Chapter 6. Free Radical Polymerization 6. 1 Introduction

Chapter 6. Free Radical Polymerization 6.1 Introduction 6.2 Free Radical Initiators. 6.3 Techniques of Free Radical Polymerization. 6.4 Kinetic and Mechanism of polymerization. 6.5 Stereochemistry of polymerization. 6.6 Polymerization of Dienes 6.7 Monomer Reactivity 6.8 Copolymerization.

A. Type of polymerization. 6. 1 Introduction Free-radical polymerization Ionic polymerization Complex coordination polymerization

B. Commercialized free-radical polymerization.

6.2 Free Radical Initiators. 6.2.1 Peroxides and Hydroperoxides A. Benzoly peroxide and other peroxides a. Thermal decomposition of BPO. b. Half-life of benzoyloxy radical : 30 min at 100℃ c. Cage effect : confining effect of solvent molecules.

d. Other peroxides. Diacetyl peroxide Di-t-butyl peroxide Diacetyl peroxide Di-t-butyl peroxide (half-life:10hours at 120℃) 6.2.1 Peroxides and Hydroperoxides

e. Promoters : Inducing initiation at lower temperature. (6.9) (6.10) + + + - 6.2.1 Peroxides and Hydroperoxides

B. Hydroperoxide a. Thermal decomposition hydroperoxide b. Cumyl hydroperoxide. 6.2.1 Peroxides and Hydroperoxides

A. α,α'-Azobis(isobutyronitrile) (AIBN). a. Decomposition of AIBN. b. Half-life of isobutyronitrile radical : 1.3 hours at 80℃. 6.2.2 Azo Compounds.

B. Side reaction : Cage effect. a. Tetramethylsuccinonitrile b. Ketenimine 6.2.2 Azo Compounds.

A. One electron transfer reaction. a. Making free radical by one electron transfer by redox reaction. b. Low-temperature reaction. c. Emulsion polymerization. B. Example of redox system. 6.2.3 Redox Initiators.

A. Peroxide and Azo compound. Photolysis and thermalysis. B. Photolabile initiator. 6.2.4 Photoinitiator

A. Polymerization without initiators. a. Dimer formation by Diels-Alder reation. 11 12 · b. Radical formation from dimer. · 6.2.5 Thermal Polymerization.

6.2.6 Electrochemical Polymerization. A. Polymerization of electrolysis. a. Cathode reaction : electron transfer to monomer ion forming radical anion (6.22) b. Anode reaction : electron transfer to anode forming radical cation (6.23) B. Coating metal surfaces with polymers.

6.3 Techniques of Free Radical Polymerization.

6.3 Techniques of Free Radical Polymerization. 6.3.1 Bulk A. Reactor charges. a. Monomer. b. Initiator (soluble in monomer). B. Problems. a. Heat transfer. b. Viscosity. c. Auto-acceleration.

6.3.2 Suspension. A. Reactor charges. a. Monomer. b. Initiator (soluble in monomer). c. Water or other liquid. d. Stabilizer: Poly(vinyl alcohol), CMC B. Vigorously stirring to keep suspension.

6.3.3 Solution. A. Reactor charges. a. Monomer (soluble in solvent). b. Initiator (soluble in solvent). c. Solvent. B. Refluxing solution.

6.3.4 Emulsion. A. Reactor charges. a. Monomer. b. Redox initiator c. Soap or emulsifier. d. Water. e. Others (cf. Table 6.3). B. Polymerization in swollen micelle. Latex products.

6.3.4 Emulsion. TABLE 6.3. Typical Emulsion Polymerization Recipesa Ingredients, Conditions Ingredients (parts by weight) Water Butadiene Styrene Ethyl acrylate 2-Chloroethyl vinyl ether p-Divinylbenzene Soap Potassium persulfate 1-Dodecanethiol Sodium pyrophosphate Conditions Time Temperature Yield aRecipes from Cooper.23 bSodium lauryl sulfate. 190 70 30 - - - 5 0.3 0.5 - 12hr 50oC 65% Styrene-Buradiene Copolymer Polyacrylate Latex 133 - - 93 5 2 3b 1 - 0.7 8hr 60oC 100%

