Lecture 25: “Living” Cationic Polymerizations, Examples of Cationic Polymerization, Isobutyl Rubber Synthesis, Polyvinyl Ethers

10.569 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) C • is very reactive, difficult to control and stabilize → 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 comes off H A • Protonic Acids (Bronsted): HA strong, but without nucleophilic counterion HClO4, CF3SO3H, H2SO4, CFCOOH → ClO4want to avoid recombination through counterion + H2C CH R H3C CH A R • Lewis Acids Often as initiator/coordination complexes helps stabilize counterions and prevent recombination BF3 + H2O ↔ [H+BF3-OH] AlCl3 + RCl ↔ [R+AlCl4-] SbF5 + HF ↔ [H+SbF6-] Equilibrium between anion-cation pair 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 SbCl5 C SbCl6 Propagation H CH2 A C C H H2 C C R R H + R initiation species H2C CH2 CHR vinyl monomer A 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 CH H2C CH H3C CH3 H C C H2 H3C C H CH3 secondary carbocation C H2 CH3 H2 C C CH3 tertiary carbocation This occurs via intramolecular hydride (H-) shifts Usually slow: If Rp ≤ rearrangement rate, will get rearranged product CH3 H2 H2 C C C CH3 If Rp > rearrangement rate, will get random copolymer H CH3 H2 H2 C C C H2 C N C m H3C CH3 CH3 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 Prof. Paula Hammond Lecture 25 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 H O H kt,comb CH2 C O C CF3 CH2 + CF3COO C R R ii) Anion Splitting H CH2 + C BF3OH H kt,s CH2 C OH + BF3 R R B) Transfer or termination to impurity or solvent O To H2O, ROR, NR3, C OR , etc. Initiator + HMNM(IZ) XA ktr,s HMNMA + X(IZ) Impurity or Solvent e.g. more stable than H CH2 C A + ROR R CH2 H R C O R R or H CH2 C A H C R A not as reactive, acts as retardant - will not propagate further H + R-OH R CH2 C R OR + H A weak acid will not initiate All these processes kill chain length. 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 25 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. Transfer (Kinetic Chain Maintained) A) Proton transfer to monomer H H C C H R A + H2C CH H H C C R H + H3C C A R R propagates B) Hydride ion transfer from monomer H H CH2 C A + H2C R C CH2 CH2 + R R propagates C) Proton transfer to counterion (“spontaneous termination”) CH2 C A ktr,ci k tr , M kp can be sizeable. CH3 CH2 C + CH2 CH3 C A R In general, chain transfer to monomer is favorable so CM → CH3 H2C propagating continues H A usually goes on to initiate again protic acid initiators are possible +Lewis acids are less likely Usually propagates (does NOT kill chain) 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 25 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 I + ZY 1. 2. Y(IZ) Y(IZ) + M ki ⎡Y + ( IZ )− ⎤ ⎦ ⇒ K = ⎣ ZY ⎡⎣I ⎤⎡ ⎤ ⎦⎣ ⎦ YM(IZ) often rate limiting If step 2 is rate determining, then − Ri = ki ⎡Y + ( IZ ) ⎤ ⎡⎣M ⎤⎦ ⎣ ⎦ ZY = ki K ⎣⎡I ⎦⎣ ⎤⎡ ⎦⎣ ⎤⎡M ⎦⎤ *Ri could be determined based on step 1. Then the expressions would be different. Propagation kp YMj(IZ) + M YMj+1(IZ) Assumption: chain length has little effect on reactivity some number of monomer − Rp = kp ⎡YM +j ( IZ ) ⎤ ⎣⎡M ⎦⎤ = kp ⎣⎡M + ⎦⎤ ⎣⎡M ⎤⎦ ⎣ ⎦ 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 kt,comb − YMIZ ⇒ YM(IZ) Rt = kt ,comb ⎡YM + ( IZ ) ⎤ = kt ,comb ⎡⎣M + ⎤⎦ ⎣ ⎦ If we assume steady state [M+] Rt = kt ⎡⎣ M + ⎤⎦ = ki K ⎣⎡M ⎤⎡ ⎤⎡ZY ⎦⎤ = Ri ⎦⎣I ⎦⎣ Steady state assumption is that Rt = Ri ki K ⎡⎣M ⎤⎡ Ri ⎦⎣I ⎤⎡ ⎦⎣ZY ⎤⎦ + ⎣⎡M ⎦⎤ = k = kt t Going back to Rp Rp = Ri k p ⎣⎡M ⎦⎤ kt R with ⎡⎣ M + ⎤⎦ = i kt = Kki k 2 p ⎡⎣I ⎤⎡ ⎦⎣ZY ⎤⎡ ⎦⎣M ⎤⎦ kt second order in [M] First order in Ri 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 25 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. (unlike free radical) PN : No transfer (to monomer, solvent, counterion) PN = Rp Rt = kp ⎣⎡M ⎦⎤ kt PN : If transfer occurs Rtr,M: to monomer → create new propagating chain → create new cationic species Rtr,S: to solvent Rtr,Ci: to counterion → recreate initiation PN = PN = 1 PN = Rp Rt + Rtr ,Ci + Rtr ,M + Rtr ,S Rtr ,Ci = ktr ,Ci ⎡⎣M + ⎤⎦ with Rtr ,M = ktr ,M ⎡⎣M + ⎤⎦ ⎣⎡M ⎦⎤ Rtr ,S = ktr ,S ⎣⎡M + ⎦⎤ ⎣⎡S ⎦⎤ k p ⎡⎣M ⎤⎦ kt + ktr ,Ci + ktr ,M ⎡⎣M ⎤⎦ + ktr ,S ⎡⎣S ⎤⎦ k ⎡S ⎤ kt + tr ,Ci + CM + CS ⎣ ⎦ k p ⎣⎡M ⎦⎤ k p ⎣⎡M ⎤⎦ ⎣⎡M ⎤⎦ ktr ,M ktr ,S kp kp Suppose transfer to solvent or impurity does not result in further propagation. e.g. NR3 ⇒ HMNM(IZ) HMNM + NR3 This must be included in steady state [M+] expression Kk i k p [I ][ZY ][M ]2 Rp = k t + k tr ,S [S ] R (IZ) N R R Is stable No further propagation term from transfer like *does not effect PN expression (do not include in PN calc) *Note: all of the above assumes the 2nd initiation step is rate determining. Validity of Steady State Assumption Not really valid - rxn rates very rapid (seconds – minutes) - often Ri > Rt 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 25 Page 6 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. - [M+] slowly increases with time - [M+] reaches maximum late in polymerization then decreases with further conversion Application of equations is merely an approximation of what really happens. 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 25 Page 7 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.