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Chap 9. Chain-growth Polymerization

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1. Free radical Initiation Processes. 2. Cationically Initiated Processes. 3. Anionically Initiated Processes. 4. Group Transfer Polymerization. 5. Coordination Polymerization. Chap 9. Chain-growth Polymerization. Chain-Growth Polymerization (Addition) Processes.

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slide1

1. Free radical Initiation Processes

  • 2. Cationically Initiated Processes
  • 3. Anionically Initiated Processes
  • 4. Group Transfer Polymerization
  • 5. Coordination Polymerization

Chap 9. Chain-growth Polymerization

Chain-Growth Polymerization (Addition) Processes

slide2

Chain-Growth Polymerization

  • 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.

Step-Growth Polymerization

  • Molecular weight increases steadily.
  • High molecular weight polymers are found at the end.
  • Long reaction time needs to synthesize high conversion and high molecular weight.
slide3

Bond Energy= 46 kcal/mole

Heat (60ºC)

UV kd

Primary radical

AIBN

Chain Growth Polymerization

(1) Initiation

kd : Initiator decomposition rate constant

: 10-4 ~ 10-6 L/mole sec

Unstable radical

slide4

Chain Growth Polymerization

(2) Propagation

(Repetition of similar reaction)

kp : 102 ~ 104 L/mole sec (much faster than step-growth polymerization)

slide5

Chain Growth Polymerization

(3) Termination

(a) Coupling or combination

slide6

Chain Growth Polymerization

(3) Termination

(b) Disproportionation

kt=ktc+ktd

106 ~ 108 L/mole sec

slide7

Chain Growth Polymerization

(4) Chain Transfer

Physical chain length

Monomer, Polymer, Solvent or Chain transfer agent

Kinetic chain length

slide8

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

slide9

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)
  • Two radicals and two kinetic chains are formed by decomposition of
  • each ROOR molecules

Kinetic Chain Reaction

slide10

Kinetic Chain Reaction

    • 2) Propagation
    •  Br• + ФCH3 HBr + ФCH2 (4)
    •  ФCH2• + Br2 ФCH2Br + Br • (5)
    • Two special features

 The number of active species is fixed.

  •  Same reactions are repeated during the kinetic chain reaction.
slide11

Kinetic Chain Reaction

  • 3) Termination
  •  2 Br• Br2
  •  2ФCH2• ФCH2 CH2Ф
  •  ФCH2• + Br • ФCH2Br + Br •
  • Net Effect of Kinetic Chain Reaction:
  • One ROOR molecule can cause formation of Br2, CH2CH2, CH2Br,HBr, ‥.
slide12

Kinetic Chain Reaction

  • Comparison between Chain Polymerization & Chain Reaction

Init.

propagation

termination

Chain reaction Ri= Rt

Reaction Rate

Steady state

Time

Induction period

In proportion to the O2 concentration

slide13

Kinetic Chain Reaction

  • 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 DPn found easily in early reaction
  • Linear Step-Growth:
  • high extent of reaction value required to obtain high DPn
slide14

Kinetic Chain Reaction

  • 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.

isobutylene

Ex)

Because of resonance stability

slide15

Kinetic Chain Reaction

  • Comparison between Free Radical Reaction &Ionic Reaction
slide16

Kinetic Chain Reaction

  • Comparison between Free Radical Reaction & Termination Step of Ionic Reaction
  • A) Free Radical Termination

Two molecules involved

= bimolecular reaction

slide17

Kinetic Chain Reaction

  • B) Cationic Termination

Anionic capture is similar to combination offree radical reaction.

But, this reaction can’t include increasing of MW because of unimolecular reaction

slide18

Kinetic Chain Reaction

  • The proton release is similar to disproportination offree radical..
  • But, one chain joins in the reaction unimolecular reaction
  • C) Anionic Termination
slide20

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

kd

ki

Primary radical species

Generally, 0.5 << f << 1

slide21

Kinetic Chain Reaction

  • A.Cage Effect –primary recombination
  • Initiator fragments surrounded by restricting cage of solvent
      • Ex)
      • (acetyl peroxide)
slide22

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.
slide23

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 obtainedhalf molecules were wasted.)

slide24

.

.

I + M

I

M

n

.

