11. Carother's Equation
11
β’ N0 β‘ # of monomers originally present in system
β’ N β‘ # of molecules present in system at any time t
β’ (N0-N) β‘ Total # of functional groups of either A or B that
have reacted at t
β’ π β‘ πΈπ₯π‘πππ‘ ππ πππππ‘πππ
=
π0 β π
π0
β π = π0 1 β π
β’ Since π π =
π0
π
Xn =
1
1 β π
Carother's Equation
12. Carother's Equation
12
p Xn
0.95 20
0.99 100
0.999 1000
Example:
Good fibers of nylon 6-6 (fishing line),
Mn=12,000 g/mol, Xn~ 110, so p should be more
than?
Ans: >0.99
13. How to control the MW
13
If the Mw
β’ Too lowβ Poor properties
β’ Too highβ Difficult to process (Melt/ solubilize)
Nylon rope trick
β’ A.Q. phase (Hexine diamine)+Organic phase (Adipoyl chloride)
15. How to control the MW
15
Control
β’ Stoichiometric imbalance
β’ Excess of one reactant in A-A/B-B system to limit MW
π π =
1 + π
1 + π β 2ππ
, r =
π0π΄οΌπ΄
π0π΅οΌB
=
π0π΄
π0π΅
# of
unreacted
functional
groups
Excess goes to denominator, r<1
16. How to control the MW
16
For βquantitativeβ reaction, P=0.999
N0AA N0BB r Xn
1 1 1 1000
1 1.05 0.952 39
π π =
1 + π
1 + π β 2ππ
, r =
π0π΄οΌπ΄
π0π΅οΌB
=
π0π΄
π0π΅
22. Practice
22
Q:
80 moles of monomers react to prepare Nylon 12.
After completion of 8 h, 4 moles of monomers are
still left. What is the number average molecular
weight of polymer system?
(the molecular weights of the repeating units of
polymer Nylon 12 is 197)
The extent of reaction is p= (80-4)/80 = 0.95
Xn=1/(1-p) = 1/(1-0.95)= 20
Mn=20*197=3940
23. Practice
23
Q:
If the value of C0 and k are 10 mol L-1 and 10-3 L mol s-1,
respectively, how long would it take to obtain a Xn of 37?
Assume itβs self-catalyzed
2C0
2kt=[
1
1βπ 2] β 1
β2C0
2kt=Xn
2β1
βt=68400 s
24. Flory distribution
24
Molar mass/ Degree of polymerization distribution
β Calculate the probability P(x) of finding a chain
comprising x units also know as the mole fraction P(x)
P(x)=
π π₯
π
=
# πβππππ
πππ‘ππ # ππππππ’πππ
(X=1 for monomer, 2 for dimer, 3 for trimer)
25. Flory distribution
25
ο¬ Step 1: Probability of random molecule being a monomer
P(x=1)=(1-p)
ο¬ Step 2: Probability of random molecule being a dimer
P(x=2)=p(1-p)
The first molecule reacted The adjacent is unreacted
At extent of reaction p for either
1. x A-A + x B-B β A-A[B-BA-A]x-1B-B
2. x A-B β A[BA]x-1B
Note:
β’ A, B are functional groups
β’ βMoleculesβ are either monomers or polymer chains
26. Flory distribution
26
P(x=2) =p(1-p), P(x=3) =p2(1-p)
βP(x)=px-1(1-p)
Since probability of x=P(x)=Mol. Fraction=
π π₯
π
π π₯ =
π π₯
π
= π π₯β1 1 β π
βπ π₯ = ππ π₯β1
1 β π
27. Flory distribution
27
Since Xn=
π0
π
β N =
π0
π π
= π0(1 β π)
β π π₯ = π0 π π₯β1
1 β π 2
In terms of mass fraction of x-mers, Wx
β π π₯ = π₯π π₯β1 1 β π 2
Where Wx=
π₯π π₯
π0
=
πππ‘ππ # ππ ππππππππ ππππππππππ‘ππ πππ‘π ππππππ’πππ π€ππ‘β πππππ‘β π₯
πππ‘ππ ππππππππ ππ π‘βπ π π¦π π‘ππ
π π₯ = ππ π₯β1 1 β π
28. Flory distribution here
28
Number-fraction distribution Weight-fraction distribution
π π₯
π
X
50 100 150 200
Wx=
π₯π π₯
π0
X
50 100 150 200
p=0.96
p=0.9875
p=0.9950
p=0.96
p=0.9875
p=0.9950
29. Note
29
β’ Average molecular weight Mn and Mw as function of p for
step-growth
π0 β‘ πππππ πππ π ππ ππππππ‘ π’πππ‘
π π =
π π
π0
=
1
1 β π
β π π =
π0
1 β π
βMw=
π0
(1+π)
1βπ
β’ Dispersity (PDI)
PDI=Δ=
π π
π π
= 1 + π
As pβ1 (reaction β)
Δβ2
31. Nature product polymers
31
β’ Living organisms make many polymers enzymatically.
