Carbocationic polymerization
“Onium ions”
Heterocycles ; Z= O, S, N, P, Si
Carbenium ions
Vinyl monomers
The kinetic chain carriers (active species) are positively charged.
Heterocyclic monomers
Cationic
Cationic / anionic
Vinyl Monomers
Initiation reaction called as “cationation of the monomer”
Propagation reaction: a nucleophilic attack of the monomer
Elementary steps in cationic polymerization
Initiation
- A Bronsted acids: highly nucleophilic counterion
Early termination
-Stable carbenium ions: too stable to initiate aliphatic olefins
- Friedel- Crafts acid systems
cation sources (HX, X2, R-X etc.) + MXn
Quasi-living or pseudo-cationic
Photoinitiation by Onium Salts*
Aryldiazonium (ArN2 +Z -), diaryliodonium (Ar2I+Z-), triarylsulfonium (Ar3S+Z-) and
N-alkoxy pyridinium salts, where Z is a nonnucleophilic and photostable anion
such as tetrafluoroborate(BF4), hexafluoroantimonate (SbF6 ),
tetraperfluorophenylborate [(C6F5)4B], and hexafluorophosphate (PF6 ), are effective
photoinitiators of cationic polymerization
Aryldiazonium salts have limited practical utility because of their inherent thermal
instability.
Diaryliodonium and triarylsulfonium salts are very stable—so stable that their
mixtures with highlyreactive monomers do not undergo polymerization on long-term
storage.
Some of these initiators have found commerical application in the
photocrosslinking of epoxy resins through cationic polymerization.
*Please refer “EXTERNALLY STIMULATED INITIATOR SYSTEMS FOR CATIONIC POLYMERIZATION”
YUSUF YAGCI, IVO REETZ Prog. Polym. Sci., Vol. 23, 1485–1538, 1998
Regarding onium salts, which are the most prominent latent cationic initiators,
direct and indirect acting systems can be differentiated. This strict differentiation
is reasonable, because the initiating species produced by either of these systems
are often not the same. In direct acting systems, the energy is absorbed by the
onium salt and this leads to its decomposition.
In contrast, with indirect acting systems, the energy is absorbed by an additional
component. After absorbing the energy, the additives can either react with the
onium salt thus producing initiating species, or transfer their energy to the onium
salt molecules. By changing the additives, one can often easily adjust to various
temperature ranges or wavelengths for thermo- and photolabile systems,
respectively
-Direct photolysis
If onium salt initiators, I, absorb light, electronically excited initiator, I*, species are
produced. The latter undergo a heterolytical or homolytical bond rupture leading to
cations C+ or radical cations C+., respectively.
Aryldiazonium salts
Upon irradiation, these salts with complex metal anions undergo a fragmentation
generating a
Lewis acid, which can initiate cationic polymerizations directly or react with a
hydrogen
donating constituent of the polymerization mixture yielding protons
Advantages:
The decomposition quantum yields of aryldiazonium salts are relatively high,
generally between 0.3 and 0.6.
Disadvantages:
-The thermal instability of the salt prevents long term storage and therefore
limits practical applications.
- Another disadvantage derives from the evolution of nitrogen. The evolved gas
leads to gas bubbles in the hardening coatings, thus making the material porous.
Diaryliodonium salts
Upon UV irradiation of diphenyl iodonium salts, the Ar–I bonds are ruptured both
hetero and homolytically. Whilst the heterolytic pathway generates a phenyl cation and
an iodobenzene molecule, eqn (8), a phenyl radical and an iodobenzene radical cation
are formed by homolytic cleavage, eqn (10). Both mechanisms involve the interaction
with a hydrogen donating solvent or monomer yielding Brønsted acid which initiates
the polymerization.
The spectral sensitivity of diaryliodonium salts is relatively poor. For
example, the simplest salt, diphenyl iodonium, possesses an
absorption maximum at 227 nm.
