri Rt 2(fkd kt []21 I )pn =ν if termination is by disproportionation process pn = 2ν if termination is by coupling Generally, (if no chain transfer): pn = 2aν 100% 100% disproportionation disproportionation Mn = M ⋅ pn where ½ ≤ a ≤ 1 n molecular weight of vinyl monomer unit What happens more often? • Coupling usually greater than disproportionation • Percent of coupling increases if: steric factors prevent effective coupling: CH3 CH3 C C CH3 CH3 or if: β-hydrogens are more reactive: 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.H C + C C H k []M Consider ν= p (fkd kt []122 I )1 =k ⎛⎜fk p []I ⎞⎟2 [] R Mp p ⎜kt ⎟⎝⎠ Increase Rp by: [M]↑, [I]↑ But increase ν → [M]↑, [I]↓ Thus you want to increase [M] Chain Transfer 1. Mn + X' Y ktr ktr =transfer constant Mn X' + Y d Y k M '−Y ] Chain transfer can occur when there dt are solvent impurities. But sometimes using chain transfer Rtr = []⋅= tr [ ]⋅[X 2. can be advantageous. ka+ MY YM 3. Chain transfer agent → CTA kp Used to decrease MW in polymerization YM+ M YMn kp >> ktr and kp ≈ ka ⇒ Rp is the same pn ↓ slightly – moderately depending on CTA kp << ktr and kp ≈ ka ⇒ Rp ∼ same pn ↓ dramatically kp >> ktr and ka < kp ⇒ Rp↓ slightly and pn ↓ slightly kp << ktr and ka < kp ⇒ Rp↓ drastically and pn ↓ drastically Transfer Types: 1. to monomer: ktr,m Mn ⋅ + M → Mn + M⋅ 2. to solvent or impurity ktr,s Mn ⋅ + S → Mn + S⋅ 10.569, Synthesis of Polymers, Fall 2006 Lecture 11 Prof. Paula Hammond Page 2 of 5 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.or CTA 3. to initiator: ktr,I Mn ⋅ + I → Mn + I⋅ All act to decrease pn : (assume coupling) R R pn = p = p Rt + R + Rt M ⋅ M []S []Itr,m Rtr,s + Rtr,I + ktr,m [ ][] + ktr,sM ⋅[] + ktr,IM ⋅[]2 2 Use resistor analogy: (resistors in series) C = transfer constant = relative rate const vs. Rp ktr,m ktr,S ktr,ICm = , CS = , CI =k k kp p p since Rp = kp[M⋅][M] 1 = Ri + Cm + CS []S + CI [I ] pn 2Rp [] []M M Additive effect of each constant Rt 2Rp ⎛1 ⎞ 1⎜⎟⎜⎟↔ 2ν⎝pn ⎠o Often only have transfer to CTA (or impurity) 1 = Ri + C []S 2RS [] pn pM (d t [])1 = fk k I 21 kp []M 2ν 10.569, Synthesis of Polymers, Fall 2006 Lecture 11 Prof. Paula Hammond Page 3 of 5 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.For a given amount of initiator [I] and monomer [M] ⎛ ⎞ [] p 1 = ⎜⎜ p 1 ⎟⎟ +CS []MS n ⎝ n ⎠o 11pnpnIncreasa e Incre seMW MWCCss11pnpno o[S][S][M][M]Useful to control MW is free radical with high kp and/or really low kt Cs values for different compounds: π orbitals • alkanes (weakest) • cyclic hydrocarbons • benzenes, aromatics H unstable negative charge → H- extraction unlikely Increasing radical stability CH2< < < resonance stabilization weakest share e- cloud with other C atoms CH2 High Cs values: • weak C—H bonds • stabilized by conjugation O N C O • weak C—Cl, C—Br, C—I 10.569, Synthesis of Polymers, Fall 2006 Lecture 11 Prof. Paula Hammond Page 4 of 5 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.R R R C Cl + R + RCl R C HH • weak S—S bonds, S—H weakest largest CS CTA (chain-transfer-agents) CS x 104 CS x 104 For styrene Vinyl acetate Benzene 0.023 1.2 Cyclohexane 0.031 7.0 Heptane 0.42 17.0 n-butyl alcohol 1.6 20.0 CHCl3 (chloroform) 3.4 150.0 Tri-methyl amine 7.1 370 n-butyl mercaptan 210,000 480,000 SH 10.569, Synthesis of Polymers, Fall 2006 Lecture 11 Prof. Paula Hammond Page 5 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology,