2.1-Types of Polymers

8/11/2019 2.1-Types of Polymers

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"I just want to say one word toyou -- just one word -- 'plastics.'"

Advice to Dustin Hoffman'scharacter in The Graduate


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Polymers: Introduction

Polymer: High molecular weight molecule madeup of a small repeat unit (monomer).

A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A

Monomer: Low molecular weight compound that

can be connected together to give a poymer Oligomer: Short polymer chain

Copolymer: polymer made up of 2 or moremonomers

Random copolymer: A-B-B-A-A-B-A-B-A-B-B-B-A-A-B

Alternating copolymer: A-B-A-B-A-B-A-B-A-B-A-B-A-B

Block copolymer: A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B


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Types of Polymers

Polymer Classifications

Thermoset: cross-linked polymer that cannot bemelted (tires, rubber bands)

Thermoplastic: Meltable plastic

Elastomers: Polymers that stretch and then return totheir original form: often thermoset polymers

Thermoplastic elastomers: Elastic polymers that canbe melted (soles of tennis shoes)

Polymer Families Polyolefins: made from olefin (alkene) monomers

Polyesters, Amides, Urethanes, etc.: monomers linkedby ester, amide, urethane or other functional groups

Natural Polymers: Polysaccharides, DNA, proteins


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Common PolyolefinsMonomer Polymer

Ethylene

H3C CH3

nRepeat unit

Polyethylene

CH3CH3

n

CH3 CH3 CH3 CH3 CH3 CH3CH3

Propylene

Polypropylene

PhCH3

n

Ph Ph Ph Ph Ph PhPhStyrene

Polystyrene

Cl

CH3

nCl Cl Cl Cl Cl ClCl

Vinyl ChloridePoly(vinyl chloride)

F2C CF2

Tetrafluoroethylene

F3C

2

CCF2

2

CCF2

2

CCF2

2

CCF2

2

CCF2

2

CCF2

CF3n

Poly(tetrafluoroethylene): Teflon


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Polyesters, Amides, and UrethanesMonomer Polymer

CO2HHO2CHO

OHO O

HO OH2C

H2C O

nTerephthalic

acidEthylene

glycol

Poly(ethylene terephthalate

H

Ester

HO OH

O O

4 H2N NH24Adipic Acid 1,6-Diaminohexane Nylon 6,6

HO NH

NH

H

O O

4 4n

CO2HHO2C

Terephthalic

acid

NH2H2N

1,4-Diamino

benzene

Kevlar

O

HO

OHN

HN H

n

Amide

HOOH

Ethyleneglycol

H2COCN NCO

4,4-diisocyantophenylmethaneSpandex

H2

C

H

N

H

N

O

HO

O

O

H2

C

H2

C O Hn

Urethane linkage


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Natural PolymersMonomer Polymer

Isoprenen

Polyisoprene:Natural rubber

O

H

HO

H

HO

H

HOHH

OH

OH

Poly(-D-glycoside):cellulose

O

H

O

H

HO

H

HO

HHOH

H

n

-D-glucose

H3N

O

O

R

Polyamino acid:protein

H3N

OHN

R1

OHN

Rn+1

O

OH

Rn+2n

Amino Acid

BaseO

OH

OP

O

O

O

oligonucleic acidDNA

NucleotideBase = C, G, T, A

BaseO

O

OP

O

O

O

DNA

DNA


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Physical Properties

Stretch

Linear Polymer

The chains can be stretched, which causesthem to flow past each other. When released,the polymer will not return to its original form.

Stretch

Cross-Linked Polymer

The cross-links hold the chains together.When released, the polymer will return to it'soriginal form.

Relax


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Polymer Synthesis

There are two major classes of polymer formationmechanisms

Addition polymerization: The polymer grows bysequential addition of monomers to a reactive site

Chain growth is linear

Maximum molecular weight is obtained early in the reaction

Step-Growth polymerization: Monomers react togetherto make small oligomers. Small oligomers makebigger ones, and big oligomers react to give polymers.

