Chap 10. Chain-growth Polymerization Chain-Growth Polymerization (Addition) Processes 1. Free radical Initiation Processes 2. Cationically Initiated Processes 3. Anionically Initiated Processes 4. Group Transfer Polymerization 5. Coordination Polymerization

Chain Growth Polymerization Characteristics Only growth reaction adds repeating units one at a time to the chain Monomer concentration decreases steadily throughout the reaction High Molecular weight polymer is formed at once; polymer molecular weight changes little throughout the reaction. Long reaction times give high yields but affect molecular weight little. Reaction mixture contains only monomer, high polymer, and about 10-8 part of growing chains.

Chain Growth Polymerization (ⅰ) Initiation kd : Initiator decomposition rate constant : 10-4 ~ 10-6 L/mole sec Heat (60ºC) UV kd Primary radical AIBN Bond Energy = 46 kcal/mole Unstable radical

(ⅱ) Propagation kp : 102 ~ 104 L/mole sec

(ⅲ) Termination (a) Coupling or combination

(b) Disproportionation kt=ktc+ktd 106 ~ 108 L/mole sec

Chain Growth Polymerization Kinetic Chain Length : kinetic chain length v of a radical chain polymerization is defined as the average number of monomer molecules consumed (polymerized) per each radical, which initiates a polymer chain. ex) Monomer # 4000 Disproportionation ν=4,000/4 =1,000 Decided by step 1,2,3. Physical Chain Length : This condition contains Step 1,2,3,4 Radical 1,2,3,4

Kinetic Chain Reaction Non-Polymerization Reaction Peroxide induced Bromination of Toluene 1) Initiation Two types of reaction  R-O-O-R 2RO• (1)  R-O• + Br2 ROBr + Br• (2)  R-O• + ФCH3 ROH + ФCH2• (3) Tow radicals and tow kinetic chains formed by decomposition of each ROOR molecules

Kinetic Chain Reaction 2) Propagation  Br• + ФCH3 HBr + ФCH2 (4)  ФCH2• + Br2 ФCH2Br + Br • (5) Two special features Number of active species is fixed  During kinetic chain reaction, same reactions was repeated

Kinetic Chain Reaction 3) Termination  2 Br• Br2  2ФCH2• ФCH2 CH2Ф  ФCH2• + Br • ФCH2Br + Br • NET EFFECT OF KINETIC Chain rexn: One ROOR molecule can cause formation of Br2, CH2CH2, CH2Br,HBr, ‥.

Kinetic Chain Reaction Comparison Chain Polymerization & Chain Reaction Init. propagation termination Chain reaction Ri == Rt Reaction Rate Steady state Time Induction period In proportion to the O2 concentration

Polymer of high DPn found easily in early reaction Linear Step-Growth: In case of Chain reaction, there are mainly induction periods, due to the inhibitor. If an active center is formed, the reaction rate accelerate and then come to steady state. The whole reaction rate is reaching plateau region. Linear Chain-Growth: Polymer of high DPn found easily in early reaction Linear Step-Growth: high extent of reaction value required to obtain high DPn Kinetic Chain Reaction

Kinetic Chain Reaction Comparison Free Radical Reaction & Ionic Reaction - Ionic Initiation – multiple bond addition, ring opening polymerization - Radical Initiation – Ring-opening polymerization has not initiation reaction. Kinetic Chain Reaction

Kinetic Chain Reaction Comparison Free Radical Reaction & Ionic Reaction

Kinetic Chain Reaction A) Free Radical Termination Kinetic Chain Reaction Comparison Free Radical Reaction & Termination Step of Ionic Reaction Two molecules involved = bimolecular reaction

Kinetic Chain Reaction B) Cationic Termination Kinetic Chain Reaction Anionic capture is analogous to combination of free radical reaction. But, this reaction can’t include increasing of MW because of unimolecular reaction

Kinetic Chain Reaction The proton release is similar to disproportion of free radical.. But, one chain join in the reaction unimolecular reaction Kinetic Chain Reaction

Kinetic Chain Reaction

Kinetic Chain Reaction Free Radical Initiated Polymerization of Unsaturated monomers Kinetic Scheme Initiation Two step sequence-Both enter into overall rate Initiator decomposition I2 2I 2. Addition of Initiator fragment to the monomer, Initiation of Chain growth. I+M IM The efficiency of Initiator - Determined by competition of desired reaction and side reaction Primary radical species Generally, 0.5 << f << 1 kd ki Kinetic Chain Reaction

Kinetic Chain Reaction A.Cage Effect –primary recombination Initiator fragments surrounded by restricting cage of solvent Ex) (acetyl peroxide) Kinetic Chain Reaction

Kinetic Chain Reaction I) Recombination possible I2 2I II) If elimination reaction occurs while the free radical in-cage, Formation of stable molecules due to Radical combination. And formation of Inactive Species. Kinetic Chain Reaction

Kinetic Chain Reaction B. Induced Decomposition –Secondary combination I) Through Radical attack on peroxide molecules R + R-O-O-R RH + ROOR R=O + RO Finally, R + ROOR ROR+ RO Total number of radical does not change, but among them half molecules were wasted. II) Chain Transfer to Solvent (In this case, since just one radical was obtained half molecules were wasted.) Kinetic Chain Reaction

Kinetic Chain Reaction III) Reaction with Chain Radical Since not all Molecules participate in the initiation → Efficiency factor f: Initiator Efficiency = mole fraction of initiator fragments that actually initiate polymer chains. 0.5 < f < 1.0 Kinetic Chain Reaction

Kinetic Chain Reaction C. Reaction Rate If [M] is representative for the concentration of chain radical, That is , M = IM or = I M f  1 Ri is unrelated with [M] f=[M] f < 1 Ri is related with [M] [M] , f [I2] , f due to induced decomposition by convention, two radical formation.

