yun sheng-step polymerization

8/3/2019 Yun Sheng-Step Polymerization

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Step Reaction Polymerization

P O L Y M E RAN INTRODUCTION

C H E M I S T R Y

Malcolm P. Stevens


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Distinguishing features of Chain- and Step Polymerizartion Mechanisms

Distinguishing features of Chain- and Step Polymerizartion Mechanisms

Step Polymerizations Chain Polymerizations

Introduction to Polymer Chemistry

Growth occurs only by addition of monomer to active chain

end.

Monomer is present throughout, but its concentration

decreases. Polymer begins to form immediately.

Chain growth is usually very rapid (second to microseconds).

MW and yield depend on mechanism details.

Only monomer and polymer are present during reaction.

Usually (but not always) polymer repeat unit has the sameatoms as had the monomer

Any two molecular species can react.

Monomer disappears early.

Polymer MW rises throughout.

Growth of chains is usually slow (minutes to days). Long reaction times increase MW, but yield of

polymer hardly changes.

All molecular species are present throughout.

Usually (but not always) polymer repeat unit has

fewer atoms than had the monomer.


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Condensation vs. Addition

Condensation vs. Addition


Carothers originally classified polymers based on a comparison of the atoms in the monomer to the atoms in the polymer

repeat unit.

Condensation polymers had fewer atoms in the repeat unit (i.e., some small molecule was emitted during

polymerization).

Addition polymers had the same atoms as their monomers.

Step polymerization by addition of alcohols to diisocyanates to form polyurethanes:

Chain polymerization (ring opening of heterocycle) with loss of CO2 to form polypeptide.


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A. Step-Reaction Polymerization - KineticsA. Step-Reaction Polymerization - Kinetics


Step-Reaction Polymerization


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A. Step-Reaction Polymerization - KineticsA. Step-Reaction Polymerization - Kinetics



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A. Kinetics of Step-Growth PolymerizationA. Kinetics of Step-Growth Polymerization



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B. Stoichiometric ImbalanceB. Stoichiometric Imbalance


Three ways to limit M. W. in step polymerization

These are polyethers that are processed to an oligomer stage and are subsequently

converted to network polymer by appropriate reactions of terminal epoxyide groups.

With polyimides for fiber applications, molecular weight must often be limited

because too high a viscosity is detrimental to extrusion of filaments through the

fine holes of a spinneret.


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C. Molecular Weight DistributionC. Molecular Weight Distribution



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14Introduction to Polymer Chemistry


Nx


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D. Network Step-Polymerization : Theory of GelationD. Network Step-Polymerization : Theory of Gelation


If monomers containing a functionality greater than two are used in step

polymerization, chain branching results.

If the reaction is carried to a high enough conversion,gelation occurs.

The onset of gelation, orgel point, is accompanied by a sudden increase in viscosity

such that the polymer undergoes an almost instantaneous change from a liquid to a

gel.


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D. Network Step PolymerizationD. Network Step Polymerization



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Branching point


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E. Step-Reaction CopolymerizationE. Step-Reaction Copolymerization



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E. Step-Reaction CopolymerizationE. Step-Reaction Copolymerization



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F. Step Polymerization TechniquesF. Step Polymerization Techniques



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G. Dendritic PolymersG. Dendritic Polymers



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Commerically Important Polymers Prepared by Step-Reaction PolymerizationCommerically Important Polymers Prepared by Step-Reaction Polymerization


Carbonyl addition-elimination

Polyesters, polycarbonates, polyamides,

polyimides...

Aromatic addition-elimination

Polysulfones, polysulfides,

polyetherketones

Carbonyl addition-condensation

Phenol-formaldehyde and related polymersPolymeric heterocycles

Addition to multiple bonds or epoxides

Polyurethanes

Epoxy polymers

Miscellaneous

Oxidative aromatic addition (polyphenylene

oxide)

Acyclic diene metathesis (ADMET)

Aryl-aryl coupling

Reductive coupling (polysilanes)

Hydrolysis coupling (silicones)

Diels-Alder cycloadditionBiradical coupling (polyxylylene)

Friedel-Crafts chemistry

SN2 reactions

and a host of others...


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Carbonyl Addition-Elimination Step Polymerization : I. PolyesterCarbonyl Addition-Elimination Step Polymerization : I. Polyester


Mechanism :

I. PolyesterSynthesis :

Structure-property relationships:


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Other commercially important polyester:I. PolyesterCarbonyl Addition-Elimination Step Polymerization : I. PolyesterCarbonyl Addition-Elimination Step Polymerization : I. PolyesterPBT

PEN

PET


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II. PolycarbonatesCarbonyl Addition-Elimination Step PolymerizationCarbonyl Addition-Elimination Step Polymerization

III. Polyamides


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IV. Polyimide Carbonyl Addition-Elimination Step PolymerizationCarbonyl Addition-Elimination Step Polymerization


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Aromatic Addition-Elimination PolymerizationAromatic Addition-Elimination Polymerization


Mechanism : This reaction is analogous to carbonyl addition-elimination, in that itis a two step process where the negative charge is accomodated by an

electron withdrawing group. To emphasize the simularity, this example

uses a ketone:

Krishnamurthy, S. J. Chem. Ed. 1982, 59, 543.

