Polymer Synthesis
CHEM 421
• Chapter 3-6 (Odian)
Oligomers
Polymer Synthesis
CHEM 421
“Oligomer” – Greek: oligos, “few”
mer, “parts”
• Find commercial application in a variety of fields:
Elastomers
- poly(ethylene oxide) oligomers in Spandex®
Coatings & Adhesives
- acrylic oligomers
Lubricants
- fluorinated oligomers used as
lubricants on satellites, disk drives, etc…
Free Radical Solution
Polymerizations
• Initiation
• Propagation
• Termination
Polymer Synthesis
CHEM 421
Routes to Oligomers
DP =
[ M ]o
Polymer Synthesis
CHEM 421
Use large amounts of initiator:
• very expensive
• high levels of azo-initiators leads
to toxic cross-coupling products
[ I ]o Use low monomer concentrations:
• low productivity
• requires lots of solvents
Alternative is to use ‘Chain Transfer” Processes…
Chain Transfer
Polymer Synthesis
CHEM 421
X—A = solvent, monomer, initiator, chain transfer agent…
Rtr = ktr [P•] [XA]
• Chain transfer is a chain-breaking step
–Decreases size of propagating chain
• Effect of chain transfer on Rp is a function of ka
Chain Transfer
Polymer Synthesis
CHEM 421
• Aliphatic hydrocarbons with strong
C—H bonds show low CT
• Benzene even lower
• Alkyl aromatics (benzylic H’s)
– t-butyl benzene↓, no benzylic H
• Butyl iodide (weak C—I bond)
• Acids, ethers, amines, alcohols
>> than aliphatics due to
heteroatom stabilization
• Weak S—S bond
• Halogenated solvents, weak
C—X bond
• Thiols the largest!
Chain Transfer Constants
Polymer Synthesis
CHEM 421
Rate of Polymerization
DP = —————————————
Σ Chain breaking steps
Rp
DP = ———————————————————————
(
Σ termination + CT to monomer + CT to solvent + CT to initiator + CT to CTA
CM
ktr,monomer
= ————
kp
CS
ktr,initiator
CI = ————
kp
ktr,solvent
= ————
kp
)
Effect on Rate of
Polymerization
Polymer Synthesis
CHEM 421
• How does Chain Transfer affect the rate of
polymerization (Rp)?
• Two competing processes:
Reinitiation
kreinitiation = ka
A
AM
+ M
vs.
Propagation
kpropagation = kp
Mn
+M
Mn+1
Effect on Degree of
Polymerization
Polymer Synthesis
CHEM 421
• How does Chain Transfer affect the degree
of polymerization (DP)?
• Two competing processes:
Transfer
ktransfer = ktr
Mn
Mn X + A
+ XA
vs.
Propagation
kpropagation = kp
Mn
+M
Mn+1
Competing Processes
Polymer Synthesis
CHEM 421
• Thus, we have three competing processes,
all of which affect Rp & DP ….
ktransfer = ktr
Transfer
Mn
Mn X + A
+ XA
kpropagation = kp
Propagation
Mn
Mn+1
+M
kreinitiation = ka
Reinitiation
A
+ M
AM
This leads to four possible scenarios…
1st Case
Polymer Synthesis
CHEM 421
“ Normal Chain Transfer ”
Rate of Propagation >> Rate of Transfer
kp >> ktr
Rate of Reinitiation ≈ Rate of Propagation
ka ≈ kp
• No effect on Rate of Polymerization (Rp)
ie. same # of monomers consumed / unit time
• Decrease in Degree of Polymerization (DP)
2nd Case
Polymer Synthesis
CHEM 421
“ Telomerization ”
Rate of Propagation << Rate of Transfer
kp << ktr
Rate of Reinitiation ≈ Rate of Propagation
ka ≈ kp
• Still no effect on Rate of Polymerization (Rp)
ie. same # of monomers consumed / unit time
• Huge Decrease in Degree of Polymerization (DP)
DP = 1-5 repeat units!!!
Unlike 1st case, transfer (kt) is more rapid than
propagation (kp) !!
3rd Case
Polymer Synthesis
CHEM 421
“ Retardation ”
Rate of Propagation >> Rate of Transfer
kp >> ktr
Rate of Reinitiation < Rate of Propagation
ka < kp
• Decrease in Rate of Polymerization (Rp)
Rp is decreased b/c reinitiation (kr) is slower!!
