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Discusses living / controlled radical polymerization (CRP) Technology Development and Application

Heistman 2013-10-11 12:34:05 Browse:9221 times


Discusses living / controlled radical
polymerization
(CRP) Technology Development and Application



Junqiang Xu Hongtao GuoShenzhen
Heistman
New
Material Technology Co., Ltd.



A block
copolymer having a special structure , it is different homopolymers together
the characteristics , such as a flexible amorphous block and a block of elastic
and rigid crystallinity and rigidity to exhibit special properties [ 1 ] . They
can be used as a thermoplastic elastomer , a surfactant, a surface modifying
agent, a dispersing agent, a solubilizer such polymer blends . Block copolymers
has attracted widespread attention polymer scientists and industrial sector has
become one of the molecular structure of the polymer structured most studied ,
the most widely used polymer types.



1.1 Block
copolymers



1.1.1 Molecular structure of the block copolymer



The polymer is a block
copolymer of two or more segments of different chemical composition are joined
together by chemical bonds prepared from these segments may even be
thermodynamically incompatible. Common block copolymer molecular structure
shown in Figure 1.1 .


 
 
 
 
 
 
 
 
 
 
 
 






1.1 Structure
of the block copolymer



Two, three or more types of monomers can form
a variety of molecular structures. From a structural point of view , block
copolymers can be divided into two categories: linear and non- linear. Linear
block copolymers include segments formed by the monomers A to the end of a
linear segment AB is formed by forming a monomer B diblock copolymer , the
monomers A, B, C or more types of monomers ABC triblock copolymer or multiblock
copolymer. In the form of non-linear block copolymers more , even to the point
where the polymer chains when three or more types of monomers to form a
star-shaped miktoarm copolymers.



Amphiphilic
block copolymer is the same in both the macromolecule and a hydrophilic segment
comprising a block copolymer comprising a hydrophobic segment , compared with
ordinary polymers , it has many special properties .



1.1.2 Characteristics of the block copolymer



Block copolymers having different monomer sequences are formed by
different structures have many unique properties , they are not only used for
special purposes in polymer modification , etc., in the development of new
polymer materials , also has an important significance . The results of the
theory of the block copolymer phase behavior , can be used to predict the
morphology of various block copolymers , micro size and width of the interface
. The development of these applications and the basic theory , but also
encourage people to block copolymerization further study , and a variety of
polymer segments appropriate combinations to obtain each having specific
mechanical , optical, electrical , and other physical properties of ionic kinds
of new polymer materials [ 2-6 ] . Block copolymers avoid macroscopic phase
separation occurs , while providing the nature of microscopic phase separation
in blends of incompatible polymers , adding a block copolymer and homopolymer
has the same structure , which can effectively provide phase capacitive , phase
stability and enhanced adhesion between the phases. As the high molecular
weight component , amphiphilic block polymer having a lower critical micelle
concentration and diffusion effects , and thus the low molecular weight
compared to conventional surfactant having a higher surface activity. Block
copolymers in solution can form micelles, vesicles , liquid crystal hexagonal ,
lamellar liquid crystal ordered self-assembly , Gene vector can be used ,
molecular recognition drug release , proteins and enzymes , selective
solubilization and chemical separation . Because of amphiphilic block
copolymers of these unique features, the polymer industry caused widespread
concern , various types of amphiphilic polymers have been synthesized and
applied to many fields. Amphiphilic block copolymer has become chemistry,
chemical engineering , petroleum, medicine, materials , physics, electronics,
life sciences and many other disciplines intersect the object of study .



1.1.3 Application block
copolymer coatings



Dispersants are in coatings, inks , adhesives and pastes and other
areas are also the largest amount of aid an integral type, efficiency and
effectiveness of dispersants directly determine the performance of the product
.



