Anatomy of Addition Polymerizations
• Initiation
– Generation of active initiator
– Reaction with monomer to form growing chains
• Propagation
– Chain extension by incremental monomer addition
• Termination
– Conversion of active growing chains to inert
polymer
• Chain Transfer
– Transfer of active growing site by terminating one
chain and reinitiating a new chain.
Polymerizability of Vinyl Monomers
Active Centers must be stable enough to persist
though multiple monomer additions
X
radical
cationic
X
• Typical vinyl monomers
X
anionic
CN
O
O
CH3
O
R
O
OEt
Polymerizability of Vinyl Monomers
Monomers Radical Cationic Anionic Complex
Metal
Ethylene
+
+
+
Propylene
+/+
1,1-Dialkyl
+
olefins
1,2-Dialkyl
olefins
-
+
-
+
1,3-Dienes
+
+
+
+
+
+
+
+
Styrenes
Polymerizability of Vinyl Monomers
Monomers Radical Cationic Anionic Complex
Metal
VCl
Vinyl esters
Acylates/
methacrylat
es
Acrylonitrile
s/
Acrylamides
Vinyl ethers
Substituted
Styrenes
+
+
+
-
+
+/-
+
-
+
-
+
+
+/-
+/-
+/-
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
Commodity Vinyl Polymers
Polystyrene (1920)
PS
Styrofoam, clear plastic cups
envelop windows, toys
Cl
Poly(vinyl chloride) (1927)
Cl
Cl
Cl
PVC
garden hose, pipe, car trim, seat covers, records,
floor tiles
Semi-Commodity Polymers
CO2CH3
Poly(methyl methacrylate) (1931)
CO2CH3CO2CH3
CO2CH3
CO2CH3
PMMA
plexiglas, embedding resin, resist for X-ray applications
F
Polytetrafluoroethylene. (1943)
teflon, non stick cookware, no grease bearings,
pipe-seal tape
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
Suspension Polymerization
Equivalent to a "mini-bulk" polymerization
Advantages
•
•
•
•
Aqueous (hydrocarbon) media provides good heat transfer
Good particle size control through agitation and dispersion agents
Control of porosity with proper additives and process conditions
Product easy to recover and transfer
Disadvantages
• Suspending Agents contaminate product
• Removal of residual monomer necessary
Suspension (Pearl) Polymerization
Process Type
Aqueous Phase
Monomers Used
Product
BEAD
Polymer Soluble in
Monomer
 1% Sol. Polymer
Suspending Agents
Cu++ Inhibitors
Styrene
Methyl
Methacrylate
Vinyl Acetate
Clear Beads
POWDER
Polmer Insoluble in
Monomer
Suspending Agents
Electrolytes
Vinyl Chloride
Acrylonitrile
Fluoroethylene
Opaque Beads or
Powders
INVERSE
Hydrocarbon Media
Monomer
Initiator
Acrylamide
Acrylic Acids
Beads

Emulsions
Suspension Polymerization of Styrene
Temp
Monomer Phase
16.6 Kg. Styrene
(0.5 kg Methacrylic Acid)
0.012 kg AIBN
0.006 kg Benzoyl Peroxide
0.015 kg tert-Butyl Perbenzoate
Aqueous Phase:
16.6 Kg of H2O
0.24 kg Ca3PO4
0.14 kg Na+ Naphthalene sulfonate
0.077 kg. 15% Sodium Polyacrylate
Polymerization Time. Hours
EMULSION POLYMERIZATION
• Advantages:
•
High rate of polymerization ~ kp[M] Npart/2
•
High molecular weights, ()   of particles/  R. sec-1
= N kp [M] / Ri
•
•
•
•
•
Few side reactions
High Conversion achieved
Efficient heat transfer
Low viscosity medium
Polymer never in solution
Low tendancy to agglomerate
Emulsified polymer may be stabilized and used directly
Disadvantages:
Polymer surface contaminated by surface active agents
Coagulation introduces salts; Poor electrical properties
Components of Emulsion
Polymerization
Monomer
Polymer
Monomer
Droplet
500-2000 A
Monomer Micelle 20 -30 A
R.
