Lecture 19: Metallocene Chemistry, Intro to New Developments from Brookhart, Others

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 H n-butyl lithium Li propagate carbanion Li This is a form of C Also have negative charge on other atoms: e.g. ring opening: H C O + H2C R H C CHR O R O H2 C H C O R new bond formed Monomer Requirements In general, for vinyl monomers, need monomer that supports a stable carbanion 1. H CH2 C Y should be electron withdrawing ⇒ stabilizes charge by sharing with C Y 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 N C H N O C 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 Prof. Paula Hammond Lecture 19 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 C O + longer alkyls O CH3 NOT acrylates: H C O O CH3 R CH3 H2 C C C Carbon that becomes anion is the one with the Y group. O Ex: methyl methacrylate O CH3 Ring Compounds: • epoxides: BUT not free radical versions of epoxy O O • lactones: O ex: get caprolactone O O • siloxanes: Si O O Si Si ex: silly putty silicone oil O Anionic Polymerization Initiators What about initiators? There are rules that govern good initiators as well: 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond Lecture 19 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) < _ < H2C C C CH2 H H (butadiene) (styrenes) N ... usually don’t introduce protic systems to help propagation (use them to terminate reaction) Increasing ease of initiation Most difficult to initiate < Easiest to initiate CH2 ... < < N vinyl pyridenes C O < O RO methacrylates oxyanions are always more stable than carbanions Types of initiators and trends of reactivities “mild” 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond reactivity “strong” Lecture 19 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. Na < < CH CH3 < < C Na alkyl CH3 benzyl Na Li or Na cumyl Na diphenyl methyl sodium another useful initiator: Aromatic radical anions: Na Nao + Na “gives” electron to Naphthalene reduction Na napthalene I YCH-CH2 I + Y HC + CH2 YC-CH2 free electron more stable ⋅ CH2 can pair with itself dimerization of (lower) radical species Y ⇒ C Na H H2 C H2 C Y C H Na dianionic species propagate in both directions _ 10.569, Synthesis of Polymers, Fall 2006 Prof. Paula Hammond _ Lecture 19 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, Date.