Pigment dispersion has used polyphosphates , silicates, carbonates
and inorganic dispersing agents, and conventional surfactants and
polycarboxylates, poly ( meth ) acrylate, polyurethane and other polymer
dispersant. The main advantage of the steric polymer compound so stable pigment
particles , better than small molecule surfactant electrostatic repulsion .
Studies have shown that many types of structures in polymer dispersant , AB
type block copolymer is the best , one of the most efficient types of effects ,
the structure shown in Figure 1.1 . From the point of view of molecular
structure , AB type block copolymer is a large number of surfactants , A blocks
and B blocks are similar to the hydrophilic head group of the surfactant and
the hydrophobic tail chain [ 7 ]



AB block copolymers take -tailed adsorption surface morphology
fillers , A block is a pro- pigment anchoring groups , B block is a pro- tail
chain solvation solvent . A block may be an acid , amine, alcohol , phenol and
other functional groups by ionic bonding , covalent bonding , coordination
bonding, hydrogen bonding and van der Waals interaction between the particles
adsorbed on the surface , the points comprising a plurality of adsorption can
be effectively prevent dispersant molecule desorption, adsorption close and
lasting. B block may be a polyether , polyester, polyolefin, polyacrylate group
, Were applied to both polar and non -polar solvents. Stable particles mainly
depends on the steric effect of the adsorption layer formed in the block B
generated , so as solvated tail chain length of the block B has high uniformity
and request that the adsorption medium and uniform thickness can be formed
layer, if the B block is too short , it can not form an effective steric layer
, and if the B block is too long , it may be from the bridging effect , causing
an increase in the viscosity of the dispersion , flocculation or precipitation.
The thickness is generally considered when the steric layer is 20nm, the best
stability results can be achieved [ 8 ] . It can be expected , a solvent having
a specific length of the chain , and the low polydispersity of the polymer
dispersing agent having excellent dispersing effect .



Synthesis of
controlled molecular structure and molecular weight of the specific AB type
block copolymer is a coating dispersion aid development



Direction .
The world's leading fine and specialty chemicals companies such as BYK, Ciba,
Dupont, Rhodia , etc. have invested tremendous financial and human resources
for structural controllability of high molecular weight distribution
development and marketing of a narrow block copolymer. BYK companies use
technology to develop a controlled polymerization of the AB type block polymer
is used for a variety of coating a dispersion aid . Has launched DISPERBYK-2000
and DISPERBYK-2001 dispersant used in automotive coatings , DISPERBYK-2010 and
DISPERBYK-2020 dispersing agent used in construction and industrial coatings ,
the former in aqueous systems , which is used for solvent-based systems . 2007
new DISPERBYK-2025 and a dispersant for baking paint , DISPERBYK-2009 solvent-
dispersant, dedicated to the wood , but also for electronic ink [ 9 ] . Ciba
NMP technology companies use synthetic AB type block copolymer , launched Ciba
EFCA4300, Ciba EFCA4330 and Ciba EFCA4340 for solvent-based coatings pigment
dispersion , in 2007 introduced new Ciba EFCA4585 for pigment dispersion in
aqueous systems , purportedly high pigment loading , low viscosity, good color
development and color intensity , also exhibit excellent resistance to flocculation,
for a variety of resin systems and a wide range of pH values [ 10 ] .



1.1.4 Synthesis of block copolymer



In addition polymerizable vinyl monomer or other active catalyst
for the polymerization usually are chains . The basic characteristics of the
polymeric chain : the active site required for the reaction , such as free
radical , anionic , cationic , etc. ; whole process can be divided into the
chain initiation , chain propagation and chain termination reactions of the
three primitives ; activation energy of the reactions vary greatly ; time has
little effect on the molecular weight , the main impact of the conversion rate
[ 11 ] .



Since the block polymer in the synthesis
process, the requirements are controlled molecular weight and structure , if
the conventional method , even if the spent time , energy, and materials can
not synthesize a specific structure of the block copolymer . Therefore, you
must use the living / controlled polymerization methods. Currently, the block
copolymers can be prepared are the following : anionic polymerization active
child , cationic living polymerization , ring-opening polymerization
disproportionation , group transfer polymerization , living / controlled
radical polymerization.





1.1.4.1 Living Anionic Polymerization



Research began in the block copolymer anionic polymerization
without termination of discovery we know of some commodities , such as PS
(polystyrene )-b-PB ( polybutadiene )-b-PS, namely SBS (Kraton) and PEO (
polyoxyethylene )-b-PPO ( polypropylene oxide )-b-PEO (Pluronic) are derived
from anionic polymerization techniques .



