PHOTOPOLYMERIZATION
A. Photoinitiated Free Radical Polymerization................top
When polymerizations are initiated by light and both the initiating
species and the growing chain ends are radicals, we speak of radical
photopolymerization. In far the most cases of photoinduced
polymerization, initiators are used to generate radicals. One has to
distinguish between two different types of photoinitiators.See animation of the
photopolymerization of the different type monomers.
Photopolymerization
of monofunctional monomers,
Photopolymerization
of bifunctional monomers
Type I Photoinitiators: Unimolecular Photoinitiators.
These substances undergo an homolytic bond cleavage upon absorption of
light. Benzoin and its derivatives are the most widely used
photoinitiators for radical polymerization of vinyl monomers.
Type II Photoinitiators Bimolecular Photoinitiators. These
photoiniating systems consist of a photoinitiator such as benzophenone
or thioxanthone and a coinitiator such as alcohol or amine. Radicals are
generated in a bimolecular process by the reduction of photoexcited
aromatic carbonyl compound by hydrogen abstraction or electron transfer
reactions
B. Photoinitiated Cationic Polymerization................top
Direct Acting Systems
Upon photolysis, these thermally
and hygroscopically stable initiators undergo irreversible
photofragmentation to produce cation radicals and Brønsted acids.
Reactive species thus
produced photochemically with onium salts initiate the cationic
polymerization of suitable monomers as illustrated below.
Indirect Acting
Systems
For practical applications,
onium salts should absorb light appreciably at wavelengths longer than
350 nm where the commercially available medium and high-pressure mercury
lamps emit much of their radiation. The indirect actions of these
systems, which extend their spectral sensitivity to longer wavelengths,
are shown in Figure 1.
Several indirect ways to expand wavelength range have
been described. All of these pathways involve (i) electron
transfer reactions either with photoexcited sensitizer or free radicals
(iii) and with the electron donor compounds in the excited charge
transfer complexes (ii)
(i)
Photosensitizer
Many aromatic hydrocarbons such
as anthracene, phenothiazine, and perylene are able to sensitize the
decomposition of onium salts via electron transfer. The irradiation of
the sensitizer is followed by the formation of a complex between excited
sensitizer 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 cation as a result of
homolytic cleavage of the corresponding onium salt. The radical cations
themselves initiate the polymerization of appropriate monomers or,
alternatively, interact with hydrogen donor constituents of the
polymerization mixture (such as solvent or monomer) resulting in the
release of Brønsted acid.
(ii)
Charge Transfer Complex
Pyridinium salts are capable of
forming charge transfer (CT) complexes with electron rich donors such as
methyl- and methoxy-substituted benzene. It was found that the CT
complexes formed between pyridinium salts and aromatic electron donors
act as photoinitiators for the cationic polymerization of cyclohexene
oxide and 4-vinyl cyclohexene oxide. The mechanism illustrated in
equations 19 and 20 for the initiation of the cationic polymerization
has been suggested.
(iii)
Free Radical Promoted Cationic
Polymerization
Among the indirectly acting
initiating systems, free radical promoted cationic polymerization is the
most flexible route, since free radical photoinitiators with a wide
range of absorption characteristics are available. Many photochemically
formed radicals
can be oxidized by onium salts.
The cations thus generated are used as initiating species for cationic
polymerization according to the following reactions.
TRANSFORMATIONS REACTIONS................top
The synthesis of block copolymers between structurally different
polymers i.e. condensation and vinyl polymers, by a single
polymerization method is rather difficult due to the nature of the
respective polymerization mechanisms. Furthermore, utilization of a
single method often excludes monomers that polymerize by other
mechanisms. In order to extend the range of monomers for synthesis
of block copolymers, transformation approach was postulated by which
the polymerization mechanism could be changed from one to another
which is suitable for the respective monomers.
Transformation
reactions are classified on the basis of interconversion between
propagation mechanisms. It can be seen that between the main living
and controlled/living polymerization methods, transformations are
accessible in both directions.
