ORGANIC AND PHYSICAL CHEMISTRY OF POLYMERS phần 6 doc

So, the param-eters that control the structure of the active centers responsible for the anionicpolymerization of ethylenic monomers are: • The nature of the substituents carried by the

one The increase in volume (µ = dV /dt) that results drives the surfactant of the

empty micelles toward the external envelope of the particles; micelles thus pear from the medium by becoming particles or by supplying surfactant molecules

disap-to the already formed particles As soon as all the micelles have been used up by one

of the mechanisms mentioned above (first period in Figure 8.10)—corresponding

to approximately 15% monomer conversion—the number of particles (N p) can beconsidered constant until the end of the polymerization The rate of polymerizationduring this second period in Figure 8.10 can be expressed by the relation

R p = k p[M]part

N p

2Thus it will be constant up to 70–80% conversion Assuming that free radicals are

generated at constant rate (d [RM•]/dt= ρ = const)) and that all of them serve to

create particles, at the time t1 corresponding to the total disappearance of micelles,

N p can be written as

N p = ρt1

At t1, a particle created at t0 will exhibit the volume

V (t1, t0) = µ(t1− t0)

with its volume at t0 (when it was a micelle) being negligible

The surface of its external envelope can be easily deduced from its volume:

Figure 8.10 Kinetics of the monomer conversion for an emulsion polymerization in a closed

batch reactor.

Because the number of particles generated for the period of time dt is ρdt, the total

external surface at time t1 can be written

A t1=

 t1

a(t1, t0) ρ dt = (36π) 1/3 0.6ρµ2/3 t15/3

Because this total surface can be directly related to the concentration [S] of the

surfactant and to its molar surface (a s),

A t1= a s[S]

one obtains the following for the expression of N p:

N p = 0.53

ρµ

0.4

a s[S]0.6

The Smith– Ewart model describes satisfactorily the polymerization of styrene,isoprene, and methyl methacrylate; for these systems, it can be used to predict thesize of the latex particles and the corresponding molar masses In contrast, it isunsuited for the case of monomers partially water-soluble or polymers insoluble

in their monomer— that is, polymerization of vinyl chloride and vinyl acetate Itaccounts neither for the fact that styrene can be polymerized in absence of surfactantnor for the fact that free radicals (RM•) can equally penetrate into a micelle or in

an already formed particle during the initial phase

Fitch has thus proposed another model which considers that initiation and theearly stages of the propagation occur in the aqueous phase, with the chains precip-itating only when a critical size is reached—that is, for degree of polymerization

of a few units to a few tens depending upon the hydrosolubility of the oligomerformed

8.6 ANIONIC POLYMERIZATION

This type of polymerization is a very old one, used at the beginning of the twentiethcentury in Germany to produce a well-known synthetic rubber named “Buna”.However, it is only in the middle of the 1950s that anionic polymerization tookall its importance when Szwarc shed a new light on this field and discovered that

it can be carried out in the absence of any transfer and termination Szwarc calledsuch polymerizations “living” (see Section 8.4), and his discovery triggered anintense research activity that culminated in the synthesis of unprecedented complexmacromolecular architectures (block copolymers, stars, etc.)

8.6.1 General Characteristics

The anionic polymerization is a chain reaction that can be schematized by

~~~~Mn−, Met+ + M ~~~~M−

n+ 1, Met+

n represents a negatively charged or polarized species carried bythe growing chain, and Met+ is a positive counterion (or a polarized species),generally a metallic cation Whatever the precise mechanism involved in this type

of polymerization, it proceeds via repeated nucleophilic reactions In the case of

vinyl and related monomers, for the propagation to occur by nucleophilic

addi-tion, an activation of the monomer double bond is generally required (see, however,

“Remark,” page 312) Electron-withdrawing substituents (–CO–OR, –CN, etc.) orthose inducing a strongly positive polarization of theβ-carbon atom of the double

bond, when neared by a nucleophilic active species,

N

fulfill this condition

Anionic polymerization also applies to heterocyclic monomers In this case,

it can occur either by nucleophilic substitution or by nucleophilic addition onto

a carbonyl group followed by an elimination (mechanism BAC2), and so on Anegative enthalpy of polymerization is a necessary condition for the monomer

to be polymerized, and thus heterocyclic monomers must be strained enough toundergo ring-opening and polymerization Another constraint of prime importancethat affects the polymerizability of monomers —in particular, that of ethylenicones —is the extreme reactivity of species that propagate the process In a firstapproach—and without mistaking between the notions of nucleophilicity andbasicity—a carbanionic species can be considered as the conjugate base of a pro-

tonic acid whose pK a can be evaluated Thus, the species formed in the anionicpolymerization of styrene

Anionic polymerization is utilized only when the “living” character of the chaingrowth can be ensured In addition, initiators are selected for their ability to give a

complete initiation (f ∼ 1) and a short period of initiation compared to that of

prop-agation, allowing a controlled polymerization to occur This situation is exploited

in macromolecular engineering to synthesize polymeric chains with well-definedstructure and narrow molar mass distribution.

8.6.2 Structure of the Propagating Species

The “living character” of the growing species formed in carbanionic polymerizationoffers an opportunity to study comprehensively their structure The concentration

of the reactive centers being always extremely low in the polymerization medium,

it is easier to carry out such structural studies on simple organometallic models ofthe “living” ends Some of these are used to initiate the polymerization and theknowledge of the parameters that determine their reactivity is interesting by itself.There is a close relationship between the structure of organometallic species(∼∼M−

n, Met+) and their reactivity In the case of species responsible for thepolymerization of ethylenic monomers, their nucleophilicity and thus their reactivityare strongly determined by the electron density on the carbanionic site

A

~~~CH2 HCd−, Metd +

This electron density depends on the polarization of the C –Met bond and the

possible delocalization of the negative charge on the substituent A So, the

param-eters that control the structure of the active centers responsible for the anionicpolymerization of ethylenic monomers are:

• The nature of the substituent(s) carried by the double bond,

• The nature of the counterion associated with the carbanionic species,

• The nature of the solvent in which the reaction is carried out and the presence

of possible additives

8.6.2.1 Effect of the Substituent A If the substituent promotes a

delocaliza-tion of the negative charge [as is the case for styrene, vinylpyridines, lates, etc.], it entails a decrease of the intrinsic reactivity of the carbanionic species.Thus, in the case of acrylates, the active center is an enolate of rather low reactivity:

(meth)acry-CH2

CH3

CHOO

−, Met+

~~~~

The intrinsic reactivity of carbanionic active centers is increased by the presence ofelectron-donating substituents and is conversely decreased by that of electron-with-drawing ones However, in the case of acrylates, the monomer double bond is moreactivated by the electron-withdrawing character of its substituent than the reactivity

of the corresponding enolate is lowered by the same substituent; this explains the

very high anionic polymerizability of these monomers Thus, the intrinsic reactivity

of the monomer determines the global reactivity of the system—that is, its merizability For example, methacrylic monomers (methyl methacrylate is shownhereafter) are characterized by a lower polymerizability than that of acrylates, inspite of the electron-donating effect of their methyl group presumed to increase theelectron density on the active center and thus its reactivity; as a matter of fact, this–CH3 group inα-position prevents (by its donor effect) a full polarization of the

poly-double bond and thus decreases the monomer reactivity

OO

CH3

CH3Methyl methacrylate

Styrene and butadiene are the two reference monomers in anionic tion Their high polymerizability is primarily due to the virtue of their double bonds

polymeriza-to undergo a positive polarization and an electron shift polymeriza-toward their substituentwhen neared by a negatively charged active center

Remark. Ethylene is a monomer with no possibility of activation of itsdouble bond However, it can be polymerized by nucleophilic addition but itsanionic polymerizability is very low, the absence of any stabilizing substituentnext to the carbanionic site making the latter particularly reactive

8.6.2.2 Effect of the Nature of the counterion Examples of

polymeriza-tions that can be carried out with nonmetallic counter-ions (quaternary ammonium,phosphonium ions, etc.) are scarce, the vast majority of them requiring the use ofalkali or alkaline-earth cations

Lithium and magnesium cations exhibit a small ionic radius which explains thepartial covalent character of their bond with carbon atoms in nonpolar solvents,provided that the carbanion is not too delocalized

With cations of higher ionic radius, the interionic distance favors the separation

of charges, and thus the corresponding species can be considered totally ionized

In polar solvating media as well as in the presence of solvating additives, theionic radius of the counterion affects its capacity to be solvated

Large cations like cesium can by no means be solvated even by solvents knownfor their strong solvating power

Lithium is by far the most used counterion known; this is primarily due to thepractical and synthetic ease that is associated with the utilization of butyllithium

as initiator, but also to the virtue of this cation to generate different configurationalstructures in the polymers formed Indeed, lithium cations can generate either par-tially covalent or totally ionic species with different regio- and stereospecificity,depending upon the solvent in which it is dispersed

8.6.2.3 Effect of the Nature of the Solvent and that of Potential additives Because of the very high reactivity of anionic reactive species, the

solvents used in anionic polymerization should not exhibit any acidic character;thus basic or neutral solvents are generally chosen

The functions of a solvent are manifold and, depending upon its structure, it canfulfill one, two or three of these functions

The first function is that of a diluent; the simultaneous generation of carbanionic

initiating/propagating sites and the monomer consumption by the latter can liberate

a considerable heat in the reaction medium that can be better removed if a solvent

is present Solvents used as diluents are always aliphatic or aromatic hydrocarbons;they do not modify or only to a little extent the structure of active centers.Organolithium compounds are aggregated species whose degrees of aggregationvary with the nature of the carbanion and sometimes with the range of concentration.For instance, polystyryllithium ion pairs are aggregated as dimers like shown below:

In the latter case, only non-aggregated species —in equilibrium with aggregatedones —are reactive and contribute to the propagation:

ActiveNon active

2 Kag

