polymer synthesis chem 421 free radical polymerizations

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Polymer SynthesisCHEM 421

Free Radical Polymerizations

Polymer SynthesisCHEM 421

Reading assignments:

Chapter 3-3, 3-4, 3-5, 3-8 (Odian)

Polymer SynthesisCHEM 421

Free Radical Solution Polymerizations

• Initiation

• Propagation

• Termination

Ikd

2 R

kiR + M R M

kiR M + n M Pn M

ktPn

Pm

+ Dead Polymer

d [R•] / dt = 2 kd [I]

- d [R•] / dt = ki [R•] [M]

rp = kp [M•] [M]

- d [P•] / dt = 2 kt [P•]2

= 2 f kd [I]

Polymer SynthesisCHEM 421

Free Radical Solution Polymerizations

• Overall rate expression

–Assumptions?

» Steady State Approximation (Ri = Rt)

Rp = kp [M] (kd f [I] / kt)1/2

2 f kd [I] = 2 kt [M•]2

Rp = kp [M] [M•]

[M•] = (kd f [I] / kt)1/2

Polymer SynthesisCHEM 421

Free Radical Solution Polymerizations

• Overall rate expression

–Rate is first order in monomer

–Rate is ½ order in initiator

• To increase Rp

Rp = kp [M] (kd f [I] / kt)1/2

Polymer SynthesisCHEM 421

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

0 0.01 0.02 0.03 0.04 0.05

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

0 0.5 1 1.5 2

Determination of Orders of ReactionR

p (

mol

/L x

sec

)

Rp/[

VF

2]ou

t (1/

sec

)

[I]out 0.5 (mol/L)[VF2]out (mol/L)

T =75 C, P= 4000 psig, CO2 = 0.74 g/ml

Rp = ([VF2]IN-[VF2]OUT)/

I*

[DEPDC]IN =3 mM

Rp kp [Iout-I*] 0.5 [VF2]out1.0

Polymer SynthesisCHEM 421

Experimental Ways of Determining Rp

• Separation and isolation of product–Precipitation–Chromatography

• Chemical analysis–Titration of un-reacted double bonds…

• Spectroscopic methods–UV/Vis, NMR, FTIR

• Refractive index changes • Dilatometry• Calorimetry

Polymer SynthesisCHEM 421

React-IR

t = 30 s, 32 scansreaction conditions: p = 5000 p.s.i., T = 35 °C, 73 mol-% PDD in feedphase separation emulsion

Polymer SynthesisCHEM 421

Dilatometer

Polymer SynthesisCHEM 421

Free Radical Solution Polymerizations

• Overall rate expression

–Rate is first order in monomer

–Rate is ½ order in initiator

• Qualitative picture…?

Rp = kp [M] (kd f [I] / kt)1/2

Polymer SynthesisCHEM 421

Free Radical Solution Polymerizations

• Kinetic Chain Length (٧)

–Average number of monomer units polymerized per chain initiated

Rp Rp

Ri Rt

٧ = — = —

kp [M]

2 kt [M•] = ———

kp [M]

2 (kt kd f [I])1/2 = —————

kp [M] [M•]

2 kt [M•]2٧ = ————

Polymer SynthesisCHEM 421

Free Radical Solution Polymerizations

• Kinetic Chain Length (٧)

–To increase molecular weight…

–To increase Rate of Polymerization

٧ =kp [M]

2 (kt kd f [I])1/2 = —————

Polymer SynthesisCHEM 421

Kinetic Chain Length (٧) vsDegree of Polymerization (DP)

• Termination by combination

• Termination by disproportionation

DP = 2٧

DP = ٧

Polymer SynthesisCHEM 421

Polymer SynthesisCHEM 421

Classes of Free Radical Initiators

Polymer SynthesisCHEM 421

Initiator Decompositions

• Differences in the decomposition rates of various initiators can be expressed in terms of initiator half life

• The rate of initiator disappearance is:

• Integrate:

or

• Half life?

- d[I] / dt = kd [I]

[I] = [I]0 e-kdt

log —— = kdt [I]0

[I]

set: [I] = —— [I]0

2t1/2 = ———

0.63

kd

Polymer SynthesisCHEM 421

Polymer SynthesisCHEM 421

Factors which Influence Initiator Choice

• Temperature of polymerization

• Solubility of the initiator

• End group control or functionalization

HO OH +

HO PBd OH

Termination by combination

Polymer SynthesisCHEM 421

Factors which Influence Initiator Choice

• End group control or functionalization

F3CF2CF2C CF2 CF2 CF2 CF CF2

OCF3

CF

O

CF2CF2CF3

CF2CF2CF2CF2Br

O CF2 CF2 CF2 CF CF2

OCF3

CF

O

SO

O

O

OSO

O

O

CF2CF2CF2CF2Br

K K

Polymer SynthesisCHEM 421

Pulsed-laser Polymerization in Homogeneous Phase: Determination of kp

Polymer SynthesisCHEM 421

Pulsed-laser Polymerization in Homogeneous Phase: Determination of kp

Polymer SynthesisCHEM 421Evaluation of kp

Polymer SynthesisCHEM 421

Kinetic Chain Length Profile in a PLP Experiment

Polymer SynthesisCHEM 421Li=i kp [M] td

Polymer SynthesisCHEM 421Li=i kp [M] td

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