10.569 Synthesis of Polymers
Prof. Paula Hammond
Lecture 17: Stereoregular Polymerizations
Final comments: copolymerization
Recall azeotrope: occurs r1, r2 < 1
1.0
⇒ f1 = F1
monomer comp = final copolymer comp
- if you have no perturbations, can get same
composition throughout full range of π
- if there is perturbation → move further and
further away from f1 = F1
F1
0
1.0
0
0.5
f1
For all copolymerizations in which
- azeotrope comp
- perfectly alternating system (f1 = F1)
We have unchanged polymer composition
BUT for all other cases (most cases), monomer compositions are continually
changing.
- must create different expressions and integrate to get polymer composition
averaged over conversion
Skeist:
1−
M
∫
Mo
[M ]
[M ]o
=π
[M ] =
dM
= ln
[M ]o
M
[M ]
π = 1−
[M ]o
f1
df 1
∫ (F − f )
( )
f1
o
1
1
α
β
⎡ f ⎤ ⎡ f ⎤ ⎡( f ) −δ ⎤
= 1− ⎢ 1 ⎥ ⎢ 2 ⎥ ⎢ 1 o
⎥
⎣ ( f 1 )o ⎦ ⎣ ( f 2 )o ⎦ ⎣ ( f 1 − δ ) ⎦
where α =
γ
1.0
f1
r2
1 − r2
r
β= 1
1 − r1
avg <F1>
mol
frac
fra
c
avg <F2>
1 − r1r2
γ =
(1 − r1 )(1 − r2 )
δ=
f2
1 − r2
(2 − r1 − r2 )
F1
0
0
F2
π
1.0
1.0
For r2 > r1
(M2 consumed more rapidly) 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.
Stereochemistry of polymer chains
R
R
R
R
isotactic (same side)
R R
RR
syndiotactic (alternating)
RR
R
R
R
atactic (no sequence, random)
Dyad tacticity
fraction m + r = 1.0
isodyad
(m = meso)
syndio
(r = racemic)
Triad tacticity
iso
(mm)
%
mm + rr + mr =1
use percentages = 100%
(use NMR to determine
percentage)
hetero
(mr)
%
syndio
(rr)
%
CH2=CHR
In “normal” solution/bulk free radical polymerization
RR H
R H
+
R
H2C
CHR
kr
H
C
R
R
km
R
H
C
R
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 17
Page 2 of 6
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.
kr
⇒ determine tacticity k m
kr
∼ 1.0 ⇒ atactic
k m
kr
<< 1.0 ⇒ isotactic k m
kr
>> 1.0 ⇒ syndiotactic
k m
Sterics prefer syndiotactic addition.
Ea slightly higher for iso addition
As T↑ iso fractions ↑
T↓ syndio fractions ↑
e.g. PMMA:
CH3
H2
C C
T p = 100oC
r = 0.73
Tp = -78oC
r = 0.87
Tp = 60oC
r = 0.5
Tp = -38oC
r = 0.6
CH3
e.g. PVC
H
H2
C C
Cl
Coordination Polymerization
Coordinated catalyst: usually metal/metal halide or oxide complex:
- promote stereospecific + controlled addition of monomer
Ziegler and Natta
(Germany) (Italy)
Nobel Prize, 1963
e.g.
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 17
Page 3 of 6
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.
CH3
CH
H2C
H2C
n
or
CH3
H2C
α-olefins have
very high
transfer rates to
monomer
C
H
propylene (propane)
Initiator:
Group I-III metal (e.g. Al)
+ transition metal (e.g. Ti)
+
G
RHC
H
C
CH2
H2
C
G
R
group
metal
escort for
insertion of new
monomer
Ziegler-Natta Catalyst
Usually TiCl4 or TiCl3
+
AlR3 + AlR2Cl
R=CH2CH3 or (CH2)nCH3
alkyl
tail
Cl
CH2
Cl
Ti
Cl
H2C
CHCH3
Cl
CH2
Cl
CHCH3
Ti
CH2
Cl
Cl
Cl
π completion
CH2
Cl
H2C
Cl
Ti
Cl
CHCH3
CHCH3
Cl
CH3
Cl
CH2
Cl
Ti
Cl
H2C
CHCH3
Cl
“Escort” process guides stereochemistry
many of Z-N catalysts
(crys) ⇒ isotactic addition
(soluble) ⇒ syndiotactic
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 17
Page 4 of 6
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.
Consider polypropylene
CH3
H2
C C
n
CH3
Atactic PP: Tg ∼ -70oC
fully noncrystalline
liquid-like, paste
Isotactic PP: Tm = 170oC
highly crystalline
amorphous region: Tg ∼ -50oC - -60oC
insoluble
Syndiotactic PP: very crystalline Tm ∼ 130oC
CH2=CH2
→
H2 H2
C C
n
regular structure
Traditional free radical polymerization
Tubular Reactors:
2-6 cm in diameter
0.5 – 1.5 km in length
CH2=CH2
heat to
gas
175oC
peak T
300oC
∼
T > Tcrit
P > 200 MPa
supercritical
fluid
flui
d
add
initiator
O2, H2O2, CTA
π = 20-30%
precipitate
T = 25oC
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 17
Page 5 of 6
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.
Chain transfer to monomer and polymer:
+
+
Branches hinder crystallinity
Low density PE (LDPE)
ρ = 0.9 g/cm3
Tg = -120oC
Tm = 105oC
Highly branched,
Irregular length
Z-N catalyst: reduce transfer
minimal branching
High density PE (HDPE)
% crystallinity: 80-90%
ρ = 0.94 – 0.96
Tm > 130oC
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 17
Page 6 of 6
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.