• Decrease in Degree of Polymerization (DP)
4th Case
Polymer Synthesis
CHEM 421
“ Degradative Chain Transfer ”
Rate of Propagation << Rate of Transfer
kp << ktr
Rate of Reinitiation < Rate of Propagation
ka < kp
• Decrease in Rate of Polymerization (Rp)
Like Retardation, re-initiation is slow…
• Large decrease in Degree of Polymerization (DP)
Different from Retardation, Transfer (kt) is rapid
Chain Transfer Constant (Cs)
Polymer Synthesis
CHEM 421
Mathematical Definition:
Cs =
ktr
kp
Transfer
Propagation
The magnitude of Cs reflects the activity of the
chain transfer agent …
Determining Cs
Polymer Synthesis
CHEM 421
Make a plot…
The Mayo Equation:
1
DP
1
=
( DPo) + C
s
[S]
1
[M]
DP
Where:
DP = Degree of Polymerization
DPo = DP in absence of chain transfer agent
[S] = Chain transfer Agent Conc.
[M] = Monomer Conc.
Cs = Chain transfer Constant
Slope = Cs
[S]
[M]
Common Chain Transfer
Agents
Polymer Synthesis
CHEM 421
Advantages
Disadvantages
• Very reactive
• Toxicity
• Commercially available
• Stench (Thiols)
• Some able to functionalize
polymer end groups
• Non-Catalytic
(ie. very low MWs
require high conc.)
• Some are inexpensive
Catalytic Chain Transfer
(CCT)
F
x
F
B
O
O
N
N
x
Advantages:
x
• Catalytic - Conc. as low as 100 ppm !
• Very low MWs easily achieved
• Non-Toxic
• High yields
• Produces vinyl functional oligomers
(macromonomers)
Co
N
N
O
O
x
Polymer Synthesis
CHEM 421
B
F
F
O
Disadvantages:
N
O
N
Co
N
N
O
O
• Air Sensitive
• Need to remove catalyst
• Only works with Methacrylates and
Sytrenes
Catalytic Cycle: MMA
Example
CH2
CH3
CH2
C
CH2
C
Propagating
Chain
Polymer Synthesis
CHEM 421
O
C
C
Vinyl-terminated
Oligomer
O
O
O
CH3
CH3
Cobalt (III) Hydride
Intermediate
L
II
Co
L
H
CoIII
New Propagating
Chain
CH3
CH3
H3C
H2C
C
C
O
O
CH3
C
C
O
O
CH3
Monomer
Cobaloxime Catalysts
Polymer Synthesis
CHEM 421
Cobaloximes are the most active CCT catalysts
H
O
X
N
H
O
N
A
X
N
O
H
O
N
Py
X
N
N
O
Nonionic
X
N
O
Py
H
O
N
Py
X
Co
Co
E
N
A-
O
O
Co
X
+
N
O
Ionic
• Choice of Acrucial
X
N
O
Py
H
N
O
Other species
• Among the most
active
By varying ligand substituents one can vary
Cs over 3 orders of magnitude !!!
X
BF2 Bridging Ligands in
CCT Catalysts
F
x
F
x
B
O
O
N
N
First reported in 1981 by
Nonaka et. al.
Co
N
N
O
x
O
x
B
F
Polymer Synthesis
CHEM 421
Nonaka, T.; Hamada, K. Bull. Chem.
Soc. Jpn. 1981, 54 (10), 3185
F
• Current CCT catalyst of choice largely because
of decreased sensitivity to oxygen
• Crucial for CCT on industrial scales
Applications of CCT?
Polymerizable end
group from CCT
Macromonomer route to
graft copolymers …
CH3
H
CH2 C
+
CH2 C
Polymer Synthesis
CHEM 421
CH2
AIBN
CH2 C
C O
C O
C O
O
O
O
R
CH3
CH3
Graft Copolymer
• Vinyl-terminated oligomers polymerize well
with acrylic monomers
Chain Transfer
Polymer Synthesis
CHEM 421
• No discussion of chain transfer to
polymer???
• Not easy to determine…
• Can not simply introduce new term into
Mayo equation:
1
1
[P]
+ CP
=
DP
DPo
[M]
(
)
since doesn’t lead to decrease in Mn
• Leads to branching…
Polyethylene
Polymer Synthesis
CHEM 421
• 20 – 30 “short”
branches per 10,000
carbons
• LDPE
–50 – 70% x-tal
–PDI = 20 – 50 (!)
–Density = 0.92 – 0.93
g/mL
–Tm ≈ 110 °C
Polyethylene
Polymer Synthesis
CHEM 421