Lecture 20: Intro to Anionic Polymerization, Monomers Applicable to Anionic Methods, Kinetics of “Nonliving” Anionic Polymerization

10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 20: Intro to Anionic Polymerization, Monomers Applicable to Anionic Methods, Kinetics of “Nonliving” Anionic Polymerization Choosing Initiators (for anionic polymerization) (must be strong enough to initiate monomer) → should not attack any monomer side groups e.g. styrene: Readily initiated by any alkyl lithiums (e.g. n-butyl Li, sec-butyl Li) Cations: (Na) (Na) CANNOT initiate with diphenyl methyl Na CH Na Compare with: e- withdrawing effect large due to inductance C O O MMA CH3 C O C O resonance O O CH3 CH3 → easy to initiate → fluorenyl Na+, diphenyl methyl Na But NOT: → side rxns with ester + O O CH3 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. Kinetics of anionic polymerization In protic media: + Termination in ionic polymerization ⇒ only by transfer or side rxn ⇒ no coupling/disproportionation NaNH2 in H2O/NH3+ initiator solvent IF transfer leads to anion that is reactive enough to initiate monomer ⇒ transfer IF transfer leads to anion that won’t initiate monomer ⇒ termination K NaNH2 Na + NH2 initiating species [NH ] = K [NaNH [Na ] − 2 + 2 ] 1. Initiation: ki NH2 + M 2. Propagation: NH2M kp NH2Mi + M 3. Transfer to solvent: NH2Mi + NH3 NH2Mi+1 ktr,NH3 solvent NH2MiH + NH2 initiates more monomers kinetic chain lives BUT chain length shortens 4. Transfer to water: (termination event) k NH2Mi + H2O tr,H2O NH2MiH + HO relatively stable, will not initiate styrene (only thing HO- will initiate is epoxides) [ ] Ri = k i NH 2− [M ] = k i K [M ][NaNH 2 ] [Na ] + 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 20 Page 2 of 4 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 p = k p M − [M ] [ ] [M ][H O] Rtr,NH 3 = k tr,NH 3 M − [NH 3 ] Rtr,H 2O = k tr,H 2O − 2 termination Steady state assumption: [M-] = constant Ri = Rtr,H2O k i K [M ][NaNH 2 ] [Na ] + [ ] = k tr,H 2O M − [H 2 O ] rearrange to get expression for [M-] ] [M ] = kk K [M[H][NaNH O ][Na ] − i tr,H 2O Rp = Kk i k p [M ]2 [NaNH 2 ] 2 + 2nd order in [M] [ ][H O] k tr,H 2O Na + 2 2 Rp ∝ [M]2 (b/c: [M-] ∝ [M]) 1 pn = C NH 3 [NH 3 ] C H 2O [H 2 O ] + [M ] [M ] transfer to solvent transfer to H2O Living Polymerizations 1. No transfer No termination events due to other side rxns that might occur due to impurities ⇒ need solvent with no protic groups (aprotic) ⇒ eliminate H2O to get dry solvent O2 CO2 Other reactive species like NH2, anything that can snatch H off “lifetime” of propagating anion can be very long (∼ hours) 2. Need system with very rapid Ri >> Rp fast initiation start polymerization at same time to get highly controlled MW + polydispersity All chains start at the same time and finish the monomer off ∼ monodisperse MW distribution 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 20 Page 3 of 4 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. Kinetics of Living Polymerizations [ ] = k [M ][M ] 1. initiation: Ri = k i I − [M ] 2. propagation: Rp Rapid initiation: [I ]o ≅ [M − p − ] 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Rp=kp[M][I]o Lecture 20 Page 4 of 4 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.