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 • Protonic Acids (Bronsted): HA comes off strong, but without nucleophilic counterion HClO4, CF3SO3H, H2SO4, CFCOOH - → ClO4 want to avoid recombination through counterion CH A H A + H2C H3CCH R R

• Lewis Acids Often as initiator/coordination complexes helps stabilize counterions and prevent recombination

+ - BF3 + H2O ↔ [H BF3 OH] + - AlCl3 + RCl ↔ [R AlCl4 ] Equilibrium between + - SbF5 + HF ↔ [H SbF6 ] 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 H H A H2 A CH2 C CH2 C C C + H2C CHR R vinyl monomer R R initiation species

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 H2C CH

CH H C 3 CH3

H CH3 C C H2 H2 C C C H3C C H H2 CH3 CH3 tertiary carbocation secondary 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 H2 C C C C C N m CH3 H3C 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 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 H H O k t,comb CH C O C CF CH2 C +CF3COO 2 3 R R

ii) Anion Splitting H H kt,s CH2 C +BF3OH CH2 C OH +BF3 R R

B) Transfer or termination to impurity or solvent O To H2O, ROR, NR3, C OR , etc.

Initiator k tr,s + HMNM(IZ) + XA HMNMA X(IZ) Impurity or Solvent e.g. more stable than H C H H R A R CH2 C + ROR CH2 C O A R R R not as reactive, acts as retardant - will not propagate further or H H A CH2 C + R-OH CH2 C OR + H A R R weak acid will not initiate

All these processes

kill chain length.

10.569, 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. Transfer (Kinetic Chain Maintained) A) Proton transfer to monomer H H H H H +HCC C C A + H2C CH C C 3 A H R R R R propagates

B) Hydride ion transfer from monomer

H H CH2 CH + H C C A CH2 C A + H2CC 2 2 R R R R propagates

k ,Mtr In general, chain transfer to monomer is favorable so CM → can be sizeable. k p

C) Proton transfer to counterion (“spontaneous termination”) CH3 CH3 k tr,ci CH2 C + H CH2 C A A

CH3 CH2 usually goes on to initiate again propagating continues protic acid initiators are possible +Lewis acids are less 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

− ⎡YIZ+ ⎤ I () 1. +ZY Y(IZ) ⇒ K = ⎣ ⎦ ⎣⎡IZY⎦⎣⎤⎡ ⎦⎤

ki 2. Y(IZ) +M YM (IZ) often rate limiting

If step 2 is rate determining, then ⎡⎤+ − RkYIZMii= ()⎣⎦⎡⎤ ⎣⎦

= kKi ⎣⎦⎣⎡⎤⎡ I ZY ⎦⎣⎦⎤⎡⎤ 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 Let [M+] = total concentration of of monomer all-size propagating carbocations − RkYMIZMkMM==⎡⎤++⎡ ⎤⎡⎡⎤⎤ (ion pairs + free ions) pp⎣⎦ j()⎣ ⎦p⎣⎦⎣⎦

Termination Must determine primary means of termination (solvent, impurities, counterion combinations, or all?)

Example case: termination by counterion combination

kt,comb − YM(IZ) YMIZ ⇒ Rk==⎡⎤YMIZk+ ⎡ M+ ⎤ ttcomb,,⎣⎦() tcomb⎣ ⎦

If we assume steady state [M+] RkM==⎡⎤+ kKMIZYR= Steady state assumption is that R = R tt⎣⎦ i⎣⎦⎣⎦⎣⎦⎡⎤⎡⎤⎡⎤ i t i kK M I ZY + Ri i ⎣⎦⎣⎦⎣⎦⎡⎤⎡⎤⎡⎤ ⎡⎤⎣⎦M == kktt

+ Ri Going back to Rp with ⎣⎦⎡⎤M = kt 2 Kk k⎡⎤⎡ I ZY ⎤⎡⎤ M Rkip⎣⎦⎡⎤ M i p⎣⎦⎣ ⎦⎣⎦ Rp == second order in [M] kktt

First order in Ri 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. (unlike free radical)

PN : No transfer (to monomer, solvent, counterion)

RkMpp⎣⎦⎡⎤ PN == Rktt

PN : 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

RkM= ⎡⎤+ tr,, Ci tr Ci ⎣⎦ R P = p with RkMM= ⎡⎤+ N tr,, M tr M ⎣⎦⎣⎡ ⎦⎤ RRttrCitrMtrS+++,, R R, RkMS= ⎡⎤+ tr,, S tr S ⎣⎦⎣⎡ ⎦⎤

kMp ⎣⎦⎡⎤ PN = kkttrCitrM++,, k⎣⎦⎡⎤ Mk + trS , ⎣⎦⎡⎤ S

1 k kS⎡ ⎤ =+++t tr, Ci CC⎣ ⎦ kM kMMS M PN pp⎣⎦⎡⎤ ⎣⎦⎡⎤ ⎣⎦⎡⎤

k k tr, M tr, S kp kp

Suppose transfer to solvent or impurity does not result in further propagation.

e.g. NR3 ⇒ R (IZ) HMNM(IZ) +NR3 HMNM N R Is stable R No further propagation This must be included in steady state [M+] expression 2 i p [][ ][]MZYIkKk R p = term from transfer like + ,Strt []Skk

*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 Lecture 25 Prof. Paula Hammond 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 Lecture 25 Prof. Paula Hammond 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.