Anionic polymerization is the first was discovered , and get
active polymer method industrial applications. It was in 1956 by the American
scientist Szwarc et al study sodium naphthalene in tetrahydrofuran initiated
polymerization of a landmark discovery styrene polymerization [ 12,13 ] .
Szwarc et al found that, under anhydrous , anaerobic , no impurities, low
temperature , the solvent is tetrahydrofuran , sodium naphthalene initiators
for anionic polymerization reaction there is no chain termination and chain
transfer reaction , the polymer solution was for several months at low
temperature, high vacuum conditions, the concentration of active species remain
unchanged
. If
more of styrene , the polymerization reaction may proceed to obtain a higher
molecular weight polystyrene . And if the addition of the second monomer (
e.g., butadiene ) , can be obtained a styrene - butadiene block copolymer.
Based on this discovery , Szwarc , who first proposed active polymerization
(Living Polymerization) concept



Refers to those so-called living polymerization chain- reaction
polymerization to any or irreversible side reactions stop the polymerization
reaction does not exist. Polymerization activity was significantly different
from the traditional four polymerization characteristics:

( 1 ) to initiate the reaction rate much faster than the propagation reaction ,
and there is no chain termination and chain transfer reaction, all polymer
chains grow at the same time , increase the number of chains remains unchanged,
thus the molecular weight distribution is very narrow , typically less than
1.1.



( 2 )Consumption is proportional to the
molecular weight of the polymer and monomer concentration than the initial
concentration of the initiator agent .

( 3 )The molecular weight of the polymer with a linear increase of the
conversion rate , the degree of polymerization may be controlled by controlling
the resulting polymer of the monomer and initiator feeding amount .

( 4 )When the conversion of the first monomer reaches 100% , then add other
monomers , a block copolymer can be synthesized with a book structure.





Anionic polymerization pay close attention to the chemical and
polymer industry workers and great interest. People have come to realize that
living polymerization polymerization method generally irreplaceable advantages
, especially in the control of a polymer structure has the advantage , for the
molecular design of the polymer provides a powerful tool . After decades of
development, the anionic polymerization not only made great progress in
theoretical research , but also in industrial production has also been a good
application , increasing product range , expanding the scope of application ,
the economic and social benefits is increasingly evident.



In SBS , for example, synthetic anionic polymerization of the
block copolymer has three methods, namely monofunctional initiators method
bifunctional initiators method and coupling method [ 11 ] .

( 1 ) monofunctional initiators : the butyl lithium as initiator , first raised
the polymerization of styrene , styrene reaction to be completed before adding
butadiene , styrene butadiene added after completion of the reaction , and
finally terminating agent terminated. Usually when the synthetic solvents
cyclohexane or benzene , the polymerization temperature is 70
.



( 2 ) bifunctional initiators : lithium
naphthalene initiator system is commonly used bifunctional initiator , dual ion
activity center can be used to form the first trigger polymerization of
butadiene , styrene polymerization initiator and then , you get the SBS.

( 3 ) coupling method : first agent SB diblock copolymer was prepared
monofunctional initiator , reaction with a difunctional coupling agent , for
example with 1,6 - dibromo- hexane to make a coupling agent , to obtain the
same three SBS block copolymer.

1.1.4.2 cationic living polymerization
(Living Cationic Polymerization)



Since the 1956 discovery since anionic polymerization , cationic
living polymerization of exploration and research has been carried out in a
difficult , but for a long time without much success , so once led the
researchers to lose confidence . Until 1984 , studies of cationic
polymerization activity a turning point , Higashimura first reported alkyl
vinyl ether cationic living polymerization , followed by Kennedy developed a
cationic living polymerization of isobutylene , cationic polymerization
achieved a landmark breakthrough [ 14,15 ] .



In -depth study of the activity of cationic polymerization, it was
found that the activity of many so-called cationic polymerization , the
polymerization activity is not the true sense. During the polymerization
reaction and the chain transfer and chain termination reactions is not
completely eliminated, but is masked to some extent , the performance of only
the upper surface of the characteristics of living polymerization , the
polymerization activity and therefore is called the apparent or quasi -living
polymerization . Present in the polymerization reaction, the reversible chain
transfer and chain termination reaction method using the reaction was slowly
added dropwise a monomer can be suppressed .



Cationic living polymerization of the block
copolymer , there are two methods, namely the macroinitiator method and order
of feeding method. Cation- active block copolymer obtained by polymerization of
a first polyisobutylene and styrene diblock copolymer , the key technology is
first prepared macroinitiator polyisobutylene , styrene polymerization
initiator and then , to give the final diblock copolymer .



Using the
sequential addition method, the addition of the second monomer to the
polymerization system , and a third monomer , the polymerization will proceed
finally obtained diblock and triblock copolymers. Kennedy et al. Successfully
prepared using this method PS-PIB-PS triblock thermoplastic elastomer, a
tensile strength of up to 20 ~ 22MPa, the glass transition temperature of 98
.



