Anionic Polymerization - Initiation and Propagation

~~~~~~~CH2 - C: + CH2 = CHRest of Chain H

- ~~~~~~~CH2 - CRest of Chain

H

-

- CH2 - CH:

NaNH2 Na + NH2 +

+ CH2 = CHNH2 H2N - CH2 - CH:

As in free radicalpolymerization, there areinitiation and propagationsteps.

Propagation proceeds in the usual manner, but there is no termination of the typethat occurs when free radicals collide. ( Why not?)

Anionic Polymerization - Chain Transfer to Solvent

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

- + NH3

~~~~~~~CH2 - CHRest of Chain H

- + NH2

If a solvent that is able to release aproton is used it can react with theactive site. Ammonia is an example ofsuch a protic solvent and the reactionresults in the formation of anegatively charged NH2 ion, which caninitiate the polymerization of a newchain. In other words, we have chaintransfer to solvent.

Anionic Living Polymerization

+ CH2 = CHNa

.CH2 - CH:

:CH - CH2 - CH2 - CH:

Na +

.CH2 - CH:2

Let’s consider the polymerization of styreneinitiated by metallic sodium in an “inert”solvent in which there are no contaminants(i.e. there are no molecules with activehydrogens around).

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

Anionic Living Polymerization

Na + Then if there is nothing for the anionto react with, there is no termination(combination with the counterionoccurs in only a few instances; the ionshang around one another and theirattractions are mediated by solvent)

This allows the synthesis of block copolymers. Because the active site staysalive, one can first polymerize styrene,for example:

A

A A

A A

A

A

A

A A A

A A

A A

A R*

A R* R* A

A A

A R-A-A-A* R-A-A-A-A*

R-A-A-A-A-A*

StyrenePolymerize

R-A-A-A*

R-A-A-A-A*

R-A-A-A-A-A* B

B B

B B B

B

B B B

B B B

B B

B B

R-A-A-A-B-B-B-B-B*

R-A-A-A-A-A-B-B-B*

R-A-A-A-A-B-B-B-B*

Anionic Living Polymerization

Add Butadiene

Butadiene

The Polymerization

Continues

Some Final Notes onAnionic Polymerization

There are a lot more interesting things about anionic polymerization - theeffect of polar groups, the fact that not all monomers can be used to makeblock copolymers, the ability to make certain polymers with very narrowmolecular weight distributions, and so on - but these topics are for moreadvanced treatments, so now we will turn our attention to cationicpolymerization .

Cationic Polymerization

~~~~~~~CH2 - C X -

Rest of Chain CH2 = CH

X -

H

- +

H A + CH2 = CH X -

+ CH3 - C X -

H

-

~~~~~~~CH2 - C X -

CH2 = CH X -

H

- +

+ A

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

CH2 - CH X -

+

As you by now have doubtlessanticipated, cationic polymerizationsinvolve an active site where there isa positive charge because, in effect,there is a deficit of one electron atthe active site.

Cationic polymerizationscan be initiated by protonicacids or Lewis acids (thelatter sometimes combinedwith certain halogens).

Propagation then proceeds in the usual way.

Cationic Polymerization -Termination and Chain Transfer

~~~~~~~CH2 - C X -

H

- + + CF3COO

~~~~~~~CH2 - CH - O - C - CF3

O - -

~~~~~~~CH2 - CH - OH + AH X -

~~~~~~~CH2 - C X -

H

-

+ H2O+ A

Unlike anionic polymerization,termination can occur by anion -cation recombination, for example, asillustrated opposite. Lots of otherside reactions can occur, with traceamounts of water, as illustratedbelow, chain transfer to monomer,and so on. This makes it much moredifficult to make a living polymerusing cationic polymerization.

Coordination Polymerization

~~~~~~~CH2 - CH

X

Rest of Chain

CH2 = CH X -

- Catalyst

Some reactions are best describedas coordination polymerizations,since they usually involve complexesformed between a transition metaland the π electrons of the monomer(many of these reactions are similarto anionic polymerizations and couldbe considered under that category).

These types of polymerizations usually lead to linear and stereo-regularchains and often use so-called Ziegler - Natta catalysts, various metal oxides,or, more recently, metallocene catalysts.

Ziegler - Natta Catalysts

CHR

CH2

CHR

CH2

Ti

Cl Cl

ClCl

CHR

CH2

CHR

CH2

Ti

Cl Cl

ClCl

+

Ziegler-Natta catalysts generally consist of a metal organic compoundinvolving a metal from groups I - III of the periodic table, such as triethylaluminium, and a transition metal compound (from groups IV - VIII), such astitanium tetrachloride. The metal organic compound acts as a weak anionicinitiator, first forming a complex whose nature is still open to debate.Polymerization proceeds by a process of insertion. The transition metal ion (Tiin this example) is connected to the end of the growing chain andsimultaneously coordinates the incoming monomer at a vacant orbital site. Twogeneral mechanisms have been proposed and for simplicity here we simplyillustrate the so -called monometallic mechanism ( the other is bimetallic)

Vacant Orbital

Ziegler - Natta Catalysts

CHR

CH2

CHR

CH2Ti

Cl Cl

ClCl

CHR

CH2

Ti

Cl Cl

ClCl

CHR

CH2

Isotactic placement can then occur if the coordinated monomer is insertedinto the chain in such a way that the growing chain remains attached to thetransition metal ion in the same position.

