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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