Water soluble initiator
Monomer Droplet
10,000 A (1 )
POLYMERS PRODUCED USING
EMULSION PROCESSES
Polymer
Applications
Styrene-Butadiene Rubber
(SBR)
Tires, Belting, Flooring,
Molded goods, Shoe soles, Electrical
insulation
Butadiene-Acrylonitrile
Fuel tanks, Gasoline hoses, Adhesives,
Impregnated paper, leather and textiles
(nitrile rubber)
Acrylonitrile-ButadieneStyrene (ABS)
Engineering plastics, household
appliances,
Automobile parts, Luggage
Polyacrylates
Water based latex paints
Ziegler-Natta (Metal-Coordinated)
Polymerization
•
•
•
•
Stereochemical Control
Polydisperse products
Statistical Compositions and Sequences
Limited set of useful monomers, i.e. olefins
• SINGLE SITE CATALYSTS
Polyolefins
• Polypropylene (1954)
•
•
•
PP
dishwasher safe plastic ware, carpet
yarn, fibers and ropes, webbing, auto parts
H
IsotacticH
X
Tacticity
H
X
X
X
X
X
All asymmetric carbons have same configuration
• Methylene hydrogens are meso
• Polymer forms helix to minimize substituent interaction
Syndiotactic
H
•
•
•
X X
X X
Asymmetric carbons have alternate configuration
Methylene hydrogens are racemic
Polymer stays in planar zig-zag conformation
Heterotactic
•
X X
(Atactic)
Asymmetric carbons have statistical variation of configuration
Ziegler’s Discovery
• 1953 K. Ziegler, E. Holzkamp, H. Breil and H. Martin
• Angew. Chemie 67, 426, 541 (1955); 76, 545 (1964).
Al(Et)3 + NiCl2
CH3CH2CH=CH2 + Ni + AlCl(Et)2
100 atm
110 C
+ Ni(AcAc)
+ Cr(AcAc)
+ Zr(AcAc)
Same result
White Ppt. (Not reported by Holzkamp)
White Ppt. (Eureka! reported by Breil)
Al(Et)3 + TiCl4
CH2CH2
1 atm
"linear"
20-70 C Mw = 10,000 - 2,000,000
Natta’s Discovery
• 1954 Guilio Natta, P. Pino, P. Corradini, and F. Danusso
•
•
J. Am. Chem. Soc. 77, 1708 (1955) Crystallographic Data on PP
J. Polym. Sci. 16, 143 (1955) Polymerization described in French
CH3
TiCl3
CH3
CH3
CH3
CH3
Al(Et)2Cl
Isotactic
CH3
VCl4 - 78 C
Al(iBu)2Cl
O
in
CH3 CH3
CH3
CH3
CH3
Syndiotactic
Ziegler and Natta awarded Nobel Prize in 1963
Polypropylene (atactic)
CH3
CH3
R
*
CH2
n
Low molecular weight oils
Formation of allyl radicals via chain transfer limits achievable
molecular weights for all a-olefins
Polypropylene (isotactic)
CH3
TiCl3
CH3
CH3
CH3
Al(Et)2Cl
Density ~ 0.9-0.91 g/cm3—very high strength to weight ratio
Tm = 165-175C: Use temperature up to 120 C
Copolymers with 2-5% ethylene—increases clarity and
toughness of films
Applications: dishwasher safe plastic ware, carpet yarn,
fibers and ropes, webbing, auto parts
CH3
Polyethylene (HDPE)
CH3
