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Polymer SynthesisCHEM 421
• Chapter 3-6 (Odian)
Polymer SynthesisCHEM 421
Oligomers
“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…
Polymer SynthesisCHEM 421
Free Radical Solution Polymerizations
• Initiation
• Propagation
• Termination
Polymer SynthesisCHEM 421
Routes to Oligomers
[ M ]o
[ I ]o
DP =
Use large amounts of initiator:
Alternative is to use ‘Chain Transfer” Processes…
Use low monomer concentrations:
Use large amounts of initiator:• very expensive• high levels of azo-initiators leads to toxic cross-coupling products
Use low monomer concentrations:• low productivity• requires lots of solvents
Polymer SynthesisCHEM 421
Chain Transfer
• Chain transfer is a chain-breaking step
–Decreases size of propagating chain
• Effect of chain transfer on Rp is a function of ka
X—A = solvent, monomer, initiator, chain transfer agent…
Rtr = ktr [P•] [XA]
Polymer SynthesisCHEM 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
Polymer SynthesisCHEM 421
Chain Transfer Constants
Rate of Polymerization
Σ Chain breaking steps DP = —————————————
Rp
Σ (termination + CT to monomer + CT to solvent + CT to initiator + CT to CTA + CT to solvent + CT to initiator + CT to CTA))DP = ———————————————————————
CM = ————ktr,monomer
kp
CS = ————ktr,solvent
kp
CI = ————ktr,initiator
kp
Polymer SynthesisCHEM 421
Effect on Rate of Polymerization
• How does Chain Transfer affect the rate of polymerization (Rp)?
• Two competing processes:
Reinitiation
vs.
Propagation
+ A MMAkreinitiation = ka
+ MMn
kpropagation = kpMn+1
Polymer SynthesisCHEM 421
Effect on Degree of Polymerization
• How does Chain Transfer affect the degree of polymerization (DP)?
• Two competing processes:
Transfer
vs.
Propagation+ MMn
kpropagation = kpMn+1
Mn +ktransfer = ktr
Mn XX A + A
Polymer SynthesisCHEM 421
• Thus, we have three competing processes, all of which affect Rp & DP ….
Transfer
Propagation
Reinitiation
+ MMn
kpropagation = kpMn+1
Mn +ktransfer = ktr
Mn XX A + A
+ A MMAkreinitiation = ka
This leads to four possible scenarios…This leads to four possible scenarios…
Competing Processes
Polymer SynthesisCHEM 421
1st Case
Rate of Propagation >> Rate of Transfer
kp >> ktr Rate of Reinitiation ≈ Rate of Propagation
ka ≈ kp
“ Normal Chain Transfer ”
• No effect on Rate of Polymerization (Rp)
ie. same # of monomers consumed / unit time
• Decrease in Degree of Polymerization (DP)
Polymer SynthesisCHEM 421
2nd Case
Rate of Propagation << Rate of Transfer
kp << ktr Rate of Reinitiation ≈ Rate of Propagation
ka ≈ kp
“ Telomerization ”
• 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) !!
Polymer SynthesisCHEM 421
3rd Case
Rate of Propagation >> Rate of Transfer
kp >> ktr Rate of Reinitiation < Rate of Propagation
ka < kp
“ Retardation ”
• Decrease in Rate of Polymerization (Rp)
Rp is decreased b/c reinitiation (kr) is slower!!
• Decrease in Degree of Polymerization (DP)
Polymer SynthesisCHEM 421
4th Case
Rate of Propagation << Rate of Transfer
kp << ktr Rate of Reinitiation < Rate of Propagation
ka < kp
“ Degradative Chain Transfer ”
• 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
Polymer SynthesisCHEM 421
Chain Transfer Constant (Cs)
Mathematical Definition:
Cs = ktr
kp
Transfer
Propagation
The magnitude of Cs reflects the activity of thechain transfer agent …
Polymer SynthesisCHEM 421Determining Cs
1
DP
[S]
[M]
Slope = Cs
The Mayo Equation:
1
DP=
1
DP( )
o+ Cs
[S]
[M]
Make a plot…
Where:
DP = Degree of PolymerizationDPo = DP in absence of chain transfer agent[S] = Chain transfer Agent Conc.[M] = Monomer Conc.Cs = Chain transfer Constant
Polymer SynthesisCHEM 421
Common Chain Transfer Agents
Advantages
• Very reactive
• Commercially available
• Some able to functionalize polymer end groups
• Some are inexpensive
Disadvantages
• Toxicity
• Stench (Thiols)
• Non-Catalytic (ie. very low MWs require high conc.)
Polymer SynthesisCHEM 421
Catalytic Chain Transfer (CCT)
N
N
O
O
N
N
O
O
Co
B
B
FF
F F
xx
x x
N N
N NCo
O
O
O
O
Advantages:
• Catalytic - Conc. as low as 100 ppm !• Very low MWs easily achieved• Non-Toxic• High yields• Produces vinyl functional oligomers (macromonomers)
Disadvantages:
• Air Sensitive• Need to remove catalyst• Only works with Methacrylates and Sytrenes
Polymer SynthesisCHEM 421
Catalytic Cycle: MMA Example
L CoII
CH2 C
CH3
C O
O
CH3
L CoIIIH
CH2 C
CH2
C O
O
CH3
H2C C
CH3
C O
O
CH3
H3C C
CH3
C O
O
CH3
Cobalt (III) HydrideIntermediate
PropagatingChain
Vinyl-terminatedOligomer
New PropagatingChain
Monomer
Polymer SynthesisCHEM 421
Cobaloxime Catalysts
N
O
N
O
N
O
N
O
Co
H
H
X
X
X
X
A
EN
O
N
O
N
O
N
O
Co
H
H
Py
Py
+ A-
N
O
N
O
N
O
N
O
Co
H
X
X
X
X
Py
Py
Cobaloximes are the most active CCT catalystsCobaloximes are the most active CCT catalysts
Nonionic Ionic• Choice of A-
crucial
Other species• Among the most active
By varying ligand substituents one can varyCs over 3 orders of magnitude !!!
Polymer SynthesisCHEM 421
BF2 Bridging Ligands in CCT Catalysts
N
N
O
O
N
N
O
O
Co
B
B
FF
F F
xx
x x
First reported in 1981 byNonaka et. al.
• Current CCT catalyst of choice largely because of decreased sensitivity to oxygen
• Crucial for CCT on industrial scales
Nonaka, T.; Hamada, K. Bull. Chem. Soc. Jpn. 1981, 54 (10), 3185
Polymer SynthesisCHEM 421
CH2 C
CH2
C O
O
CH3
CH2 C
CH3
C O
O
CH3
CH2 C
H
C O
O
R
+AIBN
Applications of CCT?
Polymerizable endgroup from CCT
Graft Copolymer
• Vinyl-terminated oligomers polymerize well with acrylic monomers
Macromonomer route to graft copolymers …
Polymer SynthesisCHEM 421
Chain Transfer
• No discussion of chain transfer to polymer???
• Not easy to determine…
• Can not simply introduce new term into Mayo equation:
since doesn’t lead to decrease in Mn
• Leads to branching…
1
DP=
1
DP( )
o+ CP
[P]
[M]
Polymer SynthesisCHEM 421
Polyethylene
• 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
Polymer SynthesisCHEM 421
Polyethylene