Cationic Polymerization

Transcription

1 Synthesis of Polymers Prof. Paula Hammond Lecture 26: Cationic ing pening Polymerization, ther ing pening Polymerization Cationic Polymerization Kk i k p [][ I ZY ][ M ] 2 = p k t Energetics of Cationic Polymerization k i k p A p A i exp E p E i + E t p k t A t T -E p -E i +E t = E P 5-10 kcal/mol smaller impact of T than in free radical - More directly impacted by T: transfer rates C M by 100x for ΔT from -30 o C to -60 o C (isobutene) ften use refrigeration for commercial processes E = E p + E t + E tr pn energy of transfer rxns Example: Cationic Polymerization of Polyisobutylene (PIB) H 2 CH3 H 2 C C C C n temp of polymerization: -100 o C to -30 o C need a lot of refrigeration! high rate of heat release want p to be lower for control (cationic is very rapid, faster than other polymerization methods) - If used a good solvent for PIB high viscosity early on (problem!) - Thus, choose a decent solvent for monomer but poor solvent for polymer PIB - Precipitation occurs as polymer is generated pencourseware ( Massachusetts Institute of Technology, Date.

2 Porous crystal particles allow precipitation of monomer continued chain growth low η slurry low T s η, T control Example: To Form Butyl ubber CH3 n H 2 C C + m H 2 C C C CH 2 H (isobutene) (isoprene) can form crosslinks with crosslinking agents introduces vinyl group into backbone to decrease % crystallinity 0.5% - 2.5% by 1,4 trans Solvent: CHCl 3 (also precipitant of polymer) Butyl rubber: solvent resistant Ex: gloves - doesn t break down in ozone ( 3 ) chemically stable - chem tank liners - chem tubes Doesn t crystallize even at low T stiff and brittle app: space shuttle Cationic: very rapid, high MW Difficult to control, PDI affected Living Cationic Polymerization - must have i >> p (steady state is not a very good assumption) - must eliminate cross-transfer processes (that limit chain growth) 1. carbocation reactivity is lowered 2. deter combination of counterion 3. eliminate other impurities Prof. Paula Hammond Page 2 of 7 pencourseware ( Massachusetts Institute of Technology, Date.

3 Examples Vinyl ethers H 2 C CH ' Initiator: HI + ZnI 2 Solvent: toluene -40 to -25 o C Ethyl styrene H 2 C CH Initiator: CCl 4 Solvent: CH 2 Cl 2 /Toluene, -78 o C - Counterion choice is important e.g. Bulky counterion that cannot recombine or counterion attacks reversibly C + X C-X - Styrenes, vinyl ethers, vinyl carbazoles, isobutylene can attach side groups Living or Quasi-living b/c never completely get rid of transfer PDI 1.1 to 1.2 but good enough to get high yield polymers ing-pening of Cyclic Ethers (best example of ring-opening family) n=2 oxirane or epoxide monomers: ethylene oxide (CH 2 ) n propylene oxide or other epoxide rings , Synthesis of Polymers, Fall 2006 Lecture 26 Prof. Paula Hammond Page 3 of 7 pencourseware ( Massachusetts Institute of Technology, Date.

4 n=3 n=4 where does not affect polymerization oxetane, trimethylene oxide oxolane, tetrahydrofuran n=6 keep increasing (for anionic polymerization: only epoxides can be polymerized) b/c it has a huge angle strain, will open using anionic or cationic initiation anionic + can be done in controlled environment (living polymerization) p n slowly with conversion ate, kinetics indicate living system app M M p = k [ ][ ] p ll [] I p () [ ] [ ] t = M o n [] I M t p = [M n ] o [] I Can initiate with any oxyanion: CH3and its analogs carboanion CH 2 CH - all other cyclic ethers only go by cationic initiation Examples: oxetanes Initiators: strong acids: H 2 S 4, CF 3 CH or Lewis acid with counterions that don t combine Prof. Paula Hammond Page 4 of 7 pencourseware ( Massachusetts Institute of Technology, Date.

5 Lewis Acid Strong Acid A or + A Strong Acid Lewis Acid H or A Propagation Step H + H 2 2 H 2 H C C C Termination Step - Will happen if A - is nucleophilic counterion combination (bad if irreversible) If H 2 is present (very good nucleophile) If :NH 2 is present Use counterions that are stable AsF 6 -, PF 6, SbCl 6 - r very strong fluorosulfonic acids CF 3 S 3 - H + nly combine reversibly and rapidly Initiate with mono- and bifunctional initiators e.g. F 3 C S 3 + H 3 C + CF 3 S 3 large, bulky, only combines reversibly if at all or Triflic anhydride CF 3 S 2 S 2 CF 3 + n (CH 2 ) 4 (CH 2 ) 4 CF 3 S 3 propagates in both directions CF 3 S 3 Prof. Paula Hammond Page 5 of 7 pencourseware ( Massachusetts Institute of Technology, Date.

6 want to avoid: BF 3 /H 2 AlCl 3 /HCl temporarily (CH 2 ) 4 + (CH 2 ) 4 (CH 2 ) 4 (CH 2 ) 4 causes shuffling Flory distribution, hurts PDI Best Living Systems Pre-existing carbenium ions - C 6 H 5 -C + = SbF 6 Carbenium ions - CH 2 -C + = PF 6 (C 6 H 5 ) 2 C + H B - F 4 Triflic systems CF 3 -S 2 --, etc Avoid: H NH 3 ther ethers (check relative reactivities) Prof. Paula Hammond Page 6 of 7 pencourseware ( Massachusetts Institute of Technology, Date.

7 Can intentionally terminate to get o-functionalities (CH 2 ) 6 H 2 (CH 2 ) 4 -H N 3 (CH2 ) 4 -N 3 tertiary amine group NH 2 primary amine (CH2 ) 4 -NH Na (CH 2 ) 4 -- LiBr (CH 2 ) 4 -Br Ch. 7 dian: ing strain Diff impacts of ring sizes Prof. Paula Hammond Page 7 of 7 pencourseware ( Massachusetts Institute of Technology, Date.