Anionic Polymerization - Initiation and Propagation~~~~~~~CH2 - C: + CH2 = CHRest of Chain H -~~~~~~~CH2 - CRest of Chain H -- CH2 - CH:NaNH2 Na + NH2 + + CH2 = CHNH2H2N - 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:2Let’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 PolymerizationNa+ 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*StyrenePolymerizeR-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 PolymerizationAdd ButadieneButadieneThe Polymerization ContinuesSome 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 XRest 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 CatalystsCHRCH2CHRCH2TiCl ClClClCHRCH2CHRCH2TiCl ClClCl+ 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 OrbitalZiegler - Natta CatalystsCHRCH2CHRCH2TiCl ClClClCHRCH2TiCl ClClClCHRCH2 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.IsotacticAdditionCHRCH2CHRCH2TiCl ClClClCHR - CH2 - CHR - CH2ClTiCl ClClSyndiotacticAddition 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 formedVacant OrbitalChain 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 ratesCH2 = CH2 CF2 = CF2 CH2 = CH - CH =CH2 CH2 = C - CH =CH2 CH3 - CH2 = C - CH =CH2 Cl - MonomerChemical 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