Monomers :Bisphenols are most often used as the nucleophillic components. The chemistry begins when

a base like NaOH or K2CO3 deprotonatea the bisphenol, as in this example for Bisphenol A:


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I. Poly(etheretherketone), PEEK

The most common form of PEEK is the one shown, derived from Bisphenol A. This polymer is a

remarkable material, highly crystalline, thermally stable, resistant to many chemicals, very tough. It

can be melt-processed at very high temperatures (>300 C), and is useful for special applications like

pipes in oil refineries and chemical plants, and parts for aerospace, where high price is not a

limitation.


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II. Polysulfone, PSF

Like polycarbonate, many other polysulfones could be synthesized, but the particular one shown here is

by far the most common commercially, so that the general term "polysulfone" usually refers to this

particular one. Worse, it is seldom called "poly(etherethersulfone)," despite its close structural similarity

to PEEK

Unlike PEEK, poly(etherethersulfone) is completely amorphous, probably a result of the relatively

large size of the sulfonyl group, and the kink in the polymer backbone caused by the narrow C-S-C

bond angle (close to 100). Therefore, it can be processed at lower temperature than PEEK, but the

material is not as resistant to heat and chemicals.


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Carbonyl Addition-Condensation PolymerizationCarbonyl Addition-Condensation Polymerization


III. Phenol-Formaldehyde Polymers IV. Polymeric Heterocycles


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Carbonyl Addition-Condensation PolymerizationCarbonyl Addition-Condensation Polymerization


The phenol-formaldehyde polymers are the oldest commercial synthetic polymers, first introduced around 100

years ago. Their inventor, Leo Bakeland, had no idea what was happening in his reaction kettles, but he was able to

work out conditions to produce a tough, light, rigid, chemically resistant solid from two inexpensive ingredients.

He soon became a rich man, in the same class as the famous industrialists of the time like Alfred Nobel, Henry Ford,

Andrew Carnegie, George Eastman, etc.

The actual chemistry is complicated, and still not competely understood. The polymers are usually thermosetting (i.e.,

crosslinked), and their insolubility limits the analytical techniques that can be brought to bear. The main reaction is

the production of methylene bridges between aromatic rings, as shown below. Many side reactions also occur, and

some of these give phenol-formaldehyde polymer its dark color.

Of course, these crosslinked polymers cannot be melted or dissolved, so their synthesis must be conducted in molds

for the actual product. In practice, the polymerization is usually carried out to somewhere below the gel point in aseparate reactor, and then the "pre-polymer" is transferred to the mold, where the reaction is completed.

Urea or melamine can be substituted for phenol. Methylene bridges can also be formed between the nitrogen atoms,

giving rise to chemical relatives of the phenol-formaldehyde polymers. The urea and melamine based materials have

much less color, and so are useful for decorative applications such as dinner plates and countertop materials

(FormicaTM).


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Addition to Multiple Bonds or EpoxidesAddition to Multiple Bonds or Epoxides


Mechanism :

The urethane linkage (often called carbamate) is usually made by adding an OH across the

C=N of an isocyanate.

The reaction is catalyzed by bases such as tertiary amines or by certain tin salts.

I. Polyurethanes

Many different polyurethanes have been synthesized, giving rise to materials with widely varying

properties. For example, rubbery polyurethanes are used for Spandex fiber and for seat cushions in

furniture and cars, while hard polyurethanes are used for wheels on roller skates, for bowling balls, and for

paints and varnishes. The hydrogen bonds between the NH and CO groups provide toughness to the

polymers.

Polyurethanes are synthesized by the reaction ofdiols with diisocyanates:


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II. Epoxy PolymersThese polymers are best known as two component thermosetting adhesives, although linear polymers

can be prepared. The term "epoxy" polymers is something of a misnomer, because the epoxy groups are in

the monomer, not in the polymer. To form the actual polymer, one reacts a multifunctional epoxide with amultifunctional nucleophile. Epoxy monomers based on Bisphenol A are by far the most common

substrates, although others can be used. The nucleophiles are most often amines or phenoxides. The

number of reactive functional groups on the components governs whether the polymer is linear or

crosslinked.

Epoxy Adhesive ChemistryThe resulting network will not dissolve in any solvents,

and resists all but the strongest chemical reagents.


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Other Epoxy Polymer

The plurality ofOH groups provides hydrogen

bonding, useful for adhesion to polar surfaces

like glass, wood, etc. Epoxy polymers are often

used to form composite structures filled withglass or carbon fiber.


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Thanks for your attention

yun sheng-step polymerization

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