anatomy of addition polymerizations initiation –generation of active initiator –reaction with...
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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.
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Polymerizability of Vinyl Monomers
Active Centers must be stable enough to persist though multiple monomer additions
• Typical vinyl monomersX X X
radical cationic anionic
OR
OO
CH3
OEt
O
CN
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Polymerizability of Vinyl Monomers
Monomers Radical Cationic Anionic Complex Metal
Ethylene + - + +Propylene - +/- - +1,1-Dialkyl olefins
- + - -
1,2-Dialkyl olefins
- + - +
1,3-Dienes + + + +Styrenes + + + +
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Polymerizability of Vinyl MonomersMonomers Radical Cationic Anionic Complex
MetalVCl + - - +/-Vinyl esters + - - -Acylates/ methacrylates
+ - + -
Acrylonitriles/ Acrylamides
+ - + -
Vinyl ethers - + - -Substituted Styrenes
+ +/- +/- +/-
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Types of Vinyl Polymerization
Method Advantages Disadvantages
Bulk (Neat) Simple equipmentRapid reactionPure polymer isolated
Heat buildupGel effectBranched or crosslinked product
Solution Good mixingReady for application
Lower mol. Wt.Low Rpoly
Solvent Recovery
Suspension(Pearl)
Low viscosityDirect bead formation
Removal of additives
Emulsion High Rpoly
Low TemperaturesHigh Mol. Wt.High surface area latex
Removal of additivesCoagulation neededLatex stability
Inverse Emulsion Water in oil latex formedInversion promotes dissolution in water
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Commodity Vinyl Polymers
Polystyrene (1920)
PSStyrofoam, clear plastic cups envelop windows, toys
Poly(vinyl chloride) (1927)
PVCgarden hose, pipe, car trim, seat covers, records, floor tiles
Cl
Cl
Cl Cl
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Semi-Commodity Polymers
Poly(methyl methacrylate) (1931)
PMMAplexiglas, embedding resin, resist for X-ray applications
Polytetrafluoroethylene. (1943)teflon, non stick cookware, no grease bearings, pipe-seal tape
CO2CH3
CO2CH3
CO2CH3
CO2CH3
CO2CH3
F
FF
F
F
F
F
F
F
F
F
F
F
F
F
F
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Suspension Polymerization
Equivalent to a "mini-bulk" polymerizationAdvantages• 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
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Suspension (Pearl) Polymerization Process Type Aqueous Phase Monomers Used Product
BEADPolymer Soluble in
Monomer
1% Sol. PolymerSuspending Agents
Cu++ Inhibitors
StyreneMethyl
MethacrylateVinyl Acetate
Clear Beads
POWDERPolmer Insoluble in
Monomer
Suspending AgentsElectrolytes
Vinyl ChlorideAcrylonitrile
Fluoroethylene
Opaque Beads or Powders
INVERSEHydrocarbon Media
MonomerInitiator
AcrylamideAcrylic Acids
Beads
Emulsions
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Suspension Polymerization of Styrene
Temp
Polymerization Time. Hours
Aqueous Phase: 16.6 Kg of H2O 0.24 kg Ca3PO4
0.14 kg Na+ Naphthalene sulfonate 0.077 kg. 15% Sodium Polyacrylate
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
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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 contaminatedby surface active agentsCoagulation introduces salts; Poor electrical properties
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Components of Emulsion Polymerization
Monomer
Monomer Micelle 20 -30 A
PolymerMonomerDroplet500-2000 A
Monomer Droplet10,000 A (1 )
R.
