1 IONIC POLYMERIZATION I. Basics A. Ionic polymerization is a type of chain polymerization. (Polymer chain grows at one end rather than two ends.) B. Cationic polymerization 1. Lewis acid induces formation of carbocation that propagates when reacting with olefin. 2. Vinyl monomer must be substituted to stabilize carbocation. H2C C + CH 3 CH 3 CH 3 CH 3 CH 3 F3B H2 C F3B BF3 H2 C C CH 3 H2 C CH 3 + C H2C C CH 3 CH 3 C CH 3 C. Anionic polymerization 1. Strong base (amide ion) or alkyl lithium induces carbanion formation. 2. Electron withdrawing groups must stabilized carbanion. H2C CH H2C C H2C CH NH 2+ H3C H C H2 C CH D. Ionic polymerizations can also known as living polymerizations. 1. Counterion “caps” chain. (gegenion) 2. Adding more monomer causes chains to grow. 2 2. Kinetics of Cationic Polymerizations A. Initiation 1. First-order in monomer and initiator. I + M M+ i ki I M 2. Initiators a. Lewis acids: BF3, AlCl3, AlBr3, w/H2O b. Triphenylmethylhalides C + C Cl c. Tropylium halides + Cl B. Propagation 1. First-order in monomer and carbocation. M + Mn+ Mn+1+ 2. Stabilization of carbocation a. Hyperconjugation H3C H3C C H3C p k p M M CH 2 > C H CH 2 > H2C CH2 Cl Cl 3 b. HC Resonance stabilization in styrenes CH 3 HC HC CH 2 HC CH 2 CH 2 OCH3 > CH3 > H > Cl for para substitution on styrene. c. Stabilization with oxonium ion H O + H2C d. CH 2 H O O H2C CH 2 H3C H2 C O CH CH 2 Solvent stabilization iii. High dielectric constant solvents help keep carbocation and gegenion separate i. Often monomer is its own solvent. ii. CH2Cl2, CH3Cl, C6H5NO2 iii. High dielectric constant solvents help keep carbocation and gegenion separate. iv. Aprotic polar solvents are best. C. Chain Transfer 1. Undesirable; charge transfer from chain to monomer prematurely ends propagation. M + Mn+ M+ + Mn Tr k Tr M M 2. Proton traps reduce chain transfer, increase molecular weight, decreases polydispersity. Example: 2,5-di(t-butyl)pyridine N D. Termination 1. First-order in carbocation. Mn+ + G- Mn t k t M 4 E. Degree of Polymerization 1. Assume that i t . k i I M k t M M k i I M kt 2. Assume that the rate of polymerization is the rate of propagation. p k p M M k p M k i I M kt k p k i I M 2 kt 3. Without chain transfer, DP is ratio of propagation rate to termination rate. p k p M M kp M kt k t M 4. If chain transfer is dominant, DP is ratio of propagation to chain transfer rate. Note DP becomes independent of monomer concentration. DP t DP p tr k p M M k tr M M kp k tr 5. If the initiation rate is much greater than the propagation rate, i >> p, M M0 ek It M 0 M DP I0 i 5 3. Kinetics of Anionic Polymerizations A. Initiation 1. Rate law is same as cationic polymerization, first-order in monomer and initiator. 2. Initiators a. Bases: OH-, NH2-, H2O b. Alkyllithium compounds: t-butyl lithium c. Alkali metals Na + M Na+ + M- B. Propagation 1. Rate law same as cationic polymerization, first-order in monomer and carbanion. 2. Stabilization of carbanion a. Electron-withdrawing groups, R = CN, CO2R, C6H5 H3C CH R b. Solvating power of solvent glyme (ethylene glycol dimethyl ether) > tetrahydrofuran > benzene > cyclohexane C. Chain Transfer 1. Carbanion can induce proton transfer from oligomer and as well as monomer. Tr k Tr M M 2. Chain transfer reduces molecular weight; therefore it is generally undesirable. 3. Chain transfer is slowed at low temperature. D. Termination 1. Often has no specific termination step, polymer is living polymer. 2. Chain transfer is slowed at low temperature. 3. Termination is proton transfer from solvent e.g., liquid ammonia. t k t NH3 M 6 E. Degree of Polymerization 1. Assume that i t k i NH 2 M k t NH 3 M M k i NH 2 M k t NH3 2. DP is ratio of propagation rate to termination rate. p k p M M k p M DP p t k i NH 2 M k t NH3 k p M M k t M NH3 kp k p k i NH 2 M 2 M k t NH3 k t NH3