6.4 Kinetic and Mechanism of polymerization. A. Mechanism of free-radical polymerization. a. Initiation. 1) Decomposition. Initiator → 2R․ 2) Addition. (6.25)

b. Propagation. (6.26) 1) Head-to-tail orientation : predominant reaction. Steric and electronic effects. 2) Examples of not exclusively head-to-tail orientation. (13-17% of head to head) (5-6% of head to head) (19% of head to head) 6.4 Kinetic and Mechanism of polymerization.

c. Termination. 1) Combination. (6.27) Polystyrene radical. (6.29) 6.4 Kinetic and Mechanism of polymerization.

2) Disproportionation. Poly(methyl methacrylate) radical. ① Repulsion of ester group. ② Easy alpha hydrogen abstraction. 3) Acrylonitrile : Combination virtually exclusively at 60℃. 4) Poly(vinyl acetate) : Disproportionation. 6.4 Kinetic and Mechanism of polymerization.

B. Kinetic of free radical polymerization. a. Assumption. 1) The rates of initiation, propagation, and termination are all different. 2) Independent of chain length. 3) Negligible end group. 4) At steady state, constant radical concentration. (steady state assumption) b. Initiation (Ri) f : Initiator efficiency. kd : Decomposition rate constant. [I] : molar concentration of initiator. [M ·] : molar concentration of radical.

c. Termination rate ( Rt ) d. Propagation rate ( Rp ) Steady state assumption. kt = ktc+ ktd [M·]= Ri=Rt 2 B. Kinetic of free radical polymerization.

e. Average kinetic chain length ( )  B. Kinetic of free radical polymerization.

f. Gel effect : Trommsdorff effect, Norris-smith effect. 1) Difficult termination reaction because of viscosity. 2) Ease propagation reaction because monomer size is small, even though high viscosity. 3) Autoacceleration by exotherm of propagation reaction. 4) To obtain extraordinary high molecular weight polymer like gel. B. Kinetic of free radical polymerization.

by hydrogen abstracting. Lowering average kinetic chain length. a. Growing radicals move to other polymer chain. b. Backbiting self polymer chain. LDPE : branching polymer. (6.33) C. Chain transfer reactions : Growing radicals move to other parts

c. Moving to initiators or monomers. d. Moving to solvent. (6.34) (6.35) (6.36) (6.37) C. Chain transfer reactions

e. Moving to chain transfer agent. Ct : Chain transfer constant. [T] : Concentration of chain transfer agent. f. Telomerization : At high concentration of transfer agent, ktr>kp. Low-molecular-weight polymers are obtained. (Telomer) (6.39) C. Chain transfer reactions

a. Copper(I) bypyridyl(bpy) complex: b. TEMPO (18) : 2,2,6,6-tetramethylpiperidinyl-1-oxy. (6.42) (6.43) (6.44) (6.45) D. Leaving free radical polymerization : Atom transfer polymerization.

c. Synthesis of block copolymers like anionic polymerization. d. Monodisperse polymerization (PI=1.05). E. Kinetics of Emulsion polymerization. a. N : the number of particles. b. 6.4 Kinetic and Mechanism of polymerization.

6.5 Stereochemistry of polymerization. A. General consideration. a. Stereoregular polymerization : Ionic and complex coordination polymerization. 1) Terminal ion pair : counter ion. 2) Terminal complex active site. 3) Low temperature. b. Stereo-irregular polymerization : Free-radical polymerization. 1) No stereoregulating radical terminal group. 2) Somewhat higher temperature.

B. Factors influencing stereochemistry in free-radical polymerization. a. Interaction between the terminal chain carbon and the approaching monomer molecule. C. Stereoregular free-radical polymerization of PMMA. (syndiotatic PMMA) a. Polymerization temperature : below 0℃. b. (6.48) 6.5 Stereochemistry of polymerization.

c. Terminal carbon : sp2( planar ) Penultimate repeating unit : Bulky ester group. d. Poly(2,4,6-triphenylbenzylmethacrylate) 1) Less syndiotatic than PMMA. 2) More polar effect than steric effect. 6.5 Stereochemistry of polymerization. 19

6.6.1 Isolated Dienes A. Crosslinked or cyclopolymerization. 6.6 Polymerization of Dienes

A. Structure of conjugated Diene monoer. Isoprene B. a. 1,2-Addition : Pendent vinyl group. b. Stereochemistry : isotactic, syndiotactic, atactic. 6.6.2 Conjugated Dienes. 23 25

C. 1,4-Addition : Delocalized double bond a. D. 3,4-Addition E. Polymerization reaction and temperature. 24 26 27 29 6.6.2 Conjugated Dienes.

