Lecture 4, PE + PP

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•Elements of Addition Polymerization

•Branching and Tacticity

•The Effect of Crystallinity on Properties

Chapters 1 & 2 in CD (Polymer Science and Engineering)

The term polyolefin embraces all polymers that are derived from simple unsaturatedaliphatic hydrocarbons that contain one double bond per monomer. Examples include:

The most important polyolefins in terms of production volume are polyethylene (PE),polypropylene (PP) and the ethylene/propylene copolymers (EP). Other significantpolyolefins include, polybut-1-ene, poly-4-methylpent-1-ene and polyisobutene (PIB).

Isobutene

CH2 CCH3

CH3

4-Methylpent-1-ene

But-1-ene

PropyleneEthylene

CH2 CH

CH2

CH3

CH3

CH

CH2 CH

CH2

CH3

CH2 CH

CH3

CH2 CH2

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~ 1890’s

CH2N2 -(CH2)n - + N2

~ 1930’s

ICI - chemical reactions underpressure; Ethylene + Benzaldehyde


A modern cable coating The original ICI pilot plant

This allowed

this to triumph


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~~~~~~~CH2 - CH - CH2 - C* Rest of Chain

~~~~~~~CH2 - C* X - CH2 = CHRest of Chain

X

-

X -

X

-

Active site

H

-

H

-

1. Initiation2. Propagation3.Termination4. Chain Transfer

Need to consider;


CH2 = CHX

CH2 = CX - CH = CH2

CH CH -- -- --

N

CO

H

O

CH2 - CH2

Various Olefins and Vinyl Monomers

Various Dienes

Acetylene

Caprolactam

Ethylene Oxide

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CH2 = C X

-

H -

.CH2 - C. X

-

H -

.CH2 - C. X

-

H -

.CH2 - C. X

-

H -

.CH2 - C. X

- H -

.CH2 - C. X -

H -

CH2 - C - X

-

H -

CH2 - C - X

-

H - CH2 - C -

X

-

H -

- CH2 - C - X

-

H -


~~~~~~~CH2 - CH - CH2 - C.Rest of Chain

CH2 = CH X -

X - X -

~~~~~~~CH2 - C.

X -

Rest of Chain H

-

H

-

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X -

X -

X -

~~~~~~~CH2 - C.

X - CH2 = CHRest of Chain

X -Anion or Cation

H

-

H

-

H

-

Free RadicalAnionicCationicCoordination (Catalyst)

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X

-

X -

X

-

Active site

H

-

H

-

1. Initiation2. Propagation3.Termination4. Chain Transfer

Need to consider;

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R - O - O - R 2 R - O.

R - O. + CH2 = CHX

R - O - CH2 - C.

Peroxide PeroxideRadical

INITIATION

H

-

X -

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~~~~~~~CH2 - CH - CH2 - C.Rest of Chain

~~~~~~~CH2 - C.

X - CH2 = CHRest of Chain

X -

X - X -

H

- H

-

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X -

X -

~~~~~~~CH2 - CH - CH - CH2~~~~~~~

X -

X -

~~~~~~~CH2 - C. Rest of Chain .C - CH2~~~~~~~Rest of Chain H

- H

-~~~~~~~CH - C.

X -

Rest of Chain .C - CH2~~~~~~~Rest of Chain

X -

H

- H

- H

-

~~~~~~~CH = CHX H2XC - CH2~~~~~~~Rest of Chain Rest of Chain

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Formation of short chainbranches in polyethylene

~~~CH2 - CH - CH2 - CH2.

C4H9-

~~~CH2 - CH CH2 - CH2

CH2.CH2H

~~~CH2 - CH CH2 - CH2

CH2.CH2H

~~~CH2 - CH - CH2 - CH2 - CH2 - CH2.

C4H9-

CH2 = CH2

CH2 = CH2

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Linear

Branched

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Let's establish a simple connection between structure and properties rightfrom the beginning. We'll explore polymer morphology in more detail later,but simplistically we can get:

RANDOM COILS

Like “cooked spaghetti”

SEMICRYSTALLINE POLYMERS

A bit like "uncooked spaghetti ”

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We will be asking how crystallinity affects

•Strength•Stiffness •Toughness•Barrier Properties•Solubility•Transparency•Thermal Properties•Etc

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Linear

Branched

Which of these is more likely to crystallize?

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Various grades of polyethylene are produced commercially and are oftenreferred to as “high density” or “low density”. Which do you think is thehigh density polyethylene

A. The linear, more crystalline stuff ? B. The (somewhat) branched less crystalline stuff ?

Crystallizesmore like this

than this

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They do not pack as closelytogether as in the crystallinestate.

