Hanyang Univ. Spring 2008 Chap 9. 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

Hanyang Univ. Spring 2008 Chain-Growth Polymerization Step-Growth Polymerization 1.Molecular weight increases steadily. 2.High molecular weight polymers are found at the end. 3.Long reaction time needs to synthesize high conversion and high molecular weight. 1.Only growth reaction adds repeating units one at a time to the chain 2.Monomer concentration decreases steadily throughout the reaction 3.High Molecular weight polymer is formed at once; polymer molecular weight changes little throughout the reaction. 4.Long reaction times give high yields but affect molecular weight little. 5.Reaction mixture contains only monomer, high polymer, and about part of growing chains. part of growing chains.

Hanyang Univ. Spring 2008 (1) Initiation k d : I k d : Initiator decomposition rate constant : ~ L/mole sec : ~ L/mole sec Unstable radical Chain Growth Polymerization Heat (60ºC) UV k d Primary radical AIBN Bond Energy = 46 kcal/mole

Hanyang Univ. Spring 2008 (2) Propagation k p : 10 2 ~ 10 4 L/mole sec (much faster than step-growth polymerization) (Repetition of similar reaction) Chain Growth Polymerization

Hanyang Univ. Spring 2008 (3) Termination (a) Coupling or combination Chain Growth Polymerization

Hanyang Univ. Spring 2008 (b) Disproportionation k t =k tc +k td 10 6 ~ 10 8 L/mole sec (3) Termination Chain Growth Polymerization

Hanyang Univ. Spring 2008 (4) Chain Transfer Kinetic chain length Monomer, Polymer, Solvent or Chain transfer agent Physical chain length Chain Growth Polymerization

Hanyang Univ. Spring 2008 Chain Growth Polymerization Kinetic Chain Length : kinetic chain length υ 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 υ =4,000/4 =1,000 Determined by steps 1, 2, 3. (Initiation, propagation, and termination) (No chain transfer) Physical Chain Length : This condition contains Step 1, 2, 3, 4. Radical 1,2,3,4

Hanyang Univ. Spring 2008 Non-Polymerization Reaction Peroxide induced Bromination of Toluene 1) Initiation Two types of reaction  R-O-O-R 2RO (1)  R-O + Br 2 ROBr + Br (2)  R-O + ФCH 3 ROH + ФCH 2 (3) Two radicals and two kinetic chains are formed by decomposition of each ROOR molecules Kinetic Chain Reaction

Hanyang Univ. Spring ) Propagation  Br + ФCH 3 HBr + ФCH 2 (4)  ФCH 2 + Br 2 ФCH 2 Br + Br (5) Two special features  The number of active species is fixed.  Same reactions are repeated during the kinetic chain reaction. Kinetic Chain Reaction

Hanyang Univ. Spring ) Termination  2 Br Br 2  2ФCH 2 ФCH 2 CH 2 Ф  ФCH 2 + Br ФCH 2 Br + Br Net Effect of Kinetic Chain Reaction: One ROOR molecule can cause formation of Br2,  CH2CH2 ,  CH2Br,HBr, ‥. Kinetic Chain Reaction

Hanyang Univ. Spring 2008 Time Reaction Rate Induction period Steady state Chain reaction R i = R t In proportion to the O 2 concentration Comparison between Chain Polymerization & Chain Reaction Kinetic Chain Reaction

Hanyang Univ. Spring 2008  In the case of Chain reaction, there are induction periods, due to the existence of inhibitor.  When an active center is formed, the reaction rate would be faster and then go to steady state.  The whole reaction rate is reaching a plateau region.  After that, reaction rate decreases due to a loss of monomers or initiators. Linear Chain-Growth: Polymer of high DP n found easily in early reaction Linear Step-Growth: high extent of reaction value required to obtain high DP n Kinetic Chain Reaction

Hanyang Univ. Spring 2008 Comparison Free Radical Reaction & Ionic Reaction - Ionic Initiation – multiple bond addition, ring opening polymerization - Radical Initiation – Ring-opening polymerization is not initiated. For the cationic initiation, it will not be free radical. Ex) Because of resonance stability Kinetic Chain Reaction isobutylene

Hanyang Univ. Spring 2008 Comparison between Free Radical Reaction & Ionic Reaction Kinetic Chain Reaction

Hanyang Univ. Spring 2008 A) Free Radical Termination Comparison between Free Radical Reaction & Termination Step of Ionic Reaction Two molecules involved = bimolecular reaction Kinetic Chain Reaction

Hanyang Univ. Spring 2008 B) Cationic Termination Anionic capture is similar to combination of free radical reaction. But, this reaction can’t include increasing of MW because of unimolecular reaction Kinetic Chain Reaction

Hanyang Univ. Spring 2008 The proton release is similar to disproportination of free radical.. But, one chain joins in the reaction unimolecular reaction C) Anionic Termination Kinetic Chain Reaction

Hanyang Univ. Spring 2008 Kinetic Chain Reaction

Hanyang Univ. Spring 2008 Free Radical Initiated Polymerization of Unsaturated monomers Kinetic Scheme Initiation Two step sequence-Both enter into overall rate 1.Initiator decomposition I 2 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 k d kiki Kinetic Chain Reaction

Hanyang Univ. Spring 2008 A.Cage Effect –primary recombination Initiator fragments surrounded by restricting cage of solvent Ex) (acetyl peroxide) Kinetic Chain Reaction

Hanyang Univ. Spring 2008 I) Recombination possible I 2 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

Hanyang Univ. Spring 2008 B. Induced Decomposition –Secondary combination I) Through Radical attack on peroxide molecules  R + R-O-O-R RH +  ROOR R=O + RO  Finally, R  + ROOR ROR+ RO  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

Hanyang Univ. Spring 2008 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 I + M. IMn. IMn. + I 2 IMnI + I.

