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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

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anatomy of addition polymerizations
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
Polymerizability of Vinyl Monomers

Active Centers must be stable enough to persist though multiple monomer additions

  • Typical vinyl monomers
commodity vinyl polymers
Commodity Vinyl Polymers

Polystyrene (1920)

PS

Styrofoam, clear plastic cups

envelop windows, toys

Poly(vinyl chloride) (1927)

PVC

garden hose, pipe, car trim, seat covers, records, floor tiles

semi commodity polymers
Semi-Commodity Polymers

Poly(methyl methacrylate) (1931)

PMMA

plexiglas, embedding resin, resist for X-ray applications

Polytetrafluoroethylene. (1943)

teflon, non stick cookware, no grease bearings,

pipe-seal tape

suspension polymerization
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 polymerization of styrene
Suspension Polymerization of Styrene

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
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 agents

Coagulation introduces salts; Poor electrical properties

components of emulsion polymerization
Components of Emulsion Polymerization

R.

Water soluble initiator

ziegler natta metal coordinated polymerization
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
Polyolefins
  • Polypropylene (1954)
  • PP
  • dishwasher safe plastic ware, carpet yarn, fibers and ropes, webbing, auto parts
tacticity
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
ziegler s discovery
Ziegler’s Discovery
  • 1953 K. Ziegler, E. Holzkamp, H. Breil and H. Martin
  • Angew. Chemie 67, 426, 541 (1955); 76, 545 (1964).

+ Ni(AcAc) Same result

+ Cr(AcAc) White Ppt. (Not reported by Holzkamp)

+ Zr(AcAc) White Ppt. (Eureka! reported by Breil)

natta s discovery
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

Isotactic

Syndiotactic

Ziegler and Natta awarded Nobel Prize in 1963

polypropylene atactic
Polypropylene (atactic)

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

polypropylene isotactic
Polypropylene (isotactic)

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

polyethylene hdpe
Polyethylene (HDPE)

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
Polyethylene (LLDPE)
  • Copolymer of ethylene witha-olefin

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
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
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
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

adjuvants used to control stereochemistry
Adjuvants used to control Stereochemistry

Phenyl trimethoxy silane

Ethyl benzoate

2,2,6,6-tetramethylpiperidine

Hindered amine (also antioxidant)

nature of active sites
Nature of Active Sites

Bimetallic site

Monometallic site

Active sites at the surface of a TiClx crystal on catalyst surface.

monometallic mechanism for propagation
Monometallic Mechanism for Propagation

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 propagation30
Monometallic Mechanism for Propagation

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
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
Modes of Termination

1. β-hydride shift

2. Reaction with H2 (Molecular weight control!)

types of monomers accessible for zn processes
Types Of Monomers Accessible for ZN Processes

1. -Olefins

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
Ethylene-Propylene Diene Rubber (EPDM)S. Cesca, Macromolecular Reviews, 10, 1-231 (1975)

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
Development of Single Site Catalysts

Z-N multisited catalyst, multiple site reactivities depending upon specific electronic and steric environments

Single site catalyst—every site has same chemical environment

slide36

Kaminsky Catalyst SystemW. Kaminsky et.al. Angew. Chem. Eng. Ed. 19, 390, (1980); Angew. Chem. 97, 507 (1985)

Linear HD PE

Al:Zr = 1000

Activity = 107 g/mol Zr

Me = Tl, Zr, Hf

Atactic polypropylene, Mw/Mn = 1.5-2.5

Activity = 106 g/mol Zr

methylalumoxane the key cocatalyst
Methylalumoxane: the Key Cocatalyst

n = 10-20

MAO

Proposed structure

nature of active catalyst
Nature of active catalyst

Transition metal alkylation

MAO

Ionization to form active sites

Noncoordinating Anion, NCA

homogeneous z n polymerization
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

synthesis of syndiotactic polystyrene n ishihara et al macromolecules 21 3356 1988 19 2462 1986
Synthesis of Syndiotactic PolystyreneN. Ishihara et.al. Macromolecules21, 3356 (1988); 19, 2462 (1986)

Styrene

syndiotactic polystyrene

m.p. = 265C