Water soluble initiator
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POLYMERS PRODUCED USING EMULSION PROCESSES
Polymer ApplicationsStyrene-Butadiene Rubber (SBR)
Tires, Belting, Flooring,
Molded goods, Shoe soles, Electrical insulation
Butadiene-Acrylonitrile
(nitrile rubber) Fuel tanks, Gasoline hoses, Adhesives, Impregnated paper, leather and textiles
Acrylonitrile-Butadiene-Styrene (ABS)
Engineering plastics, household appliances,
Automobile parts, Luggage
Polyacrylates Water based latex paints
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Ziegler-Natta (Metal-Coordinated) Polymerization
• Stereochemical Control
• Polydisperse products
• Statistical Compositions and Sequences
• Limited set of useful monomers, i.e. olefins
• SINGLE SITE CATALYSTS
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Polyolefins
• Polypropylene (1954)
•
• PP
• dishwasher safe plastic ware, carpet yarn, fibers and ropes, webbing, auto parts
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Tacticity
Isotactic
All asymmetric carbons have same configuration• Methylene hydrogens are meso• Polymer forms helix to minimize substituent interaction
Syndiotactic
• Asymmetric carbons have alternate configuration• Methylene hydrogens are racemic• Polymer stays in planar zig-zag conformation
Heterotactic (Atactic)• Asymmetric carbons have statistical variation of configuration
X X X X XH
H H
X
X XH X X XX
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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 Ni100 atm110 C
CH3CH2CH=CH2 + +AlCl(Et)2
+ Ni(AcAc) Same result
+ Cr(AcAc) White Ppt. (Not reported by Holzkamp)
+ Zr(AcAc) White Ppt. (Eureka! reported by Breil)
TiCl4 1 atm20-70 C
Al(Et)3 + CH2CH2"linear"
Mw = 10,000 - 2,000,000
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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
Al(Et)2Cl
CH3 CH3 CH3 CH3
CH3
VCl4
Al(iBu)2Cl
CH3 CH3
O inCH3
- 78 CCH3
CH3
Isotactic
Syndiotactic
Ziegler and Natta awarded Nobel Prize in 1963
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Polypropylene (atactic)
CH3 CH3
* n
R
CH2Low molecular weight oils
Formation of allyl radicals via chain transfer limits achievable molecular weights for all -olefins
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Polypropylene (isotactic)
CH3
TiCl3
Al(Et)2Cl
CH3 CH3 CH3 CH3
Density ~ 0.9-0.91 g/cm3—very high strength to weight ratio
Tm = 165-175C: 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
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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 compounds250,000-1,500,000 for pipe compounds >1,500,000 super abrasion resistance—medical implants MWD = 3-20 density = 0.94-0.96 g/cm3Tm ~ 133-138 C, X’linity ~ 80%
Applications: Bottles, drums, pipe, conduit, sheet, film
Generally opaque
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Polyethylene (LLDPE)
• Copolymer of ethylene with -olefin
Density controlled by co-monomer concentration; 1-butene (ethyl), or 1-hexene (butyl), or 1-octene (hexyl) (branch structure)
CH3
CH3 CH3
CH3
CH3
x y
Applications: Shirt bags, high strength films
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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 benzoateAl(Et)3 reduces TiCl4 to form active complexEthyl Benzoate modifies active sites to enhance stereoselectivity
Catalyst activity 50-2000 kg polypropylene/g Ti with isospecificity of > 90%
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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
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UNIPOL ProcessN. F. Brockman and J. B. Rogan, Ind. Eng. Chem. Prod. Res. Dev. 24, 278 (1985)
Temp ~ 70-105°C, Pressure ~ 2-3 MPa
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General Composition of Catalyst SystemGroup I – III Metals
Transition Metals Additives
AlEt3 TiCl4 H2
Et2AlCl
EtAlCl2
TiCl3
MgCl2 Support O2, H2O
i-Bu3Al VCl3, VoCL3,
V(AcAc)3
R-OH
Phenols
Et2Mg
Et2Zn
Titanocene dichloride
Ti(OiBu)4
R3N, R2O, R3P
Aryl esters
Et4Pb (Mo, Cr, Zr, W, Mn, Ni)
HMPA, DMF
R C CH
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Adjuvants used to control Stereochemistry
OCH2CH3
O
N
H
SiO
O
O
Ethyl benzoate2,2,6,6-tetramethylpiperidine
Hindered amine (also antioxidant)
Phenyl trimethoxy silane
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Nature of Active Sites
Ti
R ClCl
Cl Cl
AlR R
Monometallic site Bimetallic site
Active sites at the surface of a TiClx crystal on catalyst surface.