Essentially linear
structure
Few long chain branches, 0.5-3
methyl groups/ 1000 C atoms
Molecular Weights: 50,000-250,000 for molding compounds
250,000-1,500,000 for pipe compounds
>1,500,000 super abrasion resistance—medical implants
MWD = 3-20
density = 0.94-0.96 g/cm3
Tm ~ 133-138 C, X’linity ~ 80%
Generally opaque
Applications: Bottles, drums, pipe, conduit, sheet, film
Polyethylene (LLDPE)
• Copolymer of ethylene with a-olefin
CH3
CH3
CH3
CH3
CH3
x
y
Density controlled by co-monomer concentration; 1-butene (ethyl), or
1-hexene (butyl), or 1-octene (hexyl) (branch structure)
Applications: Shirt bags, high strength films
CATALYST PREPARATION
Ball mill MgCl2 (support) with TiCl4 to produce maximum
surface area and incorporate Ti atoms in MgCl2 crystals
Add Al(Et)3 along with Lewis base like ethyl benzoate
Al(Et)3 reduces TiCl4 to form active complex
Ethyl Benzoate modifies active sites to enhance
stereoselectivity
Catalyst activity 50-2000 kg polypropylene/g Ti with
isospecificity of > 90%
Catalyst Formation
AlEt3 + TiCl4 →
EtTiCl3 + Et2AlCl
Et2AlCl + TiCl4 → EtTiCl3 + EtAlCl2
EtTiCl3 + AlEt3 → Et2TiCl2 + EtAlCl2
EtTiCl3 → TiCl3 + Et. (source of radical products)
Et. + TiCl4 → EtCl + TiCl3
TiCl3 + AlEt3 → EtTiCl2 + Et2AlCl
UNIPOL Process
N. F. Brockman and J. B. Rogan, Ind. Eng. Chem. Prod. Res. Dev.
24, 278 (1985)
Temp ~ 70-105°C, Pressure ~ 2-3 MPa
General Composition of Catalyst System
Group I –
III Metals
AlEt3
Et2AlCl
EtAlCl2
i-Bu3Al
Et2Mg
Et2Zn
Et4Pb
Transition Metals
Additives
TiCl4
a,g, d TiCl3
MgCl2 Support
VCl3, VoCL3,
V(AcAc)3
Titanocene dichloride
Ti(OiBu)4
H2
O2, H2O
(Mo, Cr, Zr, W, Mn,
Ni)
HMPA, DMF
R C CH
R-OH
Phenols
R3N, R2O, R3P
Aryl esters
Adjuvants used to control Stereochemistry
O
O
Si
CH2CH3
O
O
O
N
H
Ethyl benzoate
2,2,6,6-tetramethylpiperidine
Hindered amine (also
antioxidant)
Phenyl
trimethoxy
silane
Nature of Active Sites
R
R
Al
R Cl
Cl Ti
Cl Cl
Monometallic site
H3C
d
CH2
Cl d
Ti
Cl
Cl
Al
R
R
Bimetallic site
Active sites at the surface of a TiClx crystal on catalyst
surface.
Monometallic Mechanism for Propagation
Monomer forms π -complex with vacant d-orbital
CH2
Cl Ti Cl
Cl Cl
CH3
CH2
Cl Ti Cl
Cl Cl
CH3
Alkyl chain end migrates to π -complex to
form new σ-bond to metal
H2C
CH3
Cl Ti Cl
Cl Cl
CH2
CH2
Cl Ti Cl
Cl Cl
CH3
Monometallic Mechanism for Propagation
Chain must migrate to original site to assure
formation of isotactic structure
H2C
CH3
Cl Ti Cl
Cl Cl
CH2
H3C
CH2
Cl Ti Cl
Cl Cl
If no migration occurs, syndiotactic placements
will form.
