10.569 Synthesis of Polymers Prof. Paula Hammond Lecture 19: Metallocene Chemistry, Intro to New Developments from Brookhart, Others Ionic Polymerization 1. Anionic 2. Cationic Anionic Polymerization - very aggressively charged negative group Y C4H9Li + H2C CHY C4H9 CH2 C propagate Li n-butyl lithium H carbanion Li This is a form of C Also have negative charge on other atoms: e.g. ring opening: H H2HHCO + H2C CHR CO C CO RR O R H CH2 C Y should be electron withdrawing ⇒ stabilizes charge by sharing with C Y new bond formed Monomer Requirements In general, for vinyl monomers, need monomer that supports a stable carbanion 1. Y can fit a range of electron-withdrawing groups How withdrawing impacts monomer reactivity 2. (second requirement for monomer) monomer should have no protic or acidic hydrogens 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.No: -COOH carboxyl groups R-OH alcohols -NH2 amide groups (polyacrylimide in electrophoresis) for biological experiments OH -C≡C-H and more… basically if it gives a H away easily, it’ll be a problem 3. contains no electrophilic groups: Note: carbanion is a very strong nucleophile Avoid e.g. O HN C O NC R R There are some exceptions: certain groups are electrophilic but less reactive to carbanion of interest: e.g. O C OCH3 That’s why we have ranking of reactive groups Finally 4. Carbanion generated must be able to attack its own monomer Now, good monomers for anionic polymerizations: Vinyl: CH3 • styrenes: R • vinyl aromatics: • vinyl pyridines: N 10.569, Synthesis of Polymers, Fall 2006 Lecture 19 Prof. Paula Hammond Page 2 of 5 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.Alkyl methacyrlates: CH3 +longer alkylsC O O CH3 NOT acrylates: H C O O CH3 CH3H2Carbon that becomes anion is the one with the Y group. RCC C O O Ex: methyl methacrylate CH3 Ring Compounds: • epoxides: BUT not free radical versions of epoxy O • lactones: O ex: get caprolactone O O • siloxanes: Si O O ex: silly putty Si silicone oil Si O Anionic Polymerization Initiators What about initiators? There are rules that govern good initiators as well: O 10.569, Synthesis of Polymers, Fall 2006 Lecture 19 Prof. Paula Hammond Page 3 of 5 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.- must be reactive enough to attack monomer i.e. stronger nucleophile (more aggressive) So this depends on the monomer to be initiated: • If monomer substituent Y is strongly e- withdrawing; → then activated monomer is relatively stable → relatively weaker nucleophiles can initiate it ex: epoxy: ethoxyanion initiate ring polymerization with variety of initiators • If substituent Y is weakly e- withdrawing: → need stronger nucleophile to initiate it: ⇒ need to know reactivity trends in monomers Reactivity trends: CH3 dienes: (ex: isoprene) < < <_ H2CCC CH2 < ...H H (butadiene) N (styrenes) usually don’t introduce protic systems to help propagation Most difficult Easiest to (use them to terminate reaction) to initiate initiate Increasing ease of initiation CH2 O<... < C O < N RO vinyl oxyanions are always pyridenes methacrylates more stable than carbanions Types of initiators and trends of reactivities “mild” “strong” reactivity 10.569, Synthesis of Polymers, Fall 2006 Lecture 19 Prof. Paula Hammond Page 4 of 5 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.CH3 < C < alkyl Na < CH < Na CH3 Li or Na benzyl Na cumyl Na diphenyl methyl sodium another useful initiator: Aromatic radical anions: Nao + Na “gives” electron to Naphthalene reductionNa napthalene I Na Y I+ HC CH2 free electron YCH-CH2 + YC-CH2 more stable ⋅ CH2 can pair with itself dimerization of (lower) radical species Y Y H2H2 C C C C ⇒ NaNa H H dianionic species propagate in both directions _ _ 10.569, Synthesis of Polymers, Fall 2006 Lecture 19 Prof. Paula Hammond Page 5 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology,