General Approaches to Polymer Synthesis
• 1. Addition
Current Strategies in Polymer Synthesis
Chain Growth
Polymerization of Vinyl Monomers
Ring Opening Polymerization
Heterocylics
Metathesis of Cyclic Olefins
2. Condensation
• Objectives: Precise Macromolecular Design
• 1
–
–
–
–
Step Growth
Polymerization of A-B or AA/BB Monomers
3. Modification of Preformed Polymers
. Control of: Molecular Weight
Molecular Weight Distribution
Composition
Sequence of repeat units
Stereochemistry
• 2. Versatility
Polysaccharides
Peptides and Proteins
–
Synthetic Precursors
Anatomy of Addition Polymerizations
Polymerizability of Vinyl Monomers
• Initiation
– Generation of active initiator
– Reaction with monomer to form growing chains
Active Centers must be stable enough to persist
though multiple monomer additions
• Propagation
– Chain extension by incremental monomer addition
X
radical
• Termination
– Conversion of active growing chains to inert polymer
cationic
X
anionic
X
• Typical vinyl monomers
• Chain Transfer
CN
– Transfer of active growing site by terminating one
chain and reinitiating a new chain.
O
O
CH3
O
O
R
OEt
Polymerizability of Vinyl Monomers
Polymerizability of Vinyl Monomers
Monomers
Monomers
Ethylene
Radical Cationic
Anionic Complex
Metal
+
+
+
-
+
-
+/+
1,2-Dialkyl
olefins
-
+
-
+
1,3-Dienes
+
+
+
+
+
+
+
+
Propylene
1,1-Dialkyl
olefins
Styrenes
VCl
Radical Cationic
Anionic Complex
Metal
+/+
-
+
+
+
-
Acrylonitriles
/ Acrylamides
+
-
+
-
Vinyl ethers
+
+
+/-
+/-
+/-
Vinyl esters
Acylates/
methacrylates
Substituted
Styrenes
1
Thermodynamics of Polymerization
X
X
X
X
Thermodynamics of Polymerization
Monomer
∆Gp = ∆Hp-T∆Sp
∆Hp < 0
π-bond Æ σ-bond
∆Sp < 0
Loss of translational entropy
Polymerization favored below a ceiling temperature, Tc
Tc =
∆Η
∆S
Classical Free Radical Process
Applied to wide range of monomers
Broad scope of experimental conditions
Molecular weight can be controlled
Mw/Mn > 1.5 → 2.0 →
Statistical compositions and sequences
Little stereochemical control
Types of Radical Initiators
• Application Temperatures, T1/2 = 10 hr.
150°C Hydroperoxides and Alkyl peresters
25°C AIBN + Light, Percarbonates,
Photoinitiators
0−5°C Redox Systems, ROOH + Me++
Tc, K (C)
Observed
93
155
600 (327)
400
MMA
56
104
478(205)
220
α-Methyl
styrene
35
110
318 (45)
61
Isobutylene
48
121
326 ( 123)
50
Free Radical Initiated Polymerization
•
•
•
•
•
•
•
Controlled Free Radical Polymerization
Broad range of monomers available
Accurate control of molecular weight
Mw/Mn ≅ 1.05 --Almost monodisperse
Blocks, telechelics, stars
(Controlled molecular architecture)
Statistical Compositions and Sequences
Thermal Free Radical Initiators
• Rate of Decomposition
Rd = kd [I]
80°C Benzoyl Peroxide, AIBN, Persulfates
-∆Sp,
J/K-mole
Ethylene
Free Radical Initiated Polymerization
•
•
•
•
•
•
•
-∆Hp,
kJ/mole
where k = A e -Ea/RT and A ≈ 1015 sec-1
To produce 10-7 to 10-6 radicals mole/l.sec,
Ea ≈ 30-40 kcal/mole (115-140 kJ/mole)
For 1st order reactions, half-live, τ = ln 2/k
• Temperatures giving half lives of 10 hr
considered optimum use temperatures
2
Kinetics of Polymerization
Fate of Initiator Radicals
• Initiation steps
• Rd = kd[I]
• Radical reactions
Recombination in solvent cage
Recombination in media
Reaction with polymer radicals (kt)
Reaction with initiator (MIH)
Radical abstraction from polymer chains
Reaction with solvent or inhibitor
O
R
X
ki
R
R
X
P M
+ M
Rp = kp1[M ][M]
R
M
X
R
M M
X
kp1
M + M
kp2
X
R
kp
X
Rp = kp2[MM ][M]
P M
Chain Transfer
X
+
R S
H
ktr
R
H
H
P C
H
X
R
X
S
• Reinitiation of growing chain using transferred
radical
S
+
X
R
+
ki
X
X
ka
R
2 R
X
P M
R
X
ktc
P M M P
R
X
H
P C
H
X
R
X
ktd
P
X
R
H
C
X
+
H
P C
H
X
H
C H
X
Rate Expressions for Radical Polymerization
+
R
R
• Termination by coupling,
• Rtc = 2 ktc [RPM.]2
2R
• Hydrogen transfer to growing polymer chain
H
P C
H
X
R
• Termination by disproportionation
• Rtd = 2 ktd [RPM.]2
• Assume rate of addition is independent of radical
size, Rp = kp [M.][M]
R
-CO2
O
Termination Steps
• Sequential monomer addition
X
R
• Add efficiency factor, Ri = 2 f ki [I] [M}
Propagation Steps
+ M
2
X
• Efficiency factor, f = 0.1 - 0.9
R
O
heat
R
O
R
+
O
• Ri = ki [I.] [M]
• Where [I.] = 2 kd [I]
• Chain initiation, Ri = 2 f kd [I]
R
O
S
X
kp
• Overall growth of polymer
• Rpoly = Ri + Rp – Rt
• Rpoly ~ Rp Rp= kp [M.][M]
– Assumptions: Contribution of Ri and Rt negligible for
high degrees of polymerization
– Radical concentration based upon Steady State
concentration of radicals, i.e. Ri = Rt
• 2 ki f [I][M] = 2 (ktc + ktd) [M.]2
• [M.] = {(ki f)/ (ktc + ktd)}1/2 [I]1/2 [M]
– Assume [M] on initiation is negligible
[M.] = {(ki f)/ (ktc + ktd)}1/2 [I]1/2
3
Rate Expressions for Radical Polymerization
• Overall rate of polymerization
• Rp= kp [M.][M]
• [M.] = {(ki f)/ (ktc + ktd)}1/2 [I]1/2
Control of Molecular Weight
• Impact of initiator concentration
DP ≈ ν = Rp / Rt where ν is the kinetic chain length
DP = ν =
• Then
• Rpoly = kp {(ki f)/ (ktc + ktd)}1/2[I]1/2[M]
DP =
1
=
DP
• Rate of polymerization is proportional to:
– square root of initiator concentration
– First order in monomer concentration
Rp
Ri
Rp
=
Rt
kp [M.] [M]
kt [M.]2
kt [M.]2
kp [M.] [M]
=
kd(f kd)1/2 [I]1/2
kp [M]
For termination by coupling DP = 2 ν
For termination by disproportionation, DP = ν
Calculation of Ctr
Control by Chain Transfer
• Add chain transfer processes to termination processes
1
=
DP
kt [M.]2 + ktr[SH][M.] + ktm[M][M.] + kti[I][M.]
Ctr
1/DP
kp [M.] [M]
• Assume chain transfer to monomer and initiator are small
1
=
DP
. 2
kt [M ] + ktr[SH][M.]
kp [M.] [M]
1
=
DP
=
kt[M.]
kp[M]
+
ktr[SH]
kp[M]
1
[SH]
+ Ctr [M]
DPo
• Where Ctr is the chain transfer constant
1/DPo
[SH]/[M]
Types of Vinyl Polymerization
Method
Advantages
Disadvantages
Bulk (Neat)
Simple equipment
Rapid reaction
Pure polymer isolated
Heat buildup
Gel effect
Branched or crosslinked product
Solution
Good mixing
Ready for application
Lower mol. Wt.
Low Rpoly
Solvent Recovery
Suspension
(Pearl)
Low viscosity
Direct bead formation
Removal of additives
Emulsion
High Rpoly
Low Temperatures
High Mol. Wt.
High surface area latex
Removal of additives
Coagulation needed
Latex stability
Inverse Emulsion Water in oil latex
formed
Inversion promotes
dissolution in water
4