Cationic Polymerization

Transcription

1 Cationic Polymerization Ionic Polymerization (Compared with Radical one) * Ionic propagating center & counter-ion BA B + A - B + IIA - B + + A - covalent tight ion pair loose ion pair free ion * ighly selective * No combination termination * Rapid rate (high [M + ] or [M - ]) * Extremely sensitive to impurities * Require stabilization of propagating center (solvent) * Low temperature 2017/2/ /2/21 2 1

2 1). Initiation: initiators are electrophiles Electron donating groups are needed (R= -OR, -R, -,...) 2017/2/21 3 Cationic Initiators (Init + ) * Proton acids with unreactive counterions: 2 SO 4, OSO 2 CF 3, ClO 4, * Lewis acid (co-initiator) + Initiator: aluminum chloride (AlCl 3 ), boron trifluoride (BF 3 ), tin tetrachloride (TiCl 4 ), SnCl 4, TiCl 4,... Initiator: proton donor (Protogen): water, X, RO, RCOO, carbocation donor (cationogen): Self-ionization 2017/2/21 4 2

3 K I + ZY Y + (IZ) - Y + (IZ) - + M YM + (IZ) - k i Activity: I : AlCl 3 > AlRCl 2 > AlR 2 Cl > AlR 3 Initiation (ZY: initiator) (I: co-initiator) (IZ - : counter-ion) ZY : Cl > C 3 COO > EtNO 2 > Phenol > 2 O > MeO Ex. BF O BF 3. O2 BF 3. O2 + (C 3 ) 2 =C 2 (C 3 ) 3 C + (BF 3 O) - AlCl 3 + (C 3 ) 3 CCl (C 3 ) 3 C + (AlCl 4 ) - (C 3 ) 3 C + (AlCl 4 ) - + ФC=C 2 (C 3 ) 3 CC 2 C + Ф(AlCl 4 ) - Polymerization rate increases with increasing [initiator]/[co-initiator], reach a maximum, and then either decreases or levels off. 2017/2/21 5 Decreased rate at high [ 2 O]/[SnCl 4 ]: unreactive * Inactivation of SnCl 4 by 2 O (hydrolysis). 2 O * 2 O competes with monomer. SnCl. 4 O2 ( 3 O + )(SnCl 4 O - ) 2017/2/21 6 3

4 2). Propagation M n+ (IZ) - + M M n M + (IZ) - k p Ex. -[C 2 C(C 3 ) 2 ] n+ (BF 3 O) - + (C 3 ) 2 =C 2 -[C 2 (C(C 3 ) 2 ] n -C 2 C + (C 3 ) 2 (BF 3 O) - Intramolecular rearrangement (Isomerization Polymerization): 1,2-hydride ion (: - ) or 1,2-methide (C 3 : - ) shift. 2017/2/21 7 Isomerization Polymerization 2017/2/21 8 4

5 Styrene C 2 C + C 2 C Two other resonance isomers + Propagation is usually very fast. Therefore, cationic vinyl polymerizations must often be run at low temperatures. Unfortunately, cooling large reactors is difficult and expensive. Also, the reaction can be inhibited by water if present in more than trace amounts, so careful drying of ingredients is necessary (another expense). 2017/2/21 9 3). Chain Transfer and Termination Cationic vinyl polymerization is plagued by numerous side reactions, most of which lead to chain transfer. It is difficult to achieve high MW because each initiator can give rise to many separate chains because of chain transfer. These side reactions can be minimized but not eliminated by running the reaction at low temperature. a) Chain Transfer to monomer Transfer of a - to monomer -[C 2 C(C 3 ) 2 ] n -C 2 C + (C 3 ) 2 (BF 3 O) - + C 2 =C(C 3 ) 2 (C 3 ) 3 C + (BF 3 O) - + -[C 2 C(C 3 ) 2 ] n -C 2 C(C 3 )=C 2 + [C 2 C(C 3 ) 2 ] n -C=C(C 3 ) 2 k M n M + (IZ) - tr,m + M M n+1 + M + (IZ) /2/

6 b) Spontaneous Termination: regeneration of initiater-co-initiator complex (Chain transfer to counter-ion) -[C 2 C(C 3 ) 2 ] n C 2 C + (C 3 ) 2 (BF 3 O) - BF 3. O 2 +-[C 2 C(C 3 ) 2 ] n -C 2 C(C 3 )=C 2 k ts M n M + (IZ) - M n (IZ) - c) Combination with counterion (Termination) -[C 2 C(C 3 ) 2 ] n C 2 C + (C 3 ) 2 (BF 3 O) - -[C 2 C(C 3 ) 2 ] n -C 2 C(C 3 ) 2 -O + BF 3 M n M + (IZ) - k t M n MIZ 2017/2/ /2/

