Polymer Synthesis CHEM 421 Chapter 3-6 (Odian). Polymer Synthesis CHEM 421 Oligomers “Oligomer” – Greek: oligos, “few” mer, “parts” Find commercial application.

Polymer SynthesisCHEM 421

• Chapter 3-6 (Odian)


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…


Free Radical Solution Polymerizations

• Initiation

• Propagation

• Termination


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


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]


• 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


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


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


Effect on Degree of Polymerization

• How does Chain Transfer affect the degree of polymerization (DP)?

• Two competing processes:

Transfer

vs.

Propagation+ MMn


Mn +ktransfer = ktr

Mn XX A + A


• Thus, we have three competing processes, all of which affect Rp & DP ….

Transfer

Propagation

Reinitiation

+ MMn


Mn +ktransfer = ktr

Mn XX A + A

+ A MMAkreinitiation = ka

This leads to four possible scenarios…This leads to four possible scenarios…

Competing Processes


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)


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) !!


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)


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


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


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.)


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


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


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 !!!


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


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 …


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]


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


Polyethylene

Polymer Synthesis CHEM 421 Chapter 3-6 (Odian). Polymer Synthesis CHEM 421 Oligomers “Oligomer” – Greek: oligos, “few” mer, “parts” Find commercial application.

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