polymer synthesis chem 421 free radical polymerizations
Post on 20-Dec-2015
255 views
TRANSCRIPT
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