LIQUD CRYSTALLINE POLYMERS................top
In the last few
years incorporations of a side-group liquid crystalline (LC) polymer
with another non-LC polymer into various forms of LC AB-type block
copolymers have made it possible to access a new wide range of structure
and property combinations. Comparatively, very little attention has been
devoted to LC AB-type graft copolymers. LC block and graft copolymers
are especially interesting because of the appearance of two ordering
principles at the same time. In fact, copolymers consisting of amorphous
polymer blocks and side-group LC polymer blocks present a structurally
rich environment in which morphology can affect the LC alignment and
phase behavior. The microphase separation of the different blocks may
lead to diverse morphologies in these types of polymers. In addition,
the orientation ability of mesogens with mechanical and electric fields
combined with the mechanical properties of block copolymers may lead to
applications such as in electro-optic free standing thin films,
mechano-optic materials, and piezoelectric elastomers.
CONDUCTING POLYMERS................top
Electrically conducting polymers have
received growing interest because of their wide range applications in
the areas such as rechargeable batteries, membranes, optical displays,
and electro chromic devices. These materials are often termed as
synthetic metals due to the fact that they combine chemical and
mechanical properties of the polymers with the electronic properties of
the metals and semicunductors. Polythiophene is one of the polymers
among a general type of conducting polymers that include polyacetylenes,
polypyrrole, polyanilines, polyphenylenes, polycarbazoles,
polyquinolines, and polyphtalocyanines.
Macromonomers are usually referred to reactive oligomers or polymers in
which a polymerizable functional group is incorporated to the chain
end(s). Macromonomers can conveniently be copolymerized with
conventional monomers to yield graft copolymers with well defined
structures. Structure of the polymer chain of the macromonomer affects
the properties of the graft copolymer while the end group controls the
reactivity in polymerization. Macromonomers possessing one or two
electroactive groups at the chain end(s) are usually prepared by using
functional initators or by end capping the living growing chains.
In this research, reactive intermediates based on monomeric and
polymeric structures with thiophene moieties were synthesized and
possibility of their use in electropolymerization through thiophene
groups was attempted.
BLOCK and GRAFT COPOLYMERIZATION................top
Block and
graft copolymers are the most demanded advanced materials because of
their diverse copolymer structures. In most cases, the corresponding
homopolymers do not form homogeneous phase. However, linear arrangement
of the blocks by chemical bonds results in the realization of a stable
structure with two phases separated. Each segment exerts its character
or function to the bulk of the copolymers. This way various properties
are improved or combined to give possibility of using block copolymers
as compatabilizers, impact modifiers, surface modifiers, coating
materials, antistatic agents and adhesives.
Block
Copolymers
According to the most common methodology, monomer A is polymerized
completely, after which, monomer B is introduced in the mixture and its
polymerization proceeds upon initiation by the active site of the first
block. This approach is referred to as sequential monomer addition in
controlled/living polymerization methods. For obtaining a well-defined
block copolymer, the living site of the first monomer must be effective
in initiating the polymerization of the second monomer, that is,
simultaneous initiation of all growing B chains must be ensured and the
rate of the crossover reaction must be higher than the rate of
propagation of monomer B.
Another route involves the use of a difunctional initiator and has been
employed in the preparation of ABA symmetric triblock copolymers. In
this methodology, a compound possessing two initiating sites is utilized
in the formation of the middle block first, followed by the
polymerization of the second monomer to synthesize the first and the
third blocks. This allows the preparation of the ABA blocks in two,
instead of three steps without fractionation or other purification
steps. The difunctional initiator must be chosen so as to initiate the
polymerization with the same rate from either direction.
Moreover, AB diblock or ABA triblock copolymers can be prepared by
coupling two appropriately end-functionalized chains of A and B
homopolymers or AB blocks, respectively. In the latter case, block B
should initially contain half the molecular weight of the final desired
block B. The efficiency of coupling will be high if the reaction is
rapid and the living diblock copolymer is used in excess.
Graft Copolymers
Graft copolymers can be
obtained with three general methods: (i) grafting-onto, in which side
chains are preformed, and then attached to the backbone; (ii)
grafting-from, in which the monomer is grafted from the backbone; and
(iii) grafting-through, in which the macromonomers are copolymerized.