1.1.4.3 Ring-Opening Metathesis Polymerization,RMOP



Cyclic olefin in the presence of a double bond in the molecule
cracking catalyst and process are connected end to end to be larger molecules
known ring-opening polymerization of disproportionation . Cyclic olefin is an
olefin compound is an important component , include monocyclic and polycyclic
olefin compound , the former is represented cyclopentene , norbornene and the
latter is represented .



Study disproportionation olefin ring-opening polymerization began
in the 1950s , to 1986 Gilliom et al published in the small alkyl heteroaryl
ring Ti catalyst disproportionation norbornene ring-opening polymerization ,
the molecular weight distribution is very narrow resulting product [ 16 ] .
They trace deuterated norbornene polymerization , the molecular weight of the
product was found with the new monomer and continuous increase ; no chain
transfer reaction and the chain termination reaction . This indicates the
presence of specific polymerization conditions and catalyst disproportionation
ring opening polymerization is a living polymerization .
Ring-opening polymerization generally
considered disproportionation metal carbene complex is triggered , the growth
mechanism . Ring-opening polymerization occurs disproportionation of polymer
materials, especially structural design of functional polymers and provides a
useful tool. Using the cycloolefin ring-opening polymerization of the
disproportionation of new polymer materials have been developed for a large
number of excellent properties , such as reaction injection molding
polymerization of dicyclopentadiene ( novel high impact plastic ) ,
polynorbornene and poly- cyclooctene ( New thermoplastic elastomers ) and the
like.





Disproportionation using ring-opening addition polymerization
method can be prepared by sequential block copolymer. For example, cyclobutane
derivatives titanocene first performed as a catalyst activity of norbornene
ring-opening polymerization disproportionation , followed by adding dicyclopentadiene
and norbornene can be successfully prepared ABA triblock copolymer. The
ring-opening polymerization dismutase activity polymerization can also be
converted to other aggregation preparation of block copolymers. First with the
titanocene compound as a catalyst initiator cyclobutane norbornene ring-opening
polymerization of disproportionation , and methanol with an equimolar under
mild reaction conditions and the resulting homopolymer , cyclobutane
dicarboxylic titanocene on the polymer end group into a methoxy
cyclopentadienyl titanium , then the catalytic polymerization of ethylene in
the presence EtAlCl2 get AB block copolymer.



Ring-opening
polymerization activity disproportionation combined with atom transfer radical
polymerization can be used for the preparation of block copolymers. Molybdenum
carbene complexes as catalysts for the first norbornene, dicyclopentadiene and
other cyclic olefin ring-opening polymerization of disproportionation , and the
living polymer obtained by reaction of the bromomethyl benzaldehyde, the
polymer end groups converted to a benzyl bromide group , then under CuCl2 /
bipyridyl catalytic conditions may lead atoms , styrene, methacrylate monomers
transfer radical polymerization , AB -type block copolymer finally obtained .



1.1.4.4 Group Transfer Polymeriztion, GTP



Group transfer polymerization activity of the polymerization as a
new technology, in 1983 by DuPont Webster et al first reported [ 17 ] . Group
transfer polymerization are α-, β- unsaturated esters , ketones , amides and
nitriles such compounds as monomers with silane groups , the compound germyl,
stannyl groups such as initiator , with anionic or Lewis acid -type compound as
catalyst , the choice of an appropriate organic solvent ,
By end of the catalyst with a silicon -based
initiator , germanium, tin atom ligands stimulate silicon, germanium , tin
atoms , oxygen atoms and that of the monomer binding to the carbonyl group of a
covalent bond, a double bond in the monomer and initiator double bond in an
addition reaction is completed , silyl , germyl, tin alkyl group formation
process moves to the end of the "active " compound. The above process
is repeated to give the corresponding polymer.



Using group transfer polymerization techniques to form the
features of the living polymer with an anionic polymerization , as can be
easily read by sequentially adding different monomers of the block copolymer
prepared ( sequential addition method ) . E.g. less the same activity selected
monomer ( acrylate / acrylic acid esters or methacrylic acid esters /
methacrylic acid ester ) , after the reaction of the first monomer may be added
to the second monomer The reaction was continued , AB -type block copolymer can
be formed .



The group transfer polymerization in combination with other polymerization
methods are also an effective method for preparing the block copolymer . More
block copolