Synthesis of Polymers Prof. Paula Hammond Lecture 19: Metallocene Chemistry, Intro to New Developments from Brookhart, Others H H

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1 Synthesis of Polymers Prof. Paula ammond Lecture 19: Metallocene Chemistry, Intro to ew Developments from Brookhart, thers Ionic Polymerization 1. Anionic 2. Cationic Anionic Polymerization - very aggressively charged negative group C 4 9 Li + 2 C C C 4 9 C2 C Li n-butyl lithium carbanion Li This is a form of C propagate Also have negative charge on other atoms: e.g. ring opening: 2 C + 2 C C C C C Monomer equirements new bond formed In general, for vinyl monomers, need monomer that supports a stable carbanion 1. C 2 C should be electron withdrawing stabilizes charge by sharing with C can fit a range of electron-withdrawing groups ow withdrawing impacts monomer reactivity 2. (second requirement for monomer) monomer should have no protic or acidic hydrogens Citation: Professor Paula ammond, Synthesis of Polymers Fall 2006 materials, MIT pencourseware ( Massachusetts Institute of Technology, Date.

2 o: -C carboxyl groups - alcohols -2 amide groups (polyacrylimide in electrophoresis) for biological experiments -C C- and more basically if it gives a away easily, it ll be a problem 3. contains no electrophilic groups: ote: carbanion is a very strong nucleophile Avoid e.g. C C There are some exceptions: certain groups are electrophilic but less reactive to carbanion of interest: e.g. C That s why we have ranking of reactive groups Finally 4. Carbanion generated must be able to attack its own monomer ow, good monomers for anionic polymerizations: Vinyl: styrenes: vinyl aromatics: vinyl pyridines: Prof. Paula ammond Page 2 of 5 Citation: Professor Paula ammond, Synthesis of Polymers Fall 2006 materials, MIT pencourseware ( Massachusetts Institute of Technology, Date.

3 Alkyl methacyrlates: C + longer alkyls T acrylates: C 2 C C C Carbon that becomes anion is the one with the group. Ex: methyl methacrylate ing Compounds: epoxides: BUT not free radical versions of epoxy lactones: ex: get caprolactone siloxanes: Si ex: silly putty Si Si silicone oil Anionic Polymerization Initiators What about initiators? There are rules that govern good initiators as well: Prof. Paula ammond Page 3 of 5 Citation: Professor Paula ammond, Synthesis of Polymers Fall 2006 materials, MIT pencourseware ( Massachusetts Institute of Technology, Date.

4 - 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 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 is weakly e- withdrawing: need stronger nucleophile to initiate it: need to know reactivity trends in monomers eactivity trends: dienes: (ex: isoprene) < < <_ 2 C C C C 2 < (styrenes) (butadiene)... Increasing ease of initiation usually don t introduce protic systems to help propagation Most difficult Easiest to (use them to terminate reaction) to initiate initiate C 2... < < C < vinyl pyridenes methacrylates oxyanions are always more stable than carbanions Types of initiators and trends of reactivities mild reactivity strong Prof. Paula ammond Page 4 of 5 Citation: Professor Paula ammond, Synthesis of Polymers Fall 2006 materials, MIT pencourseware ( Massachusetts Institute of Technology, Date.

5 a < C a diphenyl methyl sodium < benzyl a < C < alkyl cumyl a Li or a another useful initiator: Aromatic radical anions: a o + a a gives electron to aphthalene reduction a napthalene I I + C C 2 free electron + C-C 2 C-C 2 more stable C2 can pair with itself dimerization of (lower) radical species 2 2 C C C C a a dianionic species propagate in both directions Prof. Paula ammond Page 5 of 5 Citation: Professor Paula ammond, Synthesis of Polymers Fall 2006 materials, MIT pencourseware ( Massachusetts Institute of Technology, Date.