Sulphonium salts
Regarding the photolysis mechanism of triarylsulphonium salts, both heterolytic eqns
(13) and (14) and homolytic eqns (15) and (17) bond rupture of one sulphur–carbon
bond is evidenced. In direct irradiation of triphenylsulphonium salts, the heterolytic bond
cleavage starting from the excited singlet state is the preferred reaction pathway
Cationic polymerizations following the direct photolysis of triarylsulphonium
salts havebeen used for the industrially important UV curing of epoxy
coatings
Phosphonium salts
Benzyl or pyrenylmethyl groups containing phosphonium salts produce the
respective carbon centered cations after a heterolytic bond rupture
according to eqn (20). These cations are assumed to be the initiating
species in cationic polymerization.
The excellent initiating ability of phosphonium salts containing pyrenylmethyl groups has
been demonstrated for epoxides and vinyl monomers
In the case of phenacyltriphenyl phosphoniumsalts, however, Brønsted acid is most
likely the initiating species. Upon photolysis of these salts, the resonance stabilized ylide
and protons are formed according to eqn (21) Phenacyltriphenyl phosphonium salts
were used for the cationic polymerization of cyclohexene oxide, styrene, and p-methyl
styrene.
N-alkoxy pyridinium salts
When absorbing UV light in the presence of a cationically polymerizable monomer,
pyridinium type salts do readily initiate polymerization. The two initiation mechanisms
described are depicted in eqns (22) and (23) for N-ethoxy-2-methylpyridinium
hexafluorophosphate (EMP+PF6-)
Upon photolysis, the nitrogen–oxygen bond of the salt is ruptured forming a
pyridinium type radical cation and an alkoxy radical.
In addition to the radical cation, Brønsted acid formed in the presence
of hydrogen donors (monomer, solvent) may initiate the polymerization, as
illustrated in eqn (23).
The absorption of the pyridinium based photoinitiator lies in the far UV region. As can
be seen in Table 5, phenyl substituents shift the absorption maximum towards higher
wavelengths by ca. 40 nm.
As shown above, the spectral response of simple onium salts is only rarely
acceptable for their practical application. One possible pathway in tackling this dilemma
is the chemical attachment of chromophoric groups to the onium salt making it absorb
at higher wavelengths (see, for example, Table 2).
Besides that, appropriate chemicals may be added to the polymerization mixture. Some
aromatic compounds, like 1,2,4-trimethoxybenzene or hexamethyl benzene areable to
form charge-transfer (CT) complexes with pyridinium salts. Being formed in the
electronic ground state, these complexes exhibit higher optical absorptions than
the pyridinium salt alone. In these circumstances, the incident light is absorbed by the
CT complexes.
charge-transfer (CT) complexes
For example, the complex formed between N-ethoxy-4-cyano pyridinium
hexafluorophosphate and 1,2,4-trimethoxybenzene possesses an absorption maximum
at 420 nm. The absorption maxima of the two constituents are 270 nm and 265 nm for
the pyridinium salt and trimethoxybenzene, respectively.
Sensitization by classical energy transfer
This mechanism involves the electronic excitation of the sensitizer (S), a molecule
possessing a suitable absorption spectrum, to its excited state. Energy may be
transferred from the sensitizer (S*) to the onium salt (I) by either resonance excitation
or exchange energy transfer.
Depending on the two components involved, the energy transfer may proceed either in
the excited singlet or in the triplet state. However, in all examples discussed in this
chapter, triplet energy is transferred.
In consequence of the transfer process, the sensitizer returns to its ground state and
excited onium salt species (I*) are formed. The further reactions may also differ from
those, taking place when the onium salt is excited by direct absorption of light.
A sufficient energy transfer requires the excitation energy of the sensitizer E*(S) to be at
least as large as the excitation energy of the photoinitiator E*(I).
Ep(S) > Ep(I)
Oxidation of free radicals
Onium salts can only rarely initiate a cationic polymerization by themselves. Instead,
they may be used to oxidize free radicals according to reaction eqn (29), thus generating
reactive cations.