~~~~PS−, Li+ 2 ~~~~PS−, Li+

The second potential function of a solvent is that of a solvating agent Solvents

used for that purpose are ethers or tertiary amines whose basic character—according

to Lewis definition— entails a coordination to the Lewis acids that are the metalcations associated with the nucleophilic species This role of solvating agent canalso be played by additives (crown-ether, cryptands, tertiary diamines, etc.) used insmall amount in a hydrocarbon serving as diluent Depending upon their geometry

or their concentration, such additives can either solvate externally the ion pairs [seehereafter the case of polybutadienyllithium in the presence of tetramethylethylene-diamine (TMEDA)],

or cause a stretching of the carbon–metal bond (see hereafter the case of lithium solvated by a crown-ether):

When the dielectric constant (permittivity) of the solvent is sufficiently high, it

can play the role of dissociating agent Such a solvent can then cause the charges to

separate more markedly and induce a partial dissociation of ion pairs into free ions,

Kdiss

~~~~~Mn−, S

n− + Sx,Met+

where Sx corresponds to X molecules of solvent coordinating to the metal cation

The relation between the dissociation equilibrium constant (Kdiss) and the mittivity of the reaction medium (ε) can be written as

per-− ln Kdiss= − ln K0

diss+ e2

(r1+ r2) εkT

where K0

diss is the constant of dissociation of ion pairs in a medium of infinite

permittivity, r1 and r2 are the ionic radius of cation and anion, respectively, and

e is the electron charge This relation shows that by increasing the apparent ionic

radius of the cation and that of the interionic distance, the solvating effect favorsthe dissociation; most of high permittivity solvents exhibit also a strong solvatingpower The reactivity of free ions resulting from the dissociation of ion pairs

is extremely high and, even at relatively low concentration, they have a majorimpact on the global kinetics of polymerization In contrast to the case of freeradical polymerization, the same monomer can generate various propagating speciesdepending upon the nature of the initiator and that of the surrounding medium.The various reactive species are ranked hereafter in the increasing order of theirreactivity,

8.6.3 Initiation Step

The initiator has to be selected with care so as to ensure a short initiation step(compared to that of propagation) and the absence of side reactions The preferencemust go to initiators that are more nucleophilic than the active species resultingfrom their addition onto a monomer molecule

Two types of reaction can be utilized to generate primary active centers.The first one resorts to an electron transfer from a metal atom (generally analkali metal) to a molecule whose electron affinity is sufficiently high The role

of the electrophilic entity can be played by the monomer molecule and, in this

case, the transfer of ns electrons from the alkali metal results in the formation of

a radical-anion based on the monomer molecule:

Met+Met +

,Radical-anion

HC−A

, Met+Met+, −HC

AA

This direct initiation is rarely utilized because the formation of such a radical-anionthrough the reaction between a solid (metal) and a liquid (monomer) is generallyslow To overcome this limitation, an organic intermediate that cannot polymerizeitself but can accommodate electrons by transfer is generally utilized More often,these intermediates are polycyclic aromatic hydrocarbons; for example, naphthalene

is commonly used for this purpose; the reaction between naphthalene (in solution)and sodium (solid) is schematized hereafter:

Na+ naphthalene −−− −−−•( naphthalene)−,Na+The reaction must be carried out in a sufficiently solvating solvent (tetrahydrofuran,dimethoxyethane, etc.) for the electron transfer to occur, and, after elimination of

the metal in excess, a homogeneous and quasi-instantaneous initiation step can beobtained upon addition of monomer:

Remark Since two molecules of initiator lead to the formation of a

sin-gle chain, the relationship giving the degree of polymerization as a function

of the conversion must be modified In the case of a monofunctional

initia-tion, the relation is X n= [Mpol]/[I], whereas for a difunctional initiation we obtain X n= 2[Mpol]/[I] [M pol], representing the concentration of monomerpolymerized

More usually—and in particular in industry—initiation is obtained by the

means of strongly nucleophilic Lewis bases They are usually monofunctional and

monovalent organometallic species; compounds like benzylsodium or propylpotassium (cumylpotassium) may be utilized in research laboratories, but

phenyliso-in phenyliso-industry it is exclusively the isomers of butyllithium (n-, sec-, tert -) that are employed They are strongly aggregated in hydrocarbon media (n-BuLi is hex- americ, tert -BuLi is tetrameric, etc.) and they react only under their “unimeric”