β’ Most such natural polymers strongly resemble step-
polymerized materials.
β’ The structure ultimately being controlled by DNA.
33. Chain-growth
By radical polymerization by unpaired electrons traveling
down the chain and add new monomers as they go! Or made
by ionic polymerization (anionic or cationic)
33
46. Kinetic of chain-growth
46
β’ π·πππππ π£ β‘ πππππ‘ππ πβπππ πππππ‘β
β’ Average # of monomers reacting with active
center during its lifetime
β π£οΌ
ππππππππ πππππ/π ππ
# ππ πβππππ ππππππ/π ππ
=
π£ π
π£π‘
1. For combination: Xn=2 π£
2. For disproportionation: Xn= π£
48. Practice
48
The following are data for the polymerization of styrene in
benzene at 60Β°C with benzoyl peroxide as the initiator.
[M] = 3.34 Γ 103 mol/m3
[I] = 4.0 mol/m3
kp
2/kt = 0.95 Γ 10β6 m3/mol-s
If the spontaneous decomposition rate of benzoyl peroxide is
3.2 Γ 10β6 m3/mol-s, calculate the initial rate of polymerization.
Assume the initiator efficiency f=1
50. Practice
50
For vinyl acetate polymerized at 50 α΅C, the value of the
ratio kp
2/kt is 0.0138 l/mol-s. What is the value of chain
length, when monomer concentration is 6.53 mol/l and
rate of polymerization is 2.0Γ10-4 mol/l-s?
The expression for kinetic chain length is
π£ =
π π
2 π 2
2ππ‘π£π
=
0.0138 Γ 6.532
2 Γ (2.0 Γ 10β4)
= 1471
51. Thermodynamics
51
βπΊ π = βH π β Tβπ π
β’ βH π: negative, because from π ππππ π‘π π ππππ ππ ππ₯π‘βππ‘βπππππ
β’ ββπ» π β 30 β 150πΎπ½/πππ
β’ βπ π: negative, because we are confining monomers to the chain
β’ ββπ π β 100 β
130π½
ππππΎ
βπΊ π < 0 at normal temp.
52. Polymerization processes
52
1. Block (monomers only, no solvent)
-Efficient, eco-friendly, optically transparent
-Susceptible to auto acceleration, explosion
2. Solution (In a solvent)
-Heat dissipation, but susceptible to reaction with solvent
-Ungreen
3. Suspension (of monomers in AQ phase)
-Effectively bulk, Droplets 0.1-5mm
-Reaction must be stable to water
4. Emulsion (Much smaller particles 50nm-5πm)
-Use Micelles with a surfactant to control Mw
53. Features of free radical polymerization
53
1. High Mw formed immediately
The average of molecular weight in the beginning is low
2. Steady decrease in [M] thru out the reaction
3. Only the active center (the growing chain ) is reactive toward
other monomers
4. Long reaction time increase the yield of polymer produce but
not Mw
5. Increasing temp. increases the rate but decreases Mw
54. Chain-growth
54
1. The rate of propagation is proportional to the concentration of the monomer and the
square root of the concentration of the initiator.
2. The rate of termination is proportional to the concentration of the initiator.
3. The average molecular weight is proportional to the concentration of the monomer
and inversely proportional to the square root of the concentration of initiator.
4. The first chain that is initiated rapidly produces a high molecular weight polymer.
5. The monomer concentration decreases steadily throughout the reaction and
approaches zero at the end.
6. The increases in rates of initiation, propagation, and termination with increases in
temperature are in accord with the Arrhenius equation. The energies of activation of
initiation, propagation, and termination are approximately 35, 5, and 3kcal/mol,
respectively. Data for typical energies of activation are given in Tables8.3 and 8.4.
7. Increasing the temperature increases the concentration of free radicals and thus the
rate of reactions, but it decreases the average molecular weight.
8. If the temperature exceeds the ceiling temperature (Tc), the polymer will decompose
and no propagation will take place at temperatures above the ceiling temperature.
55. Chain growth V.S. Step growth
55
Step growth Chain growth
Reactive
sites
All molecules (monomer,
oligomer, polymer)
Only monomer react to the
βactive siteβ
Reaction
process
No termination 3 steps
1. Initiation
2. Propagation
3. Termination
Reaction
speed
Slower Faster (Initiatorβ)
Final
product
Oligomers of many sizes Polymer+monomer+very
few growing chains