IsotacticAddition

CHR

CH2

CHR

CH2Ti

Cl Cl

ClCl

CHR - CH2 - CHR - CH2

Cl

Ti

Cl Cl

Cl

SyndiotacticAddition

Ziegler - Natta Catalysts Or, if the chain becomes attached to thetransition metal ion in the position of the orbitalthat was initially vacant, syndiotactic addition willoccur. This becomes more favoured at lowertemperatures, but vinyl monomers usually formisotactic chains with these catalysts. Because ofthe heterogeneous nature of the geometry of thecatalyst surface atactic and stereoblock polymerscan also be formed

Vacant Orbital

Chain Polymerization Methods and Monomer Type

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

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

X -

X -

X

-Active site

H -

H -

As you might guess, not all monomerscan be polymerized by a given chainpolymerization method. There is aselectivity involved that depends uponchemical structure (i.e. the inductive andresonance characteristics of the groupX in the vinyl monomer shown opposite).With the exception of α-olefins likepropylene, most monomers with C=Cdouble bonds can be polymerized freeradically, although at different rates

CH2 = CH2

CF2 = CF2

CH2 = CH - CH =CH2

CH2 = C - CH =CH2

CH3 -

CH2 = C - CH =CH2

Cl

-

Monomer Chemical Structure

Ethylene

Butadiene

Chloroprene

Isoprene

Styrene

Tetrafluoro -ethylene

CH2 = CH

CH2 = CH Cl

-

CH2 = CH OCOCH3 -

CH2 = C Cl

-

Cl

-

CH2 = C-CH3

COOCH3 -

Monomer Chemical Structure

Vinyl Chloride

Vinylidene Chloride

Vinyl Acetate

Methyl Methacrylate

Acrylonitrile

Some Monomers that can be Polymerized Free Radically

CH2 = CH CN

-

CH2 = CH X δ + δ −

Electron donating substituent CH2 = CH

CH2 = C CH3

-

CH3 -

CH2 = CH OCH3 -

Monomer Chemical Structure

Isobutylene

Styrene

Vinyl Methyl Ether

Monomers are much more selectivewith respect to ionic initiators.Electron donating substituents, suchas alkyl, alkoxy and phenyl groupsincrease the electron density on theC=C double bond

and facilitate cationic polymerization

~~~~~~~CH2 - C X -

Rest of Chain CH2 = CH

X -

H

- +

Some Monomers that can be Polymerized Cationically

Chain Polymerization Methods and Monomer Type

Monomers that can be PolymerizedAnionically

While substituents that are electronwithdrawing,

such as cyano, acid or ester, facilitateanionic polymerization

~~~~~~~CH2 - C: X -

Rest of Chain CH2 = CH

X -

H

-

CH2 = CH X δ + δ −

Electron withdrawing substituent

CH2 = CH - CH =CH2

Monomer Chemical Structure

Butadiene

Styrene CH2 = CH

CH2 = CH CN

-

CH2 = C-CH3

COOCH3 -

Methyl Methacrylate

Acrylonitrile

N

CO

H

O

CH2 - CH2

Caprolactam

Ethylene Oxide

~~~~~~~CH2 - CH

X

Rest of Chain

CH2 = CH X -

- Catalyst

Monomers that can be Polymerized using Ziegler - Natta Catalysts

Finally, Ziegler - Natta catalysts areused to polymerize a variety of a-olefins (e.g. ethylene and propylene)and styrene, but many polar monomerscannot be polymerized this way asthey inactivate the initiator, eitherthrough complexation or reaction withthe metal components

POLYMERIZATION PROCESSES

TWO USEFUL DISTINCTIONS ;

•BETWEEN BATCH AND CONTINUOUS

•AND BETWEEN SINGLE - PHASE AND MULTI - PHASE

SINGLE - PHASE

Bulk or Melt Polymerization

Solution Polymerization

BATCH VS. CONTINUOUS -

Depends on polymerization time ie kinetics - coming up next!

SINGLE - PHASE

Bulk or Melt Polymerization

Solution Polymerization

MULTI - PHASE

Gas / Solid Liquid / Solid

Suspension

Emulsion

Etc

Rapid Stirring

Suspended Beads of Monomer + Initiator

Water

Rapid Stirring

Suspended Beads of Polymer

Polymerization

Schematic representation of suspension polymerization.

Polymer Processes—Free Radical Suspension Polymerization

R.R.

Water Soluble Initiator

Spherical Micelle

Micelle Swollen with Monomer

Monomer Droplet Stabilized by Surfactant

Surfactant Molecule

Hydrophilic (water loving)

Head

Hydrophobic (water hating)

Tail

Water

Schematic representation of the initial stages of an emulsion polymerization.

Polymer Processes—Free Radical Emulsion Polymerization