Hanyang Univ. Spring 2008 C. Reaction Rate If [M  ] is representative for the concentration of chain radical, That is, M  = IM  or = I [M  ] f  1 R i is unrelated with [M]  f=  [M] f < 1 R i is related with [M] [M], f [I 2 ], f due to induced decomposition by convention, two radical formation. Kinetic Chain Reaction

Hanyang Univ. Spring 2008 D. Initiator - containing compounds. Kinetic Chain Reaction Acetyl peroxide 80~100  C Benzoyl peroxide 80~100  C Cumyl peroxide 120~140  C

Hanyang Univ. Spring ~70  C AIBN 2,2 azobisisobutyronitrile Kinetic Chain Reaction Hydroperoxides, cumyl or t-butyl 80~100  C t-butyl peroxide

Hanyang Univ. Spring 2008 Propagation Termination By convention Since 2 radical elimination M. M. k tc k td Kinetic Chain Reaction

Hanyang Univ. Spring 2008 Overall Rate of Polymerzation Radical concentration Difficulty of measurement, low concentration. (~10 -8 molar) Thus, it is impractical using this therm. [M  ] elimination is desirable. (# of propagation step >>> # of initiation step) Kinetic Chain Reaction

Hanyang Univ. Spring 2008 [M  ] elimination methods Steady-State Assumption Radical concentration increases at the start, comes to steady state simultaneously and then reaction rate change becomes 0. (active centers created and destroyed at the same time) R i = R t Kinetic Chain Reaction

Hanyang Univ. Spring 2008 Mostly in case of f<1 system → [I 2 ] 1/2 (Square Root Dependence of [I 2 ]) ※ Odian Fig. 3-4 MMA using BPO Vinyl Acetate using AIBN RpRp [I 2 ] 1/2 BPO -CO C + N 2 Azobisisobutyronitrile Kinetic Chain Reaction H.

Hanyang Univ. Spring 2008 In case f < 1, but SRD is not applicable, Because f is ‘dependent’ on [M] Why? Due to induced decomposition of toluene + [I 2 ] Kinetic Chain Reaction + BPO + CH 3

Hanyang Univ. Spring 2008 At S-S assumption (1) Disproportionation Knowing that Kinetic Chain Length (KCL) (2) Coupling or combination

Hanyang Univ. Spring 2008 (3) Both (1)+(2) Kinetic Chain Length (KCL)

Hanyang Univ. Spring 2008 The more concentration of monomer, The less concentration of initiator, (1) Dispropotionation (2) Coupling Degree of Polymerization Kinetic Chain Length (KCL) Monomer consumption rate polymer formation rate

Hanyang Univ. Spring 2008 (3) Dispropotionation & Coupling Polymer formation rate Monomer consumption rate Kinetic Chain Length (KCL)

Hanyang Univ. Spring 2008 From (1),(2),(3) In case of no Chain transfer, and valid S-S assumption Kinetic Chain Length (KCL)

Hanyang Univ. Spring 2008 M  + XYMX + Y  Chain transfer agent If Chain transfer occurs R p is unchangable but has an effect on DP n ∵ Since ( ∵ Since [Y  ] instead of R p =k p [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

Hanyang Univ. Spring 2008 Inhibitor 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. Chain Transfer Inhibitor and Retarder Like this, when chain transfer condition arises

Hanyang Univ. Spring 2008 From the slope of a graph Chain transfer coefficient ‘C s ’ See Odian P [5]/[M] DP n Chain Transfer

Hanyang Univ. Spring 2008 Assume : no chain transfer Temperature Dependence of R p and DP n

Hanyang Univ. Spring 2008  slope of lnR p /T is ( + )  as T  lnR p  but Rate of Increase  as d lnR p /dT  Temperature Dependence of R p and DP n

Hanyang Univ. Spring 2008 Temperature Dependence of R p and DP n

Hanyang Univ. Spring 2008 ㆍ Ceiling Temperature Polymerization and Depolymerization are in equilibrium ΔG p = ΔH p – TΔS p ΔH p : Heat of polymerization ΔS p : Molecular arrangement changes between monomer and polymer At eq. State ΔG p =0 Monomers can no longer be persuaded to form polymers by chain a ceiling Temperature(T c ) polymerization above a certain temperature. ceiling Temperature(T c ) Ceiling Temperature Polymer-Depolymerization Equilibria

Hanyang Univ. Spring 2008 Rate Eq. of Polymerization Reactions at Depolymerization prominent Temperature If, M T c

Hanyang Univ. Spring 2008 k sec -1 k dp k p [M] k p [M]- k dp Tc : No reaction above T c Stable blow T c Ceiling Temperature Polymer-Depolymerization Equilibria

Hanyang Univ. Spring 2008 ※ Odian Fig 3-18 Entropy changes for all polymers are not so different.  S p = S p - S m (–) value of S p is higher  H p = H p - H m if (–), exothermic. Ceiling Temperature Polymer-Depolymerization Equilibria

Hanyang Univ. Spring 2008 Trommsdorff Effect or Gel Effect restricted mobility of polymer radical k p ( relative to [M] ) ( ∵ k p const. in reaction progress, k t drop off in reaction progress ) → Autoaccerelation effect The increasing viscosity limits the rate of termination because of diffusional limitations

Hanyang Univ. Spring 2008  t 80% 60% 40% 10% autoacceleratioan as [M 0 ] drastic in . Trommsdorff Effect or Gel Effect  one would expect ξ  as t  But ξ  as [[M 0 ] 