This so-called free radical promoted cationic polymerization is an elegant and fairly
flexible type of externally stimulated cationic polymerizations
Free radicals may be produced by various modes: photochemically, thermally or by
irradiating the system with high energy rays.
Table 9. continue
The efficiency of onium salts as oxidizing agents is related to their electron
affinity.
The higher the oxidation power of the onium salt, the higher (more
positive) is the reduction potential E1/2 red (On+).
As seen in Table 10, aryldiazonium salts are most suitable for the oxidation of
radicals.However, their practical application is hampered by the lack of thermal
stability.
Diphenyliodonium salts have also a relatively high reduction potential. Being very
suitable for the oxidation of free radicals, these salts have been most frequently
used for the oxidation of photogenerated free radicals1.
On the other hand, triphenylsulphonium salts have only limited potential for
radical induced cationic polymerizations due to their low reduction potential.
However, some highly nucleophilic radicals could be oxidized with sulphonium salts.
Even though they do not possess an optimal reduction potential Ered1/2 (On+),
pyridinium salts may also be used for oxidizing carbon centered free radicals.
Carbocations formed with the aid of N-ethoxy-2-methyl pyridinium (EMP+) were
used to initiate the polymerization of butylvinyl ether and cyclohexene oxide.
Sensitization via exciplexes
Electron transfer via exciplexes. Sensitizers such as anthracene, perylene or
phenothiazone form exciplexes with onium salts. Being formed in the consequence of
light absorption by the sensitizer, these energy rich complexes consist of non-excited
onium salt and electronically excited sensitizer molecules. In the complexation state,
electron transfer to the onium salt is observed, giving rise to positively charged sensitizer
species.
The excitation of the sensitizer is followed by the formation of a complex between
excitedsensitizer molecules and ground state onium salt. In this complex, one electron is
transferred from the sensitizer to the onium salt giving rise to the generation of sensitizer
radical cations.
The electron transfer (right part in eqn (52)) is energetically allowed, if DG
calculated by eqn (56) (extended Rehm–Weller equation) is negative.
According to eqn (56), the requirements are low oxidation potentials, Eox½ (S),
and relatively high excitation energies, E(S*), of the sensitizer. Besides that, only
onium salts with high (low negative) reduction potentials Ered½ (On+ ) (see Table
10), such as diphenyliodonium or alkoxy pyridinium salts are easily reduced by
the sensitizer.
The sensitization of onium salts (especially diphenyliodonium salts)by
anthracene
Addition fragmentation reactions
The mechanism of the addition fragmentation type initiation is depicted on the
example ofETM+ SbF-6 and benzoin.
The first step consists in the photogeneration of free radicals Virtually any
photolabile compound undergoing homolytic bond rupture may be used as a radical
source. The radicals add to the double bond of the allylonium salt thus producing a
radical in b position to the heteroatom of the onium salt cation. Consequently, the
molecule undergoes fragmentation yielding initiating cations.
Notably, upon irradiation of monomer solutions containing EPP+ SbF6 at 270 nm in the absence of any additional radical source, cationic polymerizations
take place
THERMOLATENT SYSTEMS
In many curing applications, the hardening of monomer containing curing
formulas by heat is desired.
Concerning energy absorption, thermal and photolytical initiations differ for the
following. In the case of thermal initiation, all chemical bonds absorb energy, whereas for
photoinitiation, the photon energy is absorbed only by suitable chromophoric groups
Sulphonium salts
Alkyl substituted sulphonium salts are thermally unstable, decomposing sometimes
already at room temperature. The reason, why alkylsulphonium salts are more thermally
reactive than arylsulphonium salts is that the former are stabilized only by
hyperconjugation, whereas the latter are stabilized by resonance.