TiCH2
Cl
H3C
AlR
R
Cl
Cl
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Monometallic Mechanism for Propagation
Ti
CH2ClCl
Cl Cl
CH3
Ti
CH2ClCl
Cl Cl
CH3
Ti
CH2ClCl
Cl Cl
CH3Ti
H2CClCl
Cl Cl CH2
CH3
Monomer forms π -complex with vacant d-orbital
Alkyl chain end migrates to π -complex to form new σ-bond to metal
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Monometallic Mechanism for Propagation
Ti
CH2ClCl
Cl Cl
H3CTi
H2CClCl
Cl Cl CH2
CH3
Chain must migrate to original site to assure formation of isotactic structure
If no migration occurs, syndiotactic placements will form.
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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
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Modes of Termination
TiCH2
R
C H
Al
CH2
TiR
Al
H
TiCH2
R
CH2
Al
1. β-hydride shift
2. Reaction with H2 (Molecular weight control!)
TiCH
R
C H
Al
CH3
TiR
Al
H
TiCH2
R
CH2
AlHH
2
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Types Of Monomers Accessible for ZN Processes
H2C CH2CH3 CH2CH3 R
1. -Olefins
2. Dienes, (Butadiene, Isoprene, CH2=C=CH2)
1.2 Disubstituted double bonds do not polymerize
trans-1,4 cis-1,4 iso- and syndio-1,2
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Ethylene-Propylene Diene Rubber (EPDM)S. Cesca, Macromolecular Reviews, 10, 1-231 (1975)
CH3
.4-.8
.5-.1 0.05
+ +
VOCl3 Et2AlClV(AcAc)3
Catalyst soluble in hydrocarbons
Continuous catalyst addition required to maintain activity
Rigid control of monomer feed ratio required to assure incorporation of propylene and diene monomers
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Development of Single Site Catalysts
Ti
R ClCl
Cl Cl Me
Z-N multisited catalyst, multiple site reactivities depending upon specific electronic and steric environments
Single site catalyst—every site has same chemical environment
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MeX
X
+ Al O
CH3
* *n
CH3
Al:Zr = 1000
Me = Tl, Zr, Hf
Linear HD PE
Activity = 107 g/mol Zr
Atactic polypropylene, Mw/Mn = 1.5-2.5
Activity = 106 g/mol Zr
Kaminsky Catalyst SystemW. Kaminsky et.al. Angew. Chem. Eng. Ed. 19, 390,
(1980); Angew. Chem. 97, 507 (1985)
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Methylalumoxane: the Key Cocatalyst
Al(CH3)3 + H2Otoluene
0 C Al O
CH3
* *n
n = 10-20
O
Al
AlAl
CH3
OO
O
Al
OAl
OAl
AlCH3
CH3 Proposed structure
MAO
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Nature of active catalyst
Cp2MeX
X+ Al O
CH3
* *n
Cp2MeCH3
X+ Al O
CH3
Al
X
Om
Cp2MeCH2
+Al O
CH3
Al
X
Om
X
Transition metal alkylation
Ionization to form active sites
MAO
Noncoordinating Anion, NCA
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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
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Evolution of single site catalysts
Date Metallocene Stereo control
Performance
1950’s None Moderate Mw PE
Some comonomer incorporation
Early
1980’s
None High MW PE
Better comonomer incorporation
Me
Me
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Synthesis of Syndiotactic PolystyreneN. Ishihara et.al. Macromolecules 21, 3356 (1988); 19, 2462 (1986)
*Al
O*
CH3
n
TiCl
Cl
Ti Cl
ClCl
Ti Cl
Cl
+
44.1%
99.2%
1.0%
syndiotactic polystyrene
m.p. = 265C
Styrene
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Evolution of single site catalysts
Date
Late 1980’s
Metallocene Stereo control
Slight
Performance
Very High Mw PE, excellent comonomer incorporation
Late 1980’s
Highly
Syndio-
tactic
Used commercially for PP
Early
1990’s
Highly
Isotactic
Used commercially for PP
N Me
R
Me
RR
Me
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Technology S-curves for polyolefin production