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The type of polyethylene that goes intomilk jugs is stronger, stiffer, but moreopaque (less optically clear) than the typeof polyethylene that is used to make filmwrap (greater optical clarity,more flexible,but less strong) . Can you figure out whichtype of polyethylene is used to make filmwrap ? A. High density B. Low density

Property

Strength

Stiffness

Toughness

Optical Clarity

Barrier Properties

Solubility

Generally increases with degree of crystallinity

Generally increases with degree of crystallinity

Generally decreases with degree of crystallinity

Generally decreases with increasing degree ofcrystallinity.Semi-crystalline polymers usually appear opaquebecause of the difference in refractive index of the amorphousand crystalline domains, which leads to scattering. Will dependupon crystallite size.

Change with Increasing Degree of Crystallinity

Small molecules usually cannot penetrate or diffuse throughthe crystalline domains, hence “barrier properties”, whichmake a polymer useful for things like food wrap, increase withdegree of crystallinity

Similarly, solvent molecules cannot penetrate the crystallinedomains, which must be melted before the polymer will dissolve.Solvent resistance increases with degree of crystallinity

High PressureHigh Temperature

Low Molecular Weight Products (Oligomers)

αααα -olefin

Propylene

CH2 CH

R

CH2 CH

CH3

High PressureHigh Temperature

CH2 CH2

Ethylene

Polyethylene (LDPE)

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Hogan and Banks synthesized polypropylene and linearpolyethylene in a low pressure process using metal oxidecatalysts.

The French can make evenpolyethylene look sexy!

““““AAAA CCCCoooommmmppppaaaannnnyyyy MMMMaaaakkkkeeeerrrr””””

The Hula Hoop!

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High Pressure Ziegler Metal OxidePolyethylene Polyethylene Polyethylene

Molecular Weight (Mn) 20000 15000 15000Number of Me groups/1000 Carbons 30 6 < 0.15

Density (g/cc) 0.92 0.95 0.96Crystalline Melting Point (°C) 108 130 133

In his lecture at the Nobel Prize awardceremony, Ziegler described his preliminaryworka thus, “The development began at the endof 1953 when Holzkamp, Breil, Martin andmyself, in a few almost dramatic days, observedthat the gas ethylene could be polymerized at100, 20, or 5 atmospheres and finally even atatmospheric pressure very rapidly in thepresence of certain, easily prepared, catalyststo give a high molecular weight plastic.”

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In the middle of 1954, Natta and his coworkers, having been informed byZiegler of the nature of his original HDPE catalyst, used it to polymerizepropylene and obtained a partially “crystalline”, essentially i sotactic,polypropylene (more on this shortly). Subsequently, Natta and his coworkersmade changes to the catalyst, replacing TiCl4 with TiCl3, which increased theisotacticity of polypropylene from 50-70 to 80-90%.

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Natta’s research in this areawas funded by the Italiancompany Montecatini and apatent was filed jointly soonthereafter. Unfortunately, thiscaused considerable ‘badblood’ between Ziegler andNatta and resulted in one ofthe longest lasting patentcases of all time.

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1950 - 1953

Ziegler-Natta CatalystsMetal Oxide Catalysts

Linear PE and stereoregular polyolefins

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~~~~~~~CH2 - CH

X

Rest of Chain

CH2 = CH X -

- Catalyst

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Two molecules are said tobe isomers if they aremade up of the samenumber and types ofatoms, but differ in thearrangement of these

atoms.

•Sequence isomerism

•Stereoisomerism (in vinyl polymers)

•Structural isomerism (in diene polymers)

Polymerization of a vinylmonomer, CH2= CHX, whereX may be a halogen, alkylor other chemical group(anything except hydrogen!)leads to polymers withmicrostructures that aredescribed in terms oftacticity.

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Meso Diad

Racemic Diad

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Part of an isotacticpolypropylene chainseen from the side

The same chain seenmore from the top

SSSSyyyynnnnddddiiiioooottttaaaaccccttttiiiicccc CCCChhhhaaaaiiiinnnnssssHere are two more polypropylene chains, both shown as if we were lookingdown from “on top”. One of these consists of units that are all racemic toone another and is called syndiotactic. The other has a random arrangementof units and we call such chains atactic. Which one is the atactic chain , Aor B ?

A

B

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Polystyrene - atactic

Polypropylene - largely isotactic

PVC - largely atactic (Some syndiotactic sequences ?)

PMMA - atactic

Documents

Lecture 4, PE + PP