(nonaggregate) form, the latter being in equilibrium with aggregates:

of vinyl and related monomers In this way it is possible to limit side reactions andthus to preserve the “living” character of the polymerization:

In hydrocarbon solvents, active centers have a strong tendency to be aggregated;

as previously seen, it is the case for the polymerization of styrene initiated by anorganolithium compound, in bulk or in a hydrocarbon solvent:

(~~~~~~PS−,Li+)

2 Kag 2 ~~~~~~PS−,Li+

Because only nonaggregated species are active, the kinetic equation for the agation step can be easily established:

prop-Kag = [(~~~PS−,Li+)

2]/[~~~PS−,Li+)]2[~~~~PS−,Li+] = {[(~~~~PS−,Li+)

2]/Kag}1/2

R p = −d[S]/dt = k p,app[~~~~PS−,Li+][S] = k

p{[(~~~~PS−,Li+)

2]/Kag}1/2[S]

where k p,app is the apparent rate constant of propagation and k pis the rate “constant”

of propagation, both of which vary with the concentration in potentially activecenters —that is, with [Li]

Because the equilibrium constant of aggregation (Kag) is very high, it cannot bemeasured generally so that the above equation can be simplified as

R p = k p[Li]1/n[S]

where n is the degree of aggregation.

Remark In the anionic polymerization of dienes, the degree of aggregation

varies with the concentration in organometallic species, which complicatesthe kinetic treatment

The addition of solvating agents in the reaction system causes a total disaggregation,without appreciably modifying the permittivity of the medium All organometallicspecies are then active and the kinetics becomes first order in active centers:

Indeed, an increase in the interionic distance decreases the electrostatic tion between the two electric charges and favors insertion of the monomer If thesolvent exhibits both a solvating power and a dissociating capacity, the two effectsplay a role For instance, in tetrahydrofuran (THF), whose permittivity is equal to7.8 at 20◦C, the solvation of ion pairs generates “loose” ion pairs, with each cationbeing surrounded by several molecules of THF Ion pairs are thus in equilibrium

interac-Table 8.15 Rate constants of propagation of styrene for

various alkali counterions (solvent: dioxane, T= 25 ◦

If i families of reactive species are simultaneously present in the reaction medium,

each one contributes through its own propagation kinetics:

Equilibrium constants of dissociation can be measured by conductometry fromsolutions containing different concentrations in organometallic species; they varywith the charge density of the anion, the interionic distance, and the permittivity of

reaction medium and are generally very low [C−∗] can thus be considered negligible

as compared to [C*] and [C∗±] can be assimilated to [C*], which means that

[C∗−]=
Kdiss[C∗]1/2

This leads to

R p = −d[M]/dt = kapp[C∗][M]=
k p±[C∗]+ k p−Kdiss1/2[C∗]1/2

[M]which can be written as

R p=
k p± + k p− Kdiss1/2[C∗]−1/2

kapp

[C∗][M]

The above relation shows that kapp varies with [C*]; as for k p± and k p− they can

be determined from the kapp versus [C*] plot (Figure 8.11) if Kdiss is known As

mentioned above, Kdiss can be measured under given experimental conditions byconductimetry on active solutions For instance, the rate constant of propagation

of ion pairs, the rate constant of free ions, and the constant of dissociation forpolystyrylsodium (PS−,Na+) in tetrahydrofuran solution at 25◦C are

k p±= 80 L·mol−1·s−1

k p−= 65,000 L·mol−1·s−1

Kdiss= 1.5 × 10−7mol·L−1

Thus, the proportion of free ions can be deduced from these values: for [C∗]∼10−4

mol·L, free ions represent only about 4%; in spite of that and because of their very

high reactivity, free ions contribute to an extent of∼97% to propagation

(a) The value of k p± measured in THF is different from that measured in

dioxane (Table 8.15) Indeed, in this last solvent, ion pairs are externallysolvated, which modifies only very little the interionic distance, whereas

in THF, ion pairs not only can be externally solvated but can partiallyalso be stretched (“loose” ion pairs) under the effect of the solvent andthese are more reactive

(b) Addition of homoionic species in the reaction medium by using solubleand highly dissociated salts causes a retrogradation of the dissociationequilibrium of reactive ion pairs; it entails a deceleration of the propaga-tion step By combining the results of the kinetic study and the value ofthe added homoionic salt dissociation constant, it is possible to calculate

the propagation rate constant of free ions (k p−).

The stereochemistry of the propagation step closely depends on the polarity of

the solvent and on the nature of the counterion For polymerizations involving free