Kinetic Chain Reaction D. Initiator - containing compounds. Acetyl peroxide, or benzoyl peroxide 80~100C Alkyl peroxide, cumyl or t-butyl peroxide 120~140C

Kinetic Chain Reaction Hydroperoxides, cumyl or t-butyl 80~100C 50~70C AIBN 2,2 azobisisobutyronitrile

Kinetic Chain Reaction Propagation Termination M . k tc td By convention Since 2 radical elimination

Kinetic Chain Reaction Overall Rate of Polymerzation Radical concentration Difficulty of measurement, low concentration. (~10-8molar) Thus it is impractical using this therm. [M] elimination is desirable. (# of propagation step >>> # of initiation step)

Kinetic Chain Reaction [M] elimination methods Steady-State Assumption Radical concentration increases at the start, comes to steady state simutaneously. and then reaction rate change becomes 0. (active centers created and destroyed at the same time) Ri = Rt

Kinetic Chain Reaction Mostly in case of f<1 system → [I2]1/2 (Square Root Dependence of [I2]) ※ Odian Fig. 3-4 MMA using BPO Vinyl Acetate using AIBN Rp [I2]1/2 BPO -CO2 2 300C + N2 Azobisisobutyronitrile

Kinetic Chain Reaction In case f < 1, but SRD is not applicable, Because f is ‘dependent’ on [M] Why? Due to induced decomposition of toluene + [I2]

Kinetic Chain Length (KCL) At S-S assumption (1) Disproportionation Knowing that (2) Coupling or combination

Kinetic Chain Length (KCL) (3) Both (1)+(2)

Kinetic Chain Length (KCL) Degree of Polymerization The more concentration of monomer, DP The less concentration of initiator, DP Monomer consumption rate polymer formation rate (1) Dispropotionation (2) Coupling

Kinetic Chain Length (KCL) (3) Dispropotionation & Coupling Kinetic Chain Length (KCL) Polymer formation rate Monomer consumption rate

Kinetic Chain Length (KCL) From (1),(2),(3) In case of no Chain transfer, and valid S-S assumption

Chain Transfer M + XY MX + Y Chain transfer agent If Chain transfer occurs Rp is unchangable but has an effect on DPn (∵ Since [Y] instead of Rp=kp[M][M] ) ex) (1) Chain transfer occurs by solvents or additives In this case, High chain transfer coefficient. (2) Transfer occurs by monomer or polymer

Chain Transfer Inhibitor and Retarder Inhibitor Retarder When Y take part in chain opening reaction, polymer moves from one site to another. In this case, hydroquinone etc. are used as inhibitor. Retarder When the reactivity of Y is low, controlling the MW of the monomer including these two materials, Mercaptan etc. are used as Retarder. Like this, when chain transfer condition arises

Chain Transfer 1 See Odian P.235 DPn From the slope of a graph Chain transfer codfficient ‘Cs’

Temperature Dependence of Rp and DPn Assume : no chain transfer

Temperature Dependence of Rp and DPn slope of lnRp/T is ( + ) as T  lnRp  but Rate of Increase  as d lnRp/dT 

Temperature Dependence of Rp and DPn

Ceiling Temperature Polymer-Depolymerization Equilibria Polymerization and Depolymerization are in equilibrium ΔGp = ΔHp – TΔSp ΔHp : Heat of polymerization ΔSp : Molecular arrangement changes between monomer and polymer At eq. State ΔGp=0 Monomers can no longer be persuaded to form polymers by chain polymerisation above a certain temperature. ceiling Temperature(Tc)

Rate Eq. of Polymerization Reactions at Depolymerization prominent Temperature. If, Eq. M Tc

Ceiling Temperature Polymer-Depolymerization Equilibria k sec-1 kdp kp[M] kp[M]- kdp Tc : No reaction above Tc Stable blow Tc 300 400 500 Tc

Ceiling Temperature Polymer-Depolymerization Equilibria ※Odian Fig 3-18 Entropy changes for all polymers are not so different. Sp= Sp- Sm (–) value of Sp is higher Hp= Hp- Hm if (–) , exothermic.

Trommsdorff Effect or Gel Effect The increasing viscosity limits the rate of termination because of diffusional limitations restricted mobility of polymer radical kp ( relative to [M] ) ( ∵ kp const. in reaction progress, kt drop off in reaction progress ) → Autoaccerelation effect

Trommsdorff Effect or Gel Effect  one would expect ξ  as t  But ξ  as [M0]   t 80% 60% 40% 10% autoacceleratioan as [M0] drastic in .