TABLE 6.6 Structure of Free Radical-Initiated Diene Polymersa polymerization Temperature (oC) -20 20 100 233 -20 -5 50 100 257 -46 46 100 Monomer Butadiene Isoprene Chloroprene cis-1,4 trans-1,4 1,2 3,4 Percent 6 22 28 43 1 7 18 23 12 5 10 13 77 58 51 39 90 82 72 66 77 94 81-86 71 17 20 21 18 5 5 5 5 2 1 2 2.4 - - - - 4 5 5 6 9 0.3 1 2.4 aData from Cooper34 p. 275. 6.6.2 Conjugated Dienes.

F. s-cis and s-trans 6.6.2 Conjugated Dienes.

A. Thermodynamic feasibility. a. ΔGp = ΔHp - TΔSp ΔGp : Gibbs free energy change of polymerization. ΔHp : Enthalpy change of polymerization. ΔSp : Entropy change of polymerization. ΔGp < 0 : favorable free energy of polymerization. b. Values of ΔH and ΔS for several monomers. c. Polypropylene and isobutylene : ΔG < 0 → unfavorable polymerization. because of kinetic feasibility 6.7 Monomer Reactivity

6.7 Monomer Reactivity

B. Factors of monomer reactivity in free radical polymerization. a. The stability of the monomer toward addition of a free radical. b. The stability of the monomer radicals. c. Order of monomer reactivity. Acrylonitrile > Styrene > Vinyl acetate. d. Order of benzoyloxy radical initiation. Syrene > Vinyl acetate > Acrylonitrile Benzoyloxy radical : Ph14CO2․ 6.7 Monomer Reactivity

C. The inverse relationship between monomer stability and polymerization rate. a. Vinyl acetate: not Stable monomer but high rate constant. b. Steric and polar effects: Not clear-cut generalization. Lower rate constant of MMA than MA. c. 1,2 disubstituted monomer difficult to polymerize in free radical. Exception: Tetrafluoroethylene. 6.7 Monomer Reactivity

D. Ceiling temperature (Tc) a. b. Definition of ceiling temperature. ΔGp = 0 : equal forward and backword reactions. c. High Tc : favorable polymerization. Low Tc : unfavorable polymerization. Exception : α-methylstyrene (Tc=66℃). 6.7 Monomer Reactivity

A. Mechanism of copolymerization. 6.8 Copolymerization.

a. b. c. let, (reactivity ratio) steady state assumption. d. solving : Copolymer equation or copolymer composition equation. d[M1]/d[M2] : the molar ratio of the two monomers in the copolymer [M1], [M2] : the initial molar concentration of monomers in the reaction mixture B. Kinetics of copolymerization. and

a. r1 = r2 = ∞ : Homopolymer. b. r1 = r2 = 0 : Alternating polymer. c. r1 = r2 = 1 : Copolymer composition depending on feeding monomers in the reaction temperature. d. r1 × r2 = 1 :Ideal copolymerization like ideal liquid vaporization. e. r1 × r2 > 1 : Azotropic copolymerization (polymer composition not depending on feeding). f. Determination of r1, r2 : Measure copolymer composition by NMR or other method at low conversion ( <10% ) C. Significance of reactivity ratio (r1, r2).

a. b. c. For styrene Q=1.0 , e=-0.8 d. Q : resonance stabilization. e : less negative values equal more electron attracting. D. Alfrey-price Q-e scheme.

E. Charge transfer complex polymerization(alternating copolymer). a. Styrene and maleic anhydride(D-A complex).

b. c. E. Charge transfer complex polymerization (alternating copolymer). (6.58) (6.57)