Due to the stabilization of the leaving benzyl cation by resonance,alkylbenzyl
sulphonium and alkylarylbenzylsulphonium salts possess a high thermal sensitivity. In
Table 18, the alkylbenzylsulphonium initiators which have been used so far are compiled
Regarding the effect of the substituent in the aromatic ring, R2, it turned out clearly that
electron donating substituents enhance the initiation activity by stabilizing the benzyl
cation evolved. For example, with the p-OCH3 derivative, for a 50% conversion in
styrene polymerization 50oC were sufficient, whereas with the unsubstituted derivative,
as much as 120oChad to be applied for obtaining the same conversion in the same time
(30 min)
Other highly thermosensitive benzylsulphonium salts are benzyl phenylalkyl sulphonium
salts
As far as the substituents in the benzyl group, R2, are concerned, electron donating
substituents were found to enhance the thermal sensitivity by stabilizing the benzyl
cation. The reactivity diminishes in the order CH3 > H = Cl > NO2
N-containing onium salts
N-Benzyl pyridinium salts, the N-containing onium salts most frequently used for thermal
cationic polymerization.
The activity for pyridinium ring p-substituents was found to decrease in the order p-CN >
H > p-CH3 > p-N(CH3) 2. This order indicates that electron accepting groups stabilize
the cation on the pyridinium N-atom, thereby diminishing the leaving ability of the
pyridinium moiety. If the pyridinium ring is substituted by o-CN instead of p-CN, the
thermal sensitivity is drastically improved, increasing the activity by a factor of 20–30.
The effect of the o-CN substituent was attributed to both steric and electronic factors.
Other N-containing onium salts
Besides N-benzyl pyridinium salts, further salts with positively charged nitrogen atoms
have been utilized as thermolatent cationic initiators.Recently, the thermal initiation by
benzyl ammonium salts having the following general structure, with R = OCH3, t-C4H9,
CH3, Cl, were investigated in detail.
The yield with which initiating cations are formed falls in the order CH3O > tC4H9 > CH3 > Cl, what is explainable in terms of the stabilization of ensuing benzyl
radicals by electron donating groups.
Hydrazinium salts have been introduced recently as thermally latent Brønsted acid
generating initiators.
The evaluation of the initiator activity in glycidyl vinyl ether polymerization
revealed that the p-NO2–C6H4 substituent (R1) is most effective in increasing the thermal
sensitivity. With initiators containing this substituent, polymerizations were performed at
50oC.
Phosphonium salts
Substituted benzylphosphonium salts of the general structure with R
being NO2, Cl,H,CH3, OCH3, have been synthesized by the reaction of
correspondingly substituted benzyl chlorides with triphenylphosphine. In a second
reaction step, the counter anion has been exchanged for the low nucleophilic
SbF-6These salts have been used for the thermal polymerization of glycidyl phenyl
ether and cyclohexene oxide at temperatures in the range between 100 and 170oC.
Interestingly, the activity of these compounds was found to raise with changing
for more electron withdrawing substituents; the order of reactivity observed is OCH3 ,
CH3 , Cl , NO2. This is in sharp contrast to benzylsulphonium and benzylpyridinium salts,
where electron withdrawing substituents reduced the thermal sensitivity by destabilizing
the benzyl cation formed upon thermolysis
Indirect acting thermolatent systems
Regarding thermal activation, the radical sources listed in Table 20 have been
successfully applied in cationic polymerization.
Thermally induced addition fragmentation reactions
high energy irradiation
In the presence of certain onium salts, the radiation induced cationic
polymerization of cationically polymerizable monomers is strongly accelerated, i.e.
polymer is obtained with relatively low exposure doses. Initiating cations may be
produced by two different ways.
(a) Onium salts may scavenge solvated electrons. Solvated electrons are
generated by the interaction of ionizing irradiation with matter. These reactive species
lead to the decomposition of onium salts and the formation of Brønsted acid.
(b) oxidation of radiolytically formed radicals (stemming from monomer or
solvent) to produce initiating entities. In eqn (93), the a-ether radical (derived from
vinylether)/ diphenyliodonium salt system is depicted as a typical example.