7 d) Chain transfer to polymer * Intramolecular electrophilic aromatic substitution (backbiting) C 2 C C 2 C + (IZ) - C 2 C + + (IZ) - * Intermolecular hydride transfer to polymer C 2 C + + C 2 C C 2 C + C 2 C + R R R R e) Other Chain Transfer and Termination Reactions K tr,s M n M + (IZ) - + XA M n MA + X + (IZ) - (XA: chain-transfer agent) M n M + (IZ) - + :NR 3 M n MN + R 3 (IZ) /2/ ) Polymerization Kinetics K I + ZY Y + (IZ) - Y + (IZ) - + M k i YM + (IZ) - rate-determining M n+ (IZ) - + M M n M + (IZ) - M n M + (IZ) - k t R i = Kk i [I][ZY][M] R t = k t [YM + (IZ) - ] k p M n MIZ Steady-state R p = k p [YM + (IZ) - ][M] = R i k p [M]/k t = Kk i k p [I][ZY][M] 2 /k t (R i = R t ) X n = R p /R i = k p [M]/k t M n M + (IZ) - + M M n+1 + M + (IZ) - k ts k tr,m M n M + (IZ) - M n (IZ) - k tr,s M n M + (IZ) - + XA M n MA + X + (IZ) /2/

8 * Second-order dependence upon [M] (rate-determining step: Y + (IZ) - + M YM + (IZ) - ) R p = Kk i k p [I][ZY][M] 2 /k t * First-order dependence upon [M] (rate-determining step: I + ZY Y + (IZ) - ) R i = k i [I][ZY] R t = k t [YM + (IZ) - ] R p = k p k i [I][ZY][M]/k t 2017/2/21 15 Number-average degree of polymerization (M n = M o X n ) X n = R p /(R t + R ts + R tr,m + R tr,s ) = k p [M]/(k t + k ts + k tr,m [M] + k tr,s [S]) R ts = k ts [YM + (IZ) - ] R tr,m = k tr,m [YM + (IZ) - ][M] R tr,s = k tr,s [YM + (IZ) - ][S] k tr,m /k p k tr,s /k p Mayo equation: 1/X n = (k t /k p [M]) + (k ts /k p [M]) + C M + (C S [S]/[M]) 1/X n = (1/X n ) o + C S [S]/[M] X n is independent of initiator concentration. C M : chain-transfer constant for monomer. C S : chain-transfer constant for chain-transfer agent. 2017/2/

9 Steady-state assumption is not valid in many case. Molecular-Weight Distribution (low conv. & steady-state) -(C 2 -C ) x (x-mer: degree of polymerization = x) N x = (1-p)p x-1 N x = N o (1-p) 2 p x-1 w x = x(1-p) 2 p x-1 X n = xn x / N x = xn x = x(1-p)p x-1 = 1/(1-p) X w = xw x = x 2 (1-p) 2 p x-1 = (1+p)/(1-p) PDI = X w /X n = 1+p Where p = R p /(R p + R t + R tr ) Rapid initiation narrows MWD. igh conversion leads to increased PDI. 2017/2/ ) Effects of Temperature R p = Kk i k p [I][ZY][M] 2 /k t k = Ae -E/RT lnk = lna (E/RT) X n = R p /R i = k p [M]/k t lnr p = ln(a i A p /A t ) + ln{k[i][zy][m] 2 } [E i + E p E t ]/RT dln(r p )/dt = (E i + E p -E t )/RT 2 = E R /RT 2 E i < E t, E p < E t, E R can be positive or negative (-20~40 kj/mole) e.g., styrene-ticl 4 / 2 O-C 2 Cl 2 system, E R < 0, T R p lnx n = ln(a p /A t ) + ln[m] - (E p E t )/RT dinx n /dt = (E p E t )/RT 2 = E Xn /RT 2 E Xn < 0, T X n 2017/2/

10 6) Solvent and Counter-Ion Effects Mn-A Mn + A - Mn + A - Mn + + A - covalent bond contact ion-pair solventseparated ion-pair free ions Free ions propagate faster than contact ion-pair Polar solvents favor ion-pair separation (e.g., C 2 Cl 2 ), k p Larger counter-ions are less strongly associated (e.g. SbCl 6- ), k p Dissociation of ions k t for ion-pair rearrangement Pseudocationic polymerizations involve propagation of polarized covalently-bonded species, e.g., C 2 CR 1 R 2 OClO /2/ ) Practical Considerations * Low temperature (thermodynamic and kinetic consideration) * All reactants and solvents should be rigorously dried and purified. Commercial Cationic Polymers 2017/2/