10.569 Synthesis of Polymers
Prof. Paula Hammond
Lecture 31: “Living” Free Radical Approaches: Stable Free Radical
Polymerization, Atom Transfer Radical Polymerization
Modification of Solid Polymer Surface
O
e.g.
the
C
OCH3 of PMMA slab
the
of PS surface
Surface reactivity
•
•
•
•
must have access to functional groups
interfacial energy may impact presentation of functional groups to surface
surfaces are dynamic → small rearrangement can occur
also have to consider surface impurities that might prevent or occlude
access to functional groups
⇒ issues around solvent choice and surface properties
Solvent compatibility
Need to have reaction solvent that wets surface but does not dissolve the solid
⇒ By altering solvents, get differing degrees of penetration into surface
(varying from hundreds of nm to Å’s)
⇒ Solvent impacts % yield and kinetics
⇒ availability of surface groups and solubility of reagent
Remember surfaces are heterogeneous
Have morphology (e.g. crystallinity)
Have potential for plasticization w/solvent
Functionalization of Surfaces with Polymers
“Grafting From” Approaches
1. Surface irradiation w/high energy + monomer
monomer in
solvent
M
M
hν
•
•
•
•
•
M
•
M
• M
•
M
•
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.
•
2. Covalent attachment of initiating species
OH
OH
O
+
O
Cl C CH2 CH2 C
OH
N
N
CN
C CH2 CH2
C Cl
CN
O
O
C
O
(CH2)2
C
N
N
CN
C
(CH2)2
C Cl
CN
O
O
heat
C
(CH2)2
C
+M
CN
+ monomer
“Grafting To”
X
Y
+
X
Y
Anionic/Cationic
O
CH2
EtO
CH
C
O
EtO
C
Step Growth
O
OH
+
Cl
C
e.g. polyester
Issues for grafting to
Lower yields
Steric constraints to full coverage
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 31
Page 2 of 5
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.
Examples (Handout)
Free Radical
- Accounts for ∼50% of all mass production of polymers
- Emulsion
- Suspension
- Bulk solution
Most robust method of chain growth
Insensitivity to solvent
Insensitivity to impurities (NH3, H2O)
Insensitivity to atmospheric conditions
Open to widest variety of chain growth monomers
- vinyl halogens
F
F
Cl
- vinyl esters
F
F
H
limited in ionic polymerization
easy in free radical polymerization
O
C
R
- acrylates
H
protic
O
RO
NOT vinyl ethers
OR
First Attempts at Living Free Radical Polymerization
(Otsa et al, 1982)
Ini fer ters:
initiator
transfer
termination
Plays all three roles
C2H5
C2H5
N
C2H5
C
S
S
S
C
S
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
N
C2H5
Lecture 31
Page 3 of 5
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.
Dissociation
R
2I
IH + nM
I-Mn
init.
I
I-Mn
reversible
+
I
M
IMn+1 + IH
I-Mn-I
"termination"
M
IMn + IM
IMn+1-I
new chain
transfer
Get increased molecular weight over time, but road PDI.
1985 = Solomon et al.
Introduced first nitroxide free radical systems.
→ act as “adducts” but not initiators
N
O
+
AIBN
CH3
CH3
H3C C N N C CH3
CN
CH2-O
CN
N
reversible bond
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 31
Page 4 of 5
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.
Original Free Radical Mechanism
I
Initiation
R
RM
M+R
Propagation
RM + rM
Termination
2 R[M]rM
R[M]rM
Combination
Disproportionation
Nitroxide Stable Free Radical
“TEMPO”
O
R
N
O
Must not act
as initiator
in polymerization!
N
R
R
Stable Free Radical Mechanism
Rad
R
Initiator
Fragment
R'
Few
Termination
Events
For "living"
system,
need forward
rate > Rp
R'
O N
Rad
R''
R''
N
Rad
O
R
Adduct breaks
and then
recombines
R
Small amount
of free radical
R'
R
R
Rad
R
O N
R''
R
Rad
O
R'
N
R''
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 31
Page 5 of 5
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.