Cationic PolymerizationKinetic Steps for Cationic PolymerizationInitiation: Use AcidsPropagationTermination and Transfer (Several Possibilities)Transfer (Kinetic Chain Maintained)Kinetic ExpressionsInititation: Assume Lewis Acid PairPropagationTerminationValidity of Steady State Assumption10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 25: “Living” Cationic Polymerizations, Examples of Cationic Polymerization, Isobutyl Rubber Synthesis, Polyvinyl Ethers Cationic Polymerization Some differences between cationic and anionic polymerization • Rates are faster for cationic (1 or more orders of magnitude faster than anionic or free radical) • is very reactive, difficult to control and stabilize C → more transfer occurs → more side reactions → more difficult to form “living” systems → hard to make polymers with low PDI or block copolymers • Living cationic only possible for a specific subset of monomers • Most industrial cationic processes are not living - recent developments are improving this Kinetic Steps for Cationic Polymerization Initiation: Use Acids • Protonic Acids (Bronsted): HA strong, but without nucleophilic counterion HClO4, CF3SO3H, H2SO4, CFCOOH comes off → ClO4-HA +H2CCHRH3CCHRAwant to avoidrecombinationthrough counterion • Lewis Acids Often as initiator/coordination complexes helps stabilize counterions and prevent recombination BF3 + H2O ↔ [H+BF3-OH] Equilibrium between anion-cation pair AlCl3 + RCl ↔ [R+AlCl4-] SbF5 + HF ↔ [H+SbF6-] 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.Carbenium salts with aromatic stabilization C Cl+SbCl5CSbCl6 Propagation CH2CHRH2C CHRA+CH2CHRAH2CCHRinitiation speciesvinyl monomer Note: rearrangements can occur, especially if a more stable carbocattion can be formed (e.g. tertiary carbocation) (most common for 1-alkenes, α olefins) e.g. 2 methyl butene H2CCHCHH3CCH3 CH2CHC HH3CCH3CH2H2C CCH3CH3secondary carbocationtertiary carbocation This occurs via intramolecular hydride (H-) shifts Usually slow: If Rp ≤ rearrangement rate, will get rearranged product H2CH2C CCH3CH3 If Rp > rearrangement rate, will get random copolymer H2CH2C CCH3CH3NH2C CHH3C CH3m As T↑, m↑ (less rearrangement) Rate of rearrangement does not increase as fast as rate of propagation. Hydride shift NOT common for conjugated monomers like: styrene, vinyl ethers and isobutylene and other tertiary carbocations. 10.569, Synthesis of Polymers, Fall 2006 Lecture 25 Prof. Paula Hammond Page 2 of 7 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.Termination and Transfer (Several Possibilities) A) Termination with counterion: kills propagating cation, kinetic chain (kt) i) Combination CH2CHR+CF3COOkt,combCH2CHRO C CF3O ii) Anion Splitting CH2CHR+BF3OHkt,sCH2CHROH+BF3 B) Transfer or termination to impurity or solvent O To H2O, ROR, NR3, , etc. C OR10.569, Synthesis of Polymers, Fall 2006 Lecture 25 Prof. Paula Hammond Page 3 of 7 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. HMNM(IZ)+XAktr,sInitiatorImpurityor SolventHMNMA+X(IZ) e.g. CH2CHR+RORCH2CHRO AARRmore stable thanCHRnot as reactive,acts as retardant- will not propagate further or CH2CHR+R-OHCH2CHROR AAweak acidwill not initiate+H All these processes kill chain length.Transfer (Kinetic Chain Maintained) A) Proton transfer to monomer CHHC AHR+H2CCHRCHCAHR+H3CCRHpropagates B) Hydride ion transfer from monomer CH2CHR+CH2CH2RAApropagates+H2CCRHH2CCR In general, chain transfer to monomer is favorable so CM → pMtrkk, can be sizeable. C) Proton transfer to counterion (“spontaneous termination”) CH2CCH3CH3CH2CAAusually goes onto initiate again+ktr,ciCH3CH2Hpropagatingcontinuesprotic acid initiatorsare possible+Lewis acids areless likely Usually propagates (does NOT kill chain) 10.569, Synthesis of Polymers, Fall 2006 Lecture 25 Prof. Paula Hammond Page 4 of 7 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.Kinetic Expressions Inititation: Assume Lewis Acid Pair 1. ⇒ ()YIZKIZY−+⎡⎤⎣⎦=⎡⎤⎡ ⎤⎣⎦⎣ ⎦ +ZYY(IZ)I 2. often rate limiting +MY(IZ)kiYM(IZ) If step 2 is rate determining, then () iiiRkYIZ MkK I ZY M−+⎡⎤=⎡⎤⎣⎦⎣⎦=⎡⎤⎡ ⎤⎡ ⎤⎣⎦⎣ ⎦⎣ ⎦*Ri could be determined based on step 1. Then the expressions would be different. Propagation +MYMj(IZ)kpYMj+1(IZ)some numberof monomer 10.569, Synthesis of Polymers, Fall 2006 Lecture 25 Prof. Paula Hammond Page 5 of 7 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. ()pp j pRkYMIZ MkMM−++⎡⎤⎡⎤==⎡⎤⎡⎤⎣⎦⎣⎦⎣⎦⎣⎦YMIZ k M− Assumption: chain length has little effect on reactivity Let [M+] = total concentration of all-size propagating carbocations (ion pairs + free ions) Termination Must determine primary means of termination (solvent, impurities, counterion combinations, or all?) Example case: termination by counterion combination ⇒ Rk(),,ttcomb tcomb++⎡⎤⎡⎤==⎣⎦⎣⎦i= If we assume steady state [M+] Steady state assumption is that Rtt iRkM kKMIZYR+⎡⎤==⎡⎤⎡⎤⎡ ⎤⎣⎦⎣⎦⎣ ⎦⎣⎦t = Ri iittkK M I ZYRMkk+⎡⎤⎡⎤⎡ ⎤⎣⎦⎣⎦⎣ ⎦⎡⎤==⎣⎦ Going back to Rp with itRMk+⎡⎤⎣⎦ = 2iip ppttKk k I ZY MRk MRkk⎡⎤⎡ ⎤⎡ ⎤⎡⎤⎣⎦⎣ ⎦⎣ ⎦⎣⎦== second order in [M] First order in RiYM(IZ)kt,combYMIZ(unlike free radical) NP: No transfer (to monomer, solvent, counterion) ppNttRkMPRk⎡⎤⎣⎦== NP: If transfer occurs Rtr,M: to monomer → create new propagating chain Rtr,S: to solvent → create new cationic species Rtr,Ci: to counterion → recreate initiation ,,pNttrCitrMtrSRPRR R R=+++, with ,,,,,,tr Ci tr Citr