Enantiomorphic Site Control Model for
Isospecific Polymerization
Stereocontrol is imposed by initiator active site alone with
no influence from the propagating chain end, i.e. no
penultimate effect
Demonstrated by:
13C analysis of isotactic structures
not
Stereochemistry can be controlled by catalyst enantiomers
Modes of Termination
1. β-hydride shift
C H
CH2
Ti
CH2
CH2 H
Al
R
Ti
Al
CH2
Ti
R
Al
R
2. Reaction with H2 (Molecular weight control!)
C H
H H
CH2
Al
Ti
R
CH2
CH3 H
Ti
Al
R
CH2
Ti
R
Al
Types Of Monomers Accessible for ZN Processes
1. a-Olefins
H2C CH2
CH2CH3
CH3
R
2. Dienes, (Butadiene, Isoprene, CH2=C=CH2)
trans-1,4
cis-1,4
iso- and syndio-1,2
1.2 Disubstituted double bonds do not polymerize
Ethylene-Propylene Diene Rubber (EPDM)
S. Cesca, Macromolecular Reviews, 10, 1-231 (1975)
+
+
CH3
Catalyst
soluble in
hydrocarbons
VOCl3
Et AlCl
V(AcAc)3 2
.4-.8
.5-.1
0.05
Continuous
catalyst addition
required to
maintain activity
Rigid control of monomer feed ratio required to assure
incorporation of propylene and diene monomers
Development of Single Site Catalysts
R Cl
Cl Ti
Cl Cl
Z-N multisited catalyst, multiple
site reactivities depending upon
specific electronic and steric
environments
Me
Single site catalyst—
every site has same
chemical environment
Kaminsky Catalyst System
W. Kaminsky et.al. Angew. Chem. Eng. Ed. 19, 390,
(1980); Angew. Chem. 97, 507 (1985)
Me
X +
X
CH3
*
Al O
*
n
Al:Zr = 1000
Linear HD PE
Activity = 107 g/mol Zr
Me = Tl, Zr, Hf
CH3
Atactic polypropylene, Mw/Mn = 1.5-2.5
Activity = 106 g/mol Zr
Methylalumoxane: the Key Cocatalyst
CH3
toluene
Al(CH3)3 + H2O
0C
Al
O
O
O
O
Al
Al
O
O
Al
Al
Al
Al O
*
n
n = 10-20
CH3
Al
*
CH3
CH3
MAO
Proposed structure
Nature of active catalyst
CH3
X
Cp2Me
+
*
X
Al O
*
n
MAO
Cp2Me
CH3
CH3
+
Al O
X
CH3
Cp2Me
CH2
+
Transition metal
alkylation
X
Al
m
O
X X
Al O Al O
m
Ionization to
form active sites
Noncoordinating Anion, NCA
Homogeneous Z-N Polymerization
Advantages:
High Catalytic Activity
Impressive control of stereochemistry
Well defined catalyst precursors
Design of Polymer microstructures, including chiral
polymers
Disadvantages:
Requires large excess of Aluminoxane (counter-ion)
Higher tendency for chain termination: β-H elimination, etc.
Limited control of molecular weight distribution
Evolution of single site catalysts
Date
Metallocene
1950’s
Stereo
control
Performance
None
Moderate Mw PE
Some comonomer
incorporation
Me
Early
1980’s
None
Me
High MW PE
Better comonomer
incorporation
Synthesis of Syndiotactic Polystyrene
N. Ishihara et.al. Macromolecules 21, 3356 (1988); 19, 2462 (1986)
Ti
*
Al
O
CH3
+
n*
Cl
Styrene
Cl
44.1%
Ti Cl
Cl
Cl
Ti
99.2%
syndiotactic polystyrene
Cl
Cl
1.0%
m.p. = 265C
Evolution of single site catalysts
Date
Metallocene
Late
1980’s
N
Stereo
control
Slight
Performance
Very High Mw PE,
excellent comonomer
incorporation
Highly
Syndiotactic
Used commercially
for PP
Me
R
Late
1980’s
R
R
Me
Early
1990’s
Highly
Used commercially
Isotactic for PP
Me
Technology S-curves for polyolefin
production