10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 26: Cationic Ring Opening Polymerization, Other Ring Opening Polymerization Cationic Polymerization Kkik p [][ ][ ] I ZY M 2 R = p kt Energetics of Cationic Polymerization R ∝ kik p ⇒ Ap Ai exp⎜⎛− Ep − Ei + Et ⎟⎞ p kt At ⎜⎝ RT ⎟⎠ -Ep-Ei+Et = ERP ∼ 5-10 kcal/mol smaller impact of T than in free radical - More directly impacted by T: transfer rates CM ↓ by 100x for ΔT from -30oC to -60oC (isobutene) Often use refrigeration for commercial processes E = −Ep + Et +∑Etrpn energy of transfer rxns Example: Cationic Polymerization of Polyisobutylene (PIB) CH3 CH3 H2H2C C C C n CH3 CH3 temp of polymerization: -100oC to -30oC need a lot of refrigeration! → high rate of heat release want Rp 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 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.Porous crystal particlesPorous crystal particles⇒ allow precipitation of monomer⇒ allow precipitation of monomer⇒ continued chain growth⇒ continued chain growth⇒ low η slurry⇒ lowηslurryη, T controlη, T controllow T’slow T’sExample: To Form Butyl Rubber CH3 CH3 n H2C C + mH2C C CCH2 H CH3 (isoprene) can form crosslinks(isobutene) with crosslinking agents introduces vinyl group into backbone to decrease % crystallinity 0.5% - 2.5% by 1,4 trans Solvent: CHCl3 Doesn’t crystallize (also precipitant of polymer) even at low T → stiff and brittle Butyl rubber: solvent resistant app: space shuttle Ex: gloves - doesn’t break down in ozone (O3) → chemically stable - chem tank liners - chem tubes Cationic: very rapid, high MW Difficult to control, PDI affected Living Cationic Polymerization - must have Ri >> Rp (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 10.569, Synthesis of Polymers, Fall 2006 Lecture 26 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.Examples Vinyl ethers H2C CH OR' Initiator: HI + ZnI2 Solvent: toluene -40 to -25oC Ethyl styrene H2C CH CH3 Initiator: CH3COClO4 Solvent: CH2Cl2/Toluene, -78oC - 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 Ring-Opening of Cyclic Ethers (best example of ring-opening family) O ⇒ oxirane or epoxide n=2 monomers: ethylene oxide O (CH2)n propylene oxide O CH3 or other epoxide rings 10.569, Synthesis of Polymers, Fall 2006 R Lecture 26 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. On=3 n=4 where R 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 +O O O O can be done in controlled environment (living polymerization) pn ↑ slowly with conversion Rate, kinetics indicate living system = k [][ −]R appM Mp p ll []I p ()t =[] [] Mo []− IMt ⇒ p =[M []]o n n I Can initiate with any oxyanion: CH3O- and its analogs carboanion CH2 CH - all other cyclic ethers only go by cationic initiation Examples: oxetanes O Initiators: strong acids: H2SO4, CF3COOH or Lewis acid with counterions that don’t combine R R 10.569, Synthesis of Polymers, Fall 2006 Lecture 26 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.Lewis Acid Strong Acid Strong Acid Lewis Acid RA RRR or + O H O or R O RRR R A A Propagation Step RR H2R2H2HO CC C O R H O + O RRR Termination Step - Will happen if A- is nucleophilic → counterion combination (bad if irreversible) If H2O is present (very good nucleophile) If :NH2 is present Use counterions that are stable AsF6-, PF6, SbCl6- Or very strong fluorosulfonic acids CF3SO3-H+ Only combine reversibly and rapidly Initiate with mono- and bifunctional initiators e.g. + OH3C OF3C SO3CH3 + O CF3SO3 large, bulky, only combines reversibly if at all or Triflic anhydride O O (CH2)4 O (CH2)4 OCF3SO2 O SO2CF3 +n propagates in both CF3SO3 directions CF3SO3 10.569, Synthesis of Polymers, Fall 2006 Lecture 26 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.want to avoid: BF3/H2O AlCl3/HCl temporarily O(CH2)4 O +(CH2)4 O O (CH2)4 (CH2)4 O → causes shuffling ⇒ Flory distribution, hurts PDI Best Living Systems Pre-existing carbenium ions -C6H5-C+=O SbF6 Carbenium ions-CH3CH2-C+=O PF6 (C6H5)2C+H B-F4 Triflic systems CF3-SO2-O-CH3, etc Avoid: ROH RNH3 ROR Other ethers (check relative reactivities) 10.569, Synthesis of Polymers, Fall 2006 Lecture 26 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.Can intentionally terminate to get o-functionalities H2O (CH2)6 O O(CH2)4-OH NR3 O(CH2)4-NR3 tertiary amine group RNH2 O(CH2)4-NHR primary amine O(CH2)4-O-O Na LiBr O(CH2)4-Br Ch. 7 Odian: Ring strain Diff impacts of ring sizes 10.569, Synthesis of Polymers, Fall 2006 Lecture 26 Prof. Paula Hammond Page 7 of 7 Citation: Professor Paula