Grafting onto methods
involve reaction of functional groups
(Y) located at the
chain ends of one kind of polymer with another functional groups
(X)
which distributed randomly
on the main chain of the other polymer
Chemical initiation has
also been used in grafting from method. In this case, a polymer backbone
made that contains some thermally cleavable bonds such as azo and
peroxide linkages (See below). If the polymers
are heated in the presence of a second monomer, initiation takes places.
In such systems, homopolymer formation is unavoidable because of the
concomitant formation of low molar mass radicals.
Macromonomers are short
polymer chains possessing a polymerizable group at the one termini. A
great variety of methods involving living polymerization techniques,
chain transfer reactions and end chain modifications, have been
developed to synthesize such species.
CONTROLLED
POLYMERIZATION METHODS
................top
Recent
developments in controlled/living radical polymerization provided
possibility to synthesize well-defined telechelic polymers with
controlled functionality also with radical routes. As it will be shown
below, all the three standard methods for controlled/living radical
polymerization, namely, Atom Transfer Radical Polymerization (ATRP),
Stable Free Radical Mediated Polymerization (SFRP), also called as
Nitroxide Mediated Polymerization (NMP), and Reversible
Addition-Fragmentation Chain Transfer Polymerization (RAFT) were
used for the preparation of telechelic polymers.
(i) Nitroxide
Mediated Polymerization
In NMP systems, stable free
radicals, such as nitroxides, are used as reversible terminating agents
to control the polymerization process. Dormant chains are generated by
reversible deactivation of the growing chains through covalent bond
formation. At high temperatures, the bond undergoes a homolytic cleavage
to produce the active growing chain and the nitroxide radical.
Activation is followed by a rapid deactivation, whereby a few monomer
units are incorporated to the propagating chain.
See animation of the
NMP
(ii) Atom Transfer Radical Polymerization
Atom Transfer Radical Polymerization, pioneered by Matyjaszewski and
Sawamoto, is based on a continuous and reversible halogen transfer
(pseudo halogen) between a dormant propagating species, Pn,
and a transition-metal (eg. Cu), complexed by a ligand (eg. bipy), in
its lower oxidation state (Scheme 6). Halogen transfer is accompanied by
a one-electron oxidation of the transition metal, whereby propagation
takes place by the addition of monomers to the activated chains.
Homogeneity of the polymeric chains is controlled by the fast and
simultaneous initiation as well as rapid deactivation of the growing
chains. Here, the rate of deactivation should be faster than the
propagation rate for an effective control. Low concentration of the
active centers, maintained by deactivation, suppresses termination and
chain transfer reactions at later stages
.
See
animation of the ATRP
Reverse
ATRP differs from ATRP in its initiation process, where a conventional
radical initiator, such as AIBN (2,2’-Azobisisobutyronitrile), is used.
As shown below, in the initiation step, once generated, the initiating
radicals or the propagating radicals, I. Or I-P.,
can abstract the halogen atom X from the oxidized transition-metal
species, XMtn+1, to form the reduced
transition-metal species, Mtn, and the dormant species, I-X or I-P1-X.
In the subsequent steps, the transition-metal species, Mtn,
promotes exactly the same ATRP process as normal ATRP where R-X/Mtn/Lx
are used as the initiation system. Instead of first activation of a
dormant species, R-X, with Mtn, as in the case of
normal ATRP, reverse ATRP originates from the deactivation reaction
between radicals, I. or I-P., and XMtn+1.
See animation of the
RATRP
(iii)
Reversible Addition-Fragmentation Chain Transfer
Polymerization
Reversible Addition-Fragmentation
Chain Transfer Polymerization (RAFT), developed by Rizzardo and
co-workers in the late 1990s, utilizes a chain-transfer- active
thiocarbonylthio moiety for the exchange between active and dormant
chains. The mechanism is illustrated in below.
The active species, such as the radicals stemming
from the decomposition of the initiator and propagating radicals (Pn.),
are transferred to the RAFT-agents, e.g. the thiocarbonylthio moiety.
