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

• Typical vinyl monomersX X X

radical cationic anionic

OR

OO

CH3

OEt

O

CN

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 MonomersMonomers Radical Cationic Anionic Complex

MetalVCl + - - +/-Vinyl esters + - - -Acylates/ methacrylates

+ - + -

Acrylonitriles/ Acrylamides

+ - + -

Vinyl ethers - + - -Substituted Styrenes

+ +/- +/- +/-

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

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

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

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

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

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

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

Components of Emulsion Polymerization

Monomer

Monomer Micelle 20 -30 A

PolymerMonomerDroplet500-2000 A

Monomer Droplet10,000 A (1 )

R.

Water soluble initiator

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

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

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

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

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

Polypropylene (atactic)

CH3 CH3

* n

R

CH2Low molecular weight oils

Formation of allyl radicals via chain transfer limits achievable molecular weights for all -olefins

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

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

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

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%

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 ProcessN. 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 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

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

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

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

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.

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

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

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

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

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

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)

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

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

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 Stereo control

Performance

1950’s None Moderate Mw PE

Some comonomer incorporation

Early

1980’s

None High MW PE

Better comonomer incorporation

Me

Me

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

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

Technology S-curves for polyolefin production