I

I

M

n

I

.

I

+

+

I

M

n

2

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
slide25

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 Riis unrelated with [M]

f=[M]

f < 1 Ri is related with [M]

[M] , f

[I2] , f due to induced decomposition

by convention, tworadical formation.

slide26

Kinetic Chain Reaction

  • D. Initiator
  • - containing compounds.

Acetyl peroxide 80~100C

Benzoyl peroxide 80~100C

  • Cumyl peroxide 120~140C
slide27

Kinetic Chain Reaction

  • t-butyl peroxide
  • Hydroperoxides, cumyl or t-butyl
  • 80~100C

50~70C

  • AIBN 2,2 azobisisobutyronitrile
slide28

.

.

M

M

k

k

tc

td

Kinetic Chain Reaction

    • Propagation
  • Termination

By convention

Since 2 radical elimination

slide29

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)

slide30

Kinetic Chain Reaction

[M]elimination methods

Steady-State Assumption

Radical concentration increases at the start, comes to steady state simultaneously

and thenreaction rate change becomes 0. (active centers created and destroyed at the same time)

Ri = Rt

slide31

MMA using BPO

Rp

Vinyl Acetate using AIBN

[I2]1/2

H

.

Kinetic Chain Reaction

Mostly in case of f<1system → [I2]1/2 (Square Root Dependence of [I2])

※ Odian Fig. 3-4

-CO2

2

300C

BPO

+ N2

Azobisisobutyronitrile

slide32

+ BPO +

C

H

3

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]

slide33

Kinetic Chain Length (KCL)

At S-S assumption

(1) Disproportionation

Knowing that

(2) Coupling or combination

slide35

Kinetic Chain Length (KCL)

Degree of Polymerization

  • The more concentration of monomer,
  • The less concentration of initiator,

Monomer consumption rate

polymer formation rate

(1) Dispropotionation

(2) Coupling

slide36

Kinetic Chain Length (KCL)

(3) Dispropotionation& Coupling

Polymer formation rate

Monomer consumption rate

slide37

Kinetic Chain Length (KCL)

  • From (1),(2),(3)
  • In case of noChain transfer, and valid S-S assumption
slide38

Chain Transfer

  • M + XY MX + Y
  • Chain transfer agent
  • If Chain transfer occurs Rpisunchangable buthas an effect on DPn
  • (∵ Since [Y] instead of Rp=kp[M][M] )

ex)

  • (1) Chain transfer occurs by solvents or additives
  • In this case,Highchain transfer coefficient.
  • (2) Transfer occurs by monomer or polymer
slide39

Chain Transfer

Inhibitor and Retarder

      • 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.
  • Like this, when chain transfer condition arises
slide40

Chain Transfer

1

See Odian P.235

DPn

From the slope of a graph

Chain transfer coefficient ‘Cs’

[5]/[M]

slide41

Temperature Dependence of Rp and DPn

  • Assume : no chain transfer
slide42

Temperature Dependence of Rp and DPn

  • slope of lnRp/T is ( + )
  • as T  lnRp
  • but Rate of Increase  as d lnRp/dT 
slide44

Ceiling Temperature Polymer-Depolymerization Equilibria

ㆍCeiling Temperature

Polymerization and Depolymerization are in equilibrium

ΔGp = ΔHp – TΔSp

ΔHp : Heatof polymerization

ΔSp : Molecular arrangement changes between monomer andpolymer

At eq. State ΔGp=0

Monomers can no longer be persuaded to form polymers by chain

polymerizationabove a certain temperature.ceiling Temperature(Tc)

slide45

Rate Eq. of Polymerization Reactions

at Depolymerization prominent Temperature

If,

M Tc

slide46

Ceiling Temperature Polymer-Depolymerization Equilibria

k

sec-1

kdp

Tc

: No reaction above Tc

Stable blow Tc

kp[M]

kp[M]- kdp

300

400

500

Tc

slide47

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.
slide48

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, ktdrop off in reaction progress)

→ Autoaccerelation effect

slide49

Trommsdorff Effect or Gel Effect

one would expect ξ  as t Butξ  as [[M0] 

80%

60%

  • autoacceleratioan

40%

as [M0] drastic in .

10%

t

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