ions, the sp2 hybridization of carbanionic species prevents any marked ulation of the propagation step and the resulting polymers are atactic In nonpolar

stereoreg-solvents, the carbanionic species exhibit generally an sp3 hybridization; with Li+and Mg++ as counterions, (meth)acrylic and similar monomers (2-vinylpyridine,etc.) polymerize under stereoregulating conditions through the combined effect ofmonomer coordination and steric hindrance High contents in either isotactic or

syndiotactic triads (mm or rr < 0.90) can be obtained.

As for the propagation step of heterocyclic monomers (oxiranes, thiiranes,

lac-tones, lactams, etc.), the general phenomena are very similar to those observed withvinyl and related monomers Although propagating species differ by the nature ofthe nucleophilic entities (oxoanions, thioanions, nitranions, etc.) involved, the cor-responding ion pairs can also be prone to aggregation, solvation, and dissociation,depending upon the nature of the solvent or that of additives introduced into thereaction medium Since the propagating species in the polymerization of heterocy-cles are much less reactive than pure carbanions, it is often necessary to activatethem in order to bring about sufficiently high rates of polymerization In general,the kinetics is complex because of the aggregation of active centers independently

of the reaction media, and the degrees of aggregation vary with their concentration.Moreover, the energies of activation of the propagation reaction are appreciablydifferent for ion pairs and free ions; depending upon the temperature, the contri-bution of the various active species to the propagation can vary in a large extentwith respect to the kinetics

As for the nature of the propagating active centers, the polymerization of ranes (epoxides) and ofε-caprolactone occurs through alkoxides, that of thiiranes

oxi-(episulfides) occurs through thiolates, and that of strained lactones occurs throughcarboxylates The corresponding reaction mechanisms are well-established (nucle-ophilic substitution, nucleophilic addition on carbonyl, etc.) For example, as estab-lished in the polymerization of ethylene oxide initiated by a potassium derivative,the propagation occurs by the following mechanism (nucleophilic substitution):

nucle-bonyl group; such a mechanism is called activated monomer polymerization, and

it is an unusual process in chain polymerization:

CNH

O

O

CO

poly-Anionic polymerization of N -carboxyanhydrides (Leuch’s anhydrides —NCA)

affords polypeptides; it proceeds by nucleophilic addition onto the carbonyl tion, followed by an elimination releasing CO :

CO2

CO2_

Relatively weak bases (primary or secondary amines, alkoxides, etc.) are used toinitiate the polymerization of such monomers

8.6.5 Anionic Copolymerization

In this section, only “statistical” copolymerizations will be considered

Differences in the reactivity of the growing species are more pronounced inanionic polymerization than in free radical polymerization In particular, alkoxides,thiolates, carboxylates, and so on, generally do not initiate the polymerization of(and do not copolymerize with) vinyl monomers Even in the latter family, thedifferences between the reactivity of active centers are such that only a very few

of them give “statistical” copolymers Among those, the styrene/butadiene system

is the best known, being industrially produced (in solution) under the trade name

of SBR

The existence of active centers under the form of various structures for eachcomonomer, with each of these structures having its own reactivity, complicatesthe kinetic treatment of such copolymerizations in comparison to the case of free

radical polymerization The reactivity ratios formalism can be utilized, but only

apparent values of rate constants, valid only under specific experimental conditions,can be obtained It is thus unrealistic to discuss the meaning of these values

As a matter of fact, anionic copolymerization is essentially utilized for thepreparation of block copolymers (see Section 9.2); in general, one operates bysequential addition of the comonomers in the order of increasing electroaffinity

8.6.6 Termination Reactions

Being mainly known and utilized for its “living” character, it may appear at firstglance misleading to mention the existence of termination reactions in the anionicpolymerization of vinyl and related monomers They indeed occur, and the condi-tions have to be found when necessary to minimize them so as to obtain the control

of the polymerization Otherwise, these termination reactions can also be exploitedfor the purpose of functionalization of the chain ends.

8.6.6.1 Spontaneous Termination Reaction They mainly depend on the

molecular structure of the active centers considered With polystyryl sodium intetrahydrofuran solution, a hydrideβ-elimination is observed in a first step:

H