Meanwhile, an intermediate radical is formed, and undergoes a fast
fragmentation reaction, giving a polymeric RAFT agent and a new
radical. The radical reinitiates the polymerization. The equilibrium is
established by successive chain transfer-fragmentation stages. Z and R
groups in the thiocarbonylthio compound represent the activating group
and homolytically leaving group, respectively. These, in turn, determine
the rates of addition and fragmentation. As a matter of fact, the choice
of RAFT agent for a specified monomer is rather significant and affects
the degree of control. See animation of the
RAFT
NANOCOMPOSITES................top
SYNTHESIS OF POLYMER/METAL
NANOCOMPOSITES
Nanocomposite materials containing metal
nanoparticles and polymer matrix may exhibit novel physical and
chemical properties that are of high scientific and technological
importance. These materials combine the physical properties of small
size metal nanoparticles with those of polymeric materials in a
beneficial manner. Many advanced optoelectronic and sensor devices
are based on the fabrication of these materials. Because
of their high surface free energy, nanoparticles tend to
agglomerate. A key challenge for a potential technological use is
the achievement of homogeneous dispersion of the thermodynamically
unstable nanoparticles. Various methods have been applied. Generally
applied methodology is detaining nanoparticles during the
preparation by adding protecting agents or setting them in an inert
environment. Among them, photochemical methods involving light
induced reduction of metal ions such as Ag and Au complexes are of
particular interest as they find a wide range of application
including synthesis of metallic colloids, and metallization and
patterning of films.
Recently,
we have introduced a novel approach for the preparation of
metal-polymer nanocomposites, in which nanoparticle formation and UV
crosslinking process were accomplished in one pot by simply
irradiating appropriate formulations, obtaining the homogeneous
distribution of the nanoparticles within the polymer network without
any macroscopic agglomeration. Silver or gold nanoparticles and
initiaing species were formed in a single redox process.
POLYMER/CLAY
NANOCOMPOSITES
The use of nanostructured
fillers in the polymer has gained significant importance in the
development of thermosetting composites. One of the more widely studied
nanocomposite strategies is the incorporation of layered silicates into
the polymer matrix. In comparison to other nanoparticles, layered
silicates belong to a unique group of nanofillers with only one
dimension on the nanometer scale. The individual platelets of this
filler are slightly below 1 nm in thickness, and the diameter of the
platelets varies between 200 and 600 nm.
Three methods have been developed over time for the synthesis of
polymer/clay nanocomposites: solution exfoliation, melt intercalation
and in situ intercalative polymerization. However, the most common
process to synthesize polyether/clay layered silicate nanocomposites is
via the in-situ polymerization, the monomer, together with the initiator
and/or catalyst, is intercalated within the silicate layers and the
polymerization is initiated either thermally or chemically in situ. The
chain growth in the clay galleries triggers the clay exfoliation and
hence the nanocomposite formation.
We have described several synthetic approaches,
namely cationic ring opening polymerization, activated monomer
polymerization, photoinitiated free radical polymerization and “click
chemistry” allowing the preparation of polymer/clay nanocomposites.
CONJUGATED POLYMERS
................top
There is a long interest to prepare soluble
conjugated polymers so that polymerization, purification,
characterization and processability to be carried out in solution,
polymers to be free defects, with a known structure and to form thin
films. Owing to their intrinsic chemical structure, conjugated
polymers are insoluble in most organic solvents and improving their
solubility and processability has been became an important objective
that focused many research efforts. From the chemistry viewpoint
this disadavantage could be surpassed by design of adequate monomers
and introduction of solubilizing side chains onto the conjugated
backbone is a very used and efficient method for solubility
improving. However, the side substituents used until now are short
alkyl chains or other functional substituents for introduction of
other supplementary properties; liquid crystallinity, optical
activity, ionic groups, etc.
The attaching of usual polymer short chains with
a well defined length; i.e., polystyrene, polytetrahydrofuran,
polylactone and poly-N-acetyl ethylenimine chains, onto
polyphenylenenes and polythiophenes and properties of branched
copolymers have reported.
