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MIT 10 569 - Lecture Notes

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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 NaCH Compare with: e-withdrawing effect large due to inductance MMA CH3 C O O C O resonanceC O OO 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 + NaNH2 in H2O/NH3+ ⇒ no coupling/disproportionation 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 Na +NH2NaNH2 initiating species K[NaNH ]− 2[NH2 ]= [Na+] 1. Initiation: kiNH2+ M NH2M 2. Propagation: kpNH2Mi + M NH2Mi+1 3. Transfer to solvent: NH2Mi + NH3 ktr,NH3 NH2MiH+ NH2 solvent initiates more monomers kinetic chain lives BUT chainlengthshortens 4. Transfer to water: (termination event) NH2Mi + H2O ktr,H2O NH2MiH + HO relatively stable, will not initiate styrene (only thing HO- will initiate is epoxides) k [ ]M = [][NaNHR = NH −[]kiK M [Na ]2 ] i i 2 + 10.569, Synthesis of Polymers, Fall 2006 Lecture 20 Prof. Paula Hammond 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.Rp = p [M ][]k − M −Rtr,NH3 = ktr,NH3 [M ][NH3 ] −Rtr,H2O = ktr,H2O [M ][H 2O] termination Steady state assumption: [M-] = constant Ri = Rtr,H2O i 2 −k K[M ][NaNH ]= ktr,H O [M ][H 2O]+ 2[Na ] rearrange to get expression for [M-] [M ]= k K[M ][NaNH +]− i 2 ktr,H 2O [H2O][Na ] Kkik p [][ M 2 NaNH2 ]R = 2nd order in [M] p Na +ktr,H 2O [ ][H2O] Rp ∝ [M]2 (b/c: [M-] ∝ [M]) 1 CNH 3 [NH3 ]CH 2O [H 2O] pn =[]+ [] M M transfer transfer to solvent 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 Lecture 20 Prof. Paula Hammond 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 I −[]1. initiation: Ri = ki []M −2. propagation: Rp = kp [M ][M ] Rp=kp[M][I]o Rapid initiation: [] ≅[M −]Io 10.569, Synthesis of Polymers, Fall 2006 Lecture 20 Prof. Paula Hammond 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,


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