~~~~

H

CH CC

∼∼∼∼MMA−,Li+ active chains is mainly an addition reaction onto the carbonyl

groups of the antepenultimate units by the growing enolates, followed by an ination reaction:

~~~~~~

n

−

Because the resulting alkoxide is unable to add onto the double bond of MMA, thenet result is a termination of the chain growth process.

A way to limit these reactions consists of replacing Li+ by a bulkier cation(quaternary ammonium, phosphonium, etc.) or by adding in the reaction medium,

a solvating agent that increases the apparent radius of the lithium ion; the attack ofcarbonyl groups by ion pairs can thus be thwarted, the probability of terminationreduced, and the control of polymerization improved in this way

8.6.6.2 Reaction with Termination Reagents Because of their very high

reactivity, carbanionic species react with many compounds —in particular, thoseexhibiting an acidic character:

∼∼∼Mn−,Met++ A–H −→ ∼∼∼MnH+ A−,Met+

Several atmospheric components can be utilized as terminating reagents:

8.6.7 Group Transfer Polymerization

It is now widely admitted that group transfer polymerization, which was unveiled in

1983 by a team from DuPont de Nemours, belongs to the category of anionic merization It applies to (meth)acrylic monomers whose Li-based anionic polymer-ization suffers from the termination by attack onto carbonyl groups as previouslyshown

poly-Group transfer polymerization of these monomers exhibits a “living” character

at ambient temperature and under normal experimental conditions

The initiator is a silylated acetal of dimethylketene trimethylsiloxypropene, indicated by TMS) which is active only in the presence

(1-methoxy-2-methyl-1-of a “catalyst.” The reaction pathway is represented hereafter:

n

C

CH3(H)H

H2C C

CH3(H)COOR2

The degree of polymerization obtained is determined by the molar ratio of[monomer] to the [initiator (TMS)], with the “catalyst” concentration determin-ing the rate of polymerization The “catalyst” can be a nucleophilic entity, with thebest effects being obtained with fluoride (F−) or bifluoride (HF2 −) anions derived

from salts soluble in the reaction medium, such as tris(dimethylamino)sulfoniumbifluoride and tetrabutylammonium fluoride These “catalysts” are particularly wellsuited to the polymerization of methacrylic monomers

Lewis acids such as zinc halides or a dialkylaluminium chloride (AlR2Cl) arepreferentially used to catalyze the polymerization of acrylics

When strongly nucleophilic entities are utilized as catalysts, the active centershave been identified as enolates (as with alkali counterions) Due to the nature ofcounterions —in particular, their size —their reactivity is strongly reduced com-pared to that of enolates associated with lithium; moreover, they are only present

in low concentration, the major part of the active species being in a “dormant” lacetal form in fast exchange with the reactive enolates The mechanism occurring

sily-in such polymerizations can be represented as below:

CO

R1O+ F

H3C CH3+ H2C

CH3 CH3

−

Remark. The mechanism of initiation reaction reveals the consumption

of the “catalyst.” It does not function as a true catalyst but rather like a

“co-initiator” whose presence is essential for the activation of the initiator

8.6.7.2 Propagation It must be stressed here that the propagation reaction

which is represented hereafter occurs through monomer addition by the carbonform of the enolate whereas the exchange of trimethylsilyl groups between dor-mant and reactive chains resorts to the oxygen form (due to “oxophilicity” of siliconatom)

The mechanism shown below is described as “dissociative” because the reactivespecies are fully ionized With weak nucleophilic catalysts, an “associative” mech-anism was proposed implying a pentacoordination of the silicon atom (discussed)

and mainly covalent active species.

accu-reflects the high efficiency (f ) of the initiators employed.

Such a good definition of the molecular dimensions associated with the tence of the active centers was extensively applied for the purpose of so-calledmacromolecular engineering, to design and construct precision macromoleculararchitectures An account of the various possibilities is described in Chapter 9,including those based on other “living” polymerizations

persis-8.6.9 Techniques of Anionic Polymerization

Due to the generation of the totality of active centers at the onset polymerizationand the very high polymerizability of vinyl and related monomers, there is no option

but to carry out anionic polymerizations in solution Generally, the solvents usedare hydrocarbons (aromatic or aliphatic) acting as diluents At a smaller scale inlaboratories, ethers (tetrahydrofuran, dioxane, dimethoxyethane, etc.) are sometimesused, for their solvating and dissociating effects in addition to their role as adiluent.

The extreme sensitivity of (carb)anionic active centers toward electrophilic rities, along with their utilization in low concentration to obtain high molar masses,requires a thorough purification of all reagents Initially, this purification stepappeared to be a limitation to the industrial development of anionic polymerization;now, it is not anymore the case as shown by the increasing number of industrialapplications of anionic polymerization

impu-Studies carried out recently on the control of the reactivity of propagatingspecies indicate that solvent-free processes may well be developed in the nearfuture

8.7 CATIONIC POLYMERIZATION

Cationic polymerization has witnessed an intense development in the middle ofthe twentieth century after it could be successfully applied to polymerize certainethylenic hydrocarbons such as isobutene, carbonyl monomers such as formalde-hyde, or cyclic ethers such as oxiranes, tetrahydrofuran, and cyclosiloxanes.Because of the very high reactivity of the cationic propagating species —in par-ticular, with ethylenic monomers —the polymerization systems that are commonlyused in industry often entail side reactions and frequent structural irregularities inthe polymers formed

The discovery of compositional and experimental conditions affording “living”cationic polymerizations has attracted much interest in particular because some