Poly(p-phenylene)
(PPP) is a typical conjugated, electroluminescent polymer for light
emitting devices in combination with excellent mechanical properties
and thermal and thermo oxidative stability. The key structural
factor in describing the supramolecular ordering of PPP is their
anisotropic shape, which follows from a rodlike architecture that
differentiates them from flexible polymers. Unfortunately, PPPs are
insoluble in many organic solvents, which limit their processability.
Therefore, attachment of conformationally mobile alkyl side chains
to the backbone has been important because it has allowed the
controlled synthesis of soluble and processable PPPs with high
molecular weight. In view of the expected large persistence length
of the main chain and of the flexibility of the side chains, such
molecules have been termed “hairy-rod” polymers. On combining a
stiff, insoluble, rod-like polymer such as PPP with a soft coil, for
example polystyrene, it is possible to form a new polymer with novel
and interesting properties.
The design process, the essence of which is the chemical control of
size and shape of PPPs, ultimately leads to conjugated polymers of
varying, controlled dimensionality.
Our studies focused on the synthesis of PPP type graft copolymers
that can present nanostructures between a conductive and an
insulating polymer, by using the macromonomer technique via
controlled polymerizations [ATRP or ROP (Ring Opening
Polymerization)] as versatile “tools”, combined with metal-catalyzed
Suzuki or Yamamoto polycondensation , specific to the obtainment of
soluble, high molecular weight, conjugated polymers.
CLICK CHEMISTRY
................top
‘‘Click chemistry’’ has recently been introduced as
a new way of categorizing organic reactions that are
highly efficient, modular and selective, and occur
with simple work-up procedures. The 1,3-dipolar cycloaddition of azides
to alkynes has become the most popular click reaction, and is widely
used in biology, chemistry and material science. By using the efficient
click reaction, different architectures of macromolecules such as
dendrimers, dendronized linear polymers, hydrogels, supra-polymers and
novel conjugated polymers can be created. Click chemistry strategy has
also been implemented for the preparation of segmented copolymers of
monomers polymerizable by different mechanisms.
BENZOXAZINE BASED
THERMOSETS................top
Among various high performance materials,
polybenzoxazine, as a recently developed thermosetting phenolic
resin, has received much interest for its unique characteristics
such as heat resistance, good flame retardance,
low moisture absorption, good mechanical properties and excellent
dimensional stability.
Polybenzoxazines are obtained
by ring opening polymerization of the corresponding monomers at
elevated temperatures without catalysts and releasing by-products
according to the following reaction.
They can also be polymerized by cationic
initiators and onium salt photoinitiators. However, in these cases,
the structures of the resulting polymers are complex and the
properties are different than those prepared by thermal means in the
absence of a catalyst. Therefore, the most of the studies on
benzoxazines focused on their thermal polymerization.
Benzoxazine monomers as the polybenzoxazine precursors can be easily
prepared from inexpensive raw materials like phenols, formaldehyde,
and primary amines.
This flexibility provides possibility of
synthesizing a wide range of benzoxazine monomers with additional
functionalities such as acetylene, nitrile, propargyl and maleimide
groups. The incorporation of benzoxazine groups into polymers is an
alternative way to further improve the properties. The benzoxazine
groups act as thermal crooslinker and while the polymer may be
accountable for the flexibility of these materials. Thus, thermally
crosslinked non-brittle polybenzoxazine films can be prepared. In
this paper, we describe our synthetic approaches proposed to prepare
benzoxazine functional polymers. The incorporation of benzoxazine
groups into polymers can proceed through two main synthetic routes
(Figure 1);
-
·
Performing polymerization of a particular
monomer using a benzoxazine derivative possessing suitable
initiating sites (functional initiator).
-
·
Synthesizing benzoxazine ring structure by
using a polymer with amino or phenol groups (monomer synthesis).
The
shortcoming associated with the first route is the interaction of
some propagating sites, i.e., cations with benzoxazine ring,
particularly with nitrogen and oxygen heteroatoms. However, in some
polymerization processes propagating species were unreactive towards
benzoxazine and the ring structure was preserved during the
polymerization. The second route is less sensitive to polymerization
conditions but requires functional (mainly amino) polymers in
advance. Classical monomer synthesis may successfully be followed
with the available amino telechelics.
Coupling Processes
