10.569 Synthesis of Polymers
Prof. Paula Hammond
Lecture 10: Introduction to Radical Polymerization
Segmented Copolymers
Segmented Polyurethanes (Prof. Hammond’s thesis)
1 “soft segment” → ends in –OH groups
- oligomer
- low Tg (liquid-like at 25oC)
OH
HO
HO
oligomeric diols
(CH2)4 O
H
simple polyether
MW:
∼ 1000 – 10,000
CH3
HO
CH2O
Si
O
CH2OH
CH3
2 Diisocyanate: (-N=C=O)
OCN-R-NCO
e.g.
OCN-(CH2)6-NCO
OCN
H2
C
NCO
3 Chain extender
- Connector between different units
- Almost always short diol
ex:
HO-(CH2)n-OH To get segmented polyurethane:
1. Endcap soft segment w/diisocyanate:
O
2 OCN
R
NCO + HO
OH
(rapid)
OCN
R
H
N
CO
O
OC
no byproduct
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.
H
N
R
NCO
2.
or
OCN
NCO
+ OCN
R
NCO
+
R'
OH
HO
soft
isocyanate
chain extender
1
2
1
1
3
2
1
4
3
-
-
-
hard segment
→ hard domains
App:
→ Nike shoe soles
→ biodegradable scaffolds
O
O C
stoichiometric
proportion
O
O
H
H
H
N R N C O R' OC N R
O
H
N C
Hard Segment
→ Can be made longer by adding diol
and diisocyanate in equal proportions
[
]3
[
]4
physical network
- held together by hydrogen bonds
- some deg of crystallinity
→ Tm (flow temp)
Step Growth Polymerization
Polymers at high π
• 2nd order kinetics
• MW↑ linearly with time ( p n = 1 + [a ]o kt )
1
• MW ∝
1−π
• All species in rxn bath are reactive • monomer activation required for
polymerization
• Need high π for high MW
• only activated monomer/polymer
growing chains are active in rxn
(v. small fraction at given time)
Chain Growth (Addition)
• growing chains get large rapidly then
terminate, deactivates chain
• new monomer is activated
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 10
Page 2 of 6
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.
R* + M → RM*
RM* + M → RMM*
RMn*
high M
MW
W
chains +
monomer
monomer
monomer
high π
• have only monomer
high MW polymer
growing chains
• MW ≠ f(π)
unless “living” system
although it is f[M]o
Addition monomers are:
vinyl groups (C=C)
R
O
H
ketones
(C=O)
aldehydes
O
O
O
heterocyclic ring monomers
(strained)
Propagating (active) species:
-
anionic
C
cationic
C
free radical
C•
+
Processes in Addition Polymerization:
1.
2.
3.
4.
Initiation
Propagation
Termination
Transfer of charge or active species from one chain to another
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 10
Page 3 of 6
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.
(but not always present)
Free Radical Polymerization
Kinetics: I → 2R•
1. Initiation:
R
+ H2C
CH
R
H
H2
C C
R'
initiation
fragment
R'
2. Propagation Step:
R
RM
+ M
RM2
RMn
+ M
RMn+1
H
H2
C C
+ H2C
R
CHR'
H
H2
C C
R'
R'
H
H2
C
C
R'
3. Termination:
Happens one of 2 ways:
a. coupling
RMn
+ RMp
RMn+p
R
doubling size of polymer
b. disproportionation
C
H2
CH
+
R'
C
H2
CH
R'
ktd
C
H
CH +
R'
C
H2
CH2
R'
4. Chain Transfer:
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 10
Page 4 of 6
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.
H
C
H2
+
C
C
H2
HC R"
R'
CH2
R'
+ R" C
propagating
species
impacts MW
Kinetic Rate Expression
Initiation:
I
R
kd
dissociation (rate determining)
2R
+ M
RM
d [RM ⋅]
d [R ⋅]
= f
dt
dt
efficiency factor −
d [I ]
1 d [R
⋅]
= k d [I ] =
2 dt
dt
create 2
fragments
d [RM ⋅]
d [R ⋅]
l
= f
= 2 fk d [I ] kd ∼ 10-4 – 10-6
mol ⋅ sec
dt
dt
Propagation
RMn
+ M
Rp = −
d [M ]
= k p [M ⋅][M ]
dt
RMn+1
[M ⋅] ≡ [Mn ⋅]
any active monomer
(assume equal reactivity for all M⋅ species)
kp ∼ 102 – 104
l
mol ⋅ sec
Termination
Mi + Mj
Mi+j
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 10
Page 5 of 6
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.
Rt = −
d [M ⋅]
= 2k[M ⋅]2
dt
assume same disproportionation:
let k t = k tc + k td
l
kt ∼ 106 – 108
mol ⋅ sec
How fast are you creating polymer?
Polymerization rate
−
d [M ⋅]
= R p = k p [M ⋅][M ]
dt
Assume steady state free radical concentration [M⋅]
⇒ Ri = Rt
2k t [M ⋅]2 = 2k d f [I ]
Solve for [M⋅]:
1
⎛
⎞2
[M ⋅] = ⎜⎜ k d f [I ] ⎟⎟ plug into Rp expression
⎝ kt ⎠
1
⎛ k f [I ] ⎞ 2
⎟⎟ [M
]
R p = k p ⎜⎜ d
⎝ k t ⎠
Generic Form:
1
⎛ k 2p
⎞2
Ri ⎟ [M ]
Rp = ⎜
⎜ 2k t ⎟
⎝
⎠
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 10
Page 6 of 6
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.