of unsaturated and heterocyclic monomers concerned can only be polymerized bycationic means

In this equation, ∼∼∼M+

n represents a positively charged (or polarized) speciescarried by growing chains, and A− represents a negative counterion (or a nega-tively polarized species) ensuring the neutralization of the positive charge With

ethylenic monomers, the propagation reaction is an electrophilic addition onto the

polymerizable double bond; the first step is the coordination of the double bond

onto the carbocationic site:

This reaction is all the more facile as the nucleophilic character of the monomer ispronounced: electron-donating substituents increase the cationic polymerizability,and in turn the intrinsic reactivity of the carbocationic site formed is reduced bythe effect of such substituents Thus, as in anionic polymerization, an increase

of reactivity of the monomer has more influence on its polymerizability than adecrease in reactivity of the corresponding active center Because of the strongLewis acid character of the active species, for a monomer to be polymerized,strongly nucleophilic sites must not be present

The mechanism is similar in the case of carbonylated monomers or n-donor

heteronuclear double bonds with an attack by the cationic active center onto the

oxygen atom of the carbonyl group:

Cationic polymerization is also utilized with heterocyclic monomers In this case,

disregarding the thermodynamic constraints, polymerization proceeds by ophilic attack of the hetero-element of a monomer molecule on the electron-deficient

nucle-α-carbon atom of the onium ion:

Oxiranes Oxetanes Other ethers and acetals

O

A

OA

but also aziridines, thiiranes, siloxanes, phosphazenes, and so on, all monomerswhose polymerization leads to polymeric materials with various molecular struc-tures and thus of different physical properties.

Most of the concepts concerning the structure of active species —aggregation,ionization, solvation, dissociation—which were described in the section on anionicpolymerization, apply to cationic polymerizations; in particular, the more pro-nounced the ionic character of the species and the longer the interionic distance,the more prominent the reactivity of active centers

Solvents that can be utilized in cationic polymerization must be inert with respect

to strongly electrophilic active sites They can play the role of diluent (aliphatichydrocarbons) and that of solvating agents for electrophilic species (nitroparaf-fins) and/or of dissociating medium of ion pairs into free ions (dichloromethane:

εCH2Cl2= 8.93 at 25◦C)

8.7.2 Initiation of Cationic Polymerizations

Numerous are the initiators that can be used in cationic polymerization, themonomer polymerizability determining their choice

8.7.2.1 Protonic Acids (Br ¨onsted Acids):

A−,H+−→ A−+ H+

These acids are all the more efficient as they are dissociated in the reaction medium

More important than the pK a in aqueous solution (of little interest), Table 8.16

gives the pKa values of various protonic acids in acetic acid and acetonitrile Itcan be noticed that acids which are reputed strong in aqueous solution are notdissociated in organic media The most used Br¨onsted acids to initiate cationicpolymerizations are:

Trifluoromethylsulfonic (triflic) acid H–SO3–CF3

Depending upon the nature of the solvent used and, in particular, its basicity whichrepresents its aptitude to trap protons, these initiators will be themselves more orless good proton donors Among all the systems shown in Table 8.16, perchloricacid in acetonitrile solution is the best one

Certain protonic acids add easily onto the monomer double bond, but whenthe associated counterion is more nucleophilic than the monomer, they form acovalent bond unable to propagate the reaction Such a situation often occurs withhydracids:

HCl+ H2C=CHR −→ H–CH2–HRC+,Cl−−→ H3C–HRC–Cl

Table 8.16 pK avalues for some protonic acids in two

different organic solvents

HI + n

NR

, I

+ −

N CH2 CH2R

H

n− 1NR

but can only protonate oxiranes:

HI

OR

The protonic initiators that are the most used to polymerize heterocycles are oromethylsulfonic (“triflic”) and fluorosulfonic acids

triflu-The kinetics of initiation of the polymerization of ethylenic monomers by tonic acids

pro-A−, H++ HC CHR H3C–HRC+, A−

generally exhibits a first-order variation (expected) with respect to monomer and asecond-order variation with respect to protonic acid

This phenomenon is accounted for by a mechanism involving two acid molecules

in the transition state which corresponds, for the case of HCl, to

H Cl

H Cl

The higher the rate constant of addition of acid molecule onto the monomer double

bond (k i), the greater the nucleophilicity of the monomer Thus, the rate constant ofinitiation by trifluoromethylsulfonic acid at 0◦C in dichloromethane (CH2Cl2) varies

from k i= 10 L·mol−1·s−1 for styrene, to k

i= 103L·mol−1·s−1for α-methylstyrene

and k i= 5 × 104·L mol−1·s−1 for p-methoxystyrene; these three monomers are

ranked in the order of increasing nucleophilicity

8.7.2.2 Lewis Acids BF3, AlCl3, TiCl4, SnCl4, and SbCl5 are the most erally used Lewis acids In a few cases, it was shown that these Lewis acids caninitiate polymerizations by themselves For example, aluminum halides self-ionizefrom (generally) dimeric aggregates:

gen-2AlCl3−−−→←−−− (AlCl3)2−−−→←−−− AlCl2+,AlCl

cannot be polymerized for steric reasons, undergoes dimerization in the presence

of SbCl5 according to the mechanism shown hereafter:

SbCl5 ,+

3

from which propagation occurs

However, Lewis acids are more often active in the presence of either a weak acid

or a cationizing agent (co-initiator) The reaction between the two components of

the initiating system yields an extremely strong acid complex, the actual initiator

is either the proton donor or the cationizing agent, and the Lewis acid serves

as activator The most common proton donors are water, alcohols, amines, andamides For example, TiCl4/H2O system is formed according to the scheme shownhereafter:

TiCl4+ H2O−→ TiCl4OH−,H+