Chain growth is exponential

Maximum molecular weight is obtained late in the reaction


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Addition Polymerization

In* AInitiation

In A* A


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Addition PolymerizationPropagation

In*A

InitiationIn A A* A


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Addition PolymerizationPropagation

AIn*Initiation

In A A A*


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Addition Polymerization

Propagation

n

In A A A AnA*

A A A A Am

In A A A AnA

*A A A A Am

Combination

*A A A A Am

In A A A A n A

B A A A Am

Disproportionation

Termination

Reactive site is consumed

A

In A A A AnA

A*

Chain TransferNew reactive siteis produced

MWkpropagation

ktermination

MW

% conversion0 100

In*Initiation

In A A A A*


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Types of Addition Polymerizations

Ph

Anionic

C3H7 LiC4H9

Ph

Li+ Phn

C4H9

Ph Ph

Li+

n

Ph

Radical

PhCO2Ph

n

Ph

Cationic

Cl3Al OH2H

PhHOAlCl3

PhnH

Ph Phn

HOAlCl3

PhCO2

Ph

PhCO2

Ph Phn


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Step-Growth Polymerization

Stage 1

Consumptionof monomer

n n

Stage 2

Combinationof small fragments

Stage 3

Reaction ofoligomers to givehigh molecularweight polymer


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Step-Growth Polymerization

Because high polymer does not form until the end

of the reaction, high molecular weight polymer isnot obtained unless high conversion of monomeris achieved.

Xn1

1p

Xn = Degree of polymerizationp= mole fraction monomerconversion

1

10

100

1000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Mole Fraction Conversion (p)


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Nylon-6,6

Cl Cl4

H2N NH24

Adipoyl chloride 1,6-Diaminohexane

Cl NH

NH

H

O O

4 4

NaOH

HO NH

NH

H

O O

4 4n

6 carbondiacid

6 carbondiamine

Nylon-6,6Diamine, NaOH, in H2O

Adipoyl chloridein hexane

Nylon 6,6


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Nylon-6,6

Diamine, NaOH, in H2O

Adipoyl chloridein hexane

Nylon 6,6

Since the reactants are in differentphases, they can only react at the

phase boundary. Once a layer ofpolymer forms, no more reactionoccurs. Removing the polymer allowsmore reaction to occur.


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Molecular Weight of PolymersUnlike small molecules, polymers are typically a mixture of differentlysized molecules. Only an average molecular weight can be defined.

Measuring molecular weight

Size exclusion chromatography

Viscosity

Measurements of average molecular

weight (M.W.) Number average M.W. (Mn): Total

weight of all chains divided by # ofchains

Weight average M.W. (Mw):

Weighted average. Always largerthan Mn

Viscosity average M.W. (Mv):Average determined by viscositymeasurements. Closer to Mwthan

Mn

# of molecules

Mn

Mw

increasing molecular weight

Mv


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What the Weights MeanMn: This gives you the true average weight

Let's say you had the following polymer sample:2 chains: 1,000,000 Dalton 2,000,000

5 chains: 700,000 Dalton 3,500,000

10 chains: 400,000 Dalton 4,000,000

4 chains: 100,000 Dalton 400,000

2 chains: 50,000 Dalton 100,00010,000,000

10,000,000/23 = 435,000 Dalton

1 Dalton = 1 g/mole


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Weight Average Molecular Weight

Mw: Since most of the polymer mass is in the heavier fractions, this

gives the average molecular weight of the most abundant polymerfraction by mass.

2,000,000

10,000,000 0.201,000,000 200,000

3,500,00010,000,000

0.35 700,000 245,000

4,000,000

10,000,000 0.40 400,000 160,000

400,000

10,000,000 0.04100,000 4,000

100,000

10,000,000 0.01 50,000 500

Total 609,500


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Polymer Microstructure

Polyolefins with side chains have stereocenters on every other carbon

CH3n

CH3 CH3 CH3 CH3 CH3 CH3CH3

With so many stereocenters, the stereochemistry can be complex.

There are three main stereochemical classifications for polymers.Atactic: random orientation

Isotactic: All stereocenters have same orientation

Syndiotactic: Alternating stereochemistry


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How to Determine Microstructure?

13C NMR is a very powerful way to determine the microstructure of

a polymer.

13C NMR shift is sensitive to the two

stereocenters on either side on sptectrometers

> 300 MHz. This is called pentad resolution.

r mm rmr

mmrm pentad

m = meso (same orientation)r = racemic (opposite orientation)

12 1 2

13C NMR spectrum of CH3region

of atactic polypropylene


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Why is this important?

Tacticity affects the physical properties

Atactic polymers will generally be amorphous, soft,flexible materials

Isotactic and syndiotactic polymers will be morecrystalline, thus harder and less flexible

Polypropylene (PP) is a good example

Atactic PP is a low melting, gooey material

Isoatactic PP is high melting (176), crystalline, tough

material that is industrially useful Syndiotactic PP has similar properties, but is very

clear. It is harder to synthesize

2.1-Types of Polymers

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