TiCl4OH−,H++ H2C=CHR −→ H3C–HRC+,TiCl4OH−

There are many other systems that are excellent initiators for both unsaturatedand heterocyclic monomers Several reactions are shown below to demonstratesome of the possibilities offered by these systems With a metal halide such asMetXn serving as Lewis acid, the various possibilities to generate a carbocationicinitiator are:

MetXn+ RX −−−→←−−− R+,MetX

n+1−

MetXn+ R–OR−−−→←−−− R+,MetXnOR−

MetXn+ R–O–CO–R−−−→←−−− R+,MetX

nO–CO–R−

MetXn+ R–O–SO2R−−−→←−−− R+,MetX

nO–SO2R−

Then, polymerization occurs by an electrophilic attack of the carbocation R+ ontoeither the double bond or the heteroatom of a heterocyclic monomer.

8.7.2.3 Other Initiators Compared to carbon, silicon is strongly electropositive

and can thus be used in electrophilic reactions with strong nucleophiles serving asintermediates:

Me3Si –O–SO2–CF3+ Me2C=O −→ Me3Si –O–Me2C+,−SO

3–CF3then,

Advantage can be taken of the ionizing and dissociating effects produced by

a solvent to activate inert molecules and initiate a polymerization For instance,triphenylmethyl chloride in pure sulfuric acid solution undergoes an instantaneousionization and produces a triphenylmethylium cation with a characteristic red color:

R-CO-Cl + Ag+,ClO4− R-CO+,ClO4− + AgCl

8.7.3 Propagation of Cationic Polymerizations

In the case of unsaturated monomers, the propagating active center is a tionic species (carbonium ion) reacting by electrophilic addition To this carbocation

carboca-is associated a negative counterion, and both may excarboca-ist under the same ionic species(aggregates, ion pairs, free ions) as the carbanionic homologs Thus, depending onthe nature of monomers, the counterion and the solvent and depending on thetemperature of the medium, growing chain ends may be more or less polarized,aggregated, ionized, solvated, or dissociated with a reactivity growing in the sameorder The kinetics of polymerization are generally complex and reflect the multiplestructures taken by the reactive species The effect of temperature on the apparentreactivity of active species may sometimes result in overall negative (apparent)activation energies As in anionic polymerization, the influence of temperature on

the solvation of active centers and on the permittivity of the reaction medium andthe fact that they vary in a different manner with the temperature are responsiblefor this unusual behavior.

When cationic polymerizations are initiated by γ radiations, propagation

pro-ceeds by means of free ions and it is then possible to evaluate the monomerpolymerizability from determination of the corresponding rate constants of prop-agation It can be observed that, contrary to anionic polymerizations whose rateconstants of propagation of free ions are ∼104 times higher than those measuredfor ion pairs, the same ratio of rate constants is only about 10 (or even less) incationic polymerization Such a difference is due to the faculty of certain solvents

to solvate free cations and thus reduce their intrinsic reactivity

The high reactivity of carbocationic species can be also responsible for rangement reactions during the propagation step of certain monomers For instance,

rear-in the polymerization of 3-methylbut-1-ene, structural irregularities could beformed:

Table 8.17 Energy (in kJ ·mol −1 ) related to the strength of various heterocycles with

variable number of links

vari-of various natures and geometries are given in Table 8.17

Certain heterocycles polymerize by very peculiar reaction pathways Forexample, the cationic polymerization of lactams proceeds by activation of themonomer, as for their anionic polymerization:

8.7.4 Transfer and Termination Reactions

They occur because of the high reactivity of cationic species and the necessity

to form more stable species In the case of transfer, the resulting species are,however, sufficiently reactive to reinitiate the polymerization whereas, in the case

of termination, the species formed are totally inactive

8.7.4.1 Transfer Reactions If the reinitiation is very slow, the transfer step

slows down the entire kinetics of polymerization but such incidence would be ligible if the systems were to produce polymers with high degree of polymerization.For polymerization of unsaturated monomers, two main types of transfer reac-tions can be distinguished; the first type occurs via protonβ-elimination as shown

neg-hereafter for the polymerization of styrene:

The elimination of H+,A−occurs with high probability whenever the acid released

by transfer is of high stability—that is, the basicity of the associated anion (A−) isstrong As already mentioned, the basicity of the solvent can influence the tendency

of active centers to participate in such a reaction

The second type of transfer reaction implies the presence of aromatic moietiesthat undergo Friedel–Crafts reactions These aromatic rings can be carried by themonomer (styrene, α-methylstyrene, indene, coumarone, etc.) or be part of the

solvent (toluene, etc.), the initiator or an impurity These electrophilic substitutionreactions also produce H+,A− In the cationic polymerization of styrene, theseFriedel–Crafts reactions can occur either intra- or intermacromolecularly and inthe latter case, monomer or polymer sites will be involved The various transferreactions are shown hereafter:

~~~~Mn

+, A−

The spontaneous intramolecular reaction generates an indanylene group whereasthe two intermolecular ones that follow correspond, for the first, to a transfer to

monomer with formation of a macromonomer (macromolecule containing a

poly-merizable group at one end):

~~~~Mn

~~~~Mn

Macromonomer+

+

H+, A− +, A−

and, for the second, to a transfer to another chain, resulting in an intermolecularcoupling

side reaction in the cationic polymerization of styrene

Transfer reactions to polymer occur extensively in the cationic tion of heterocyclic monomers; they do this either intra- or intermacromolecularlythrough the attack of the electron-deficient α-carbon atom of the “onium” active

polymeriza-center by a heteroatom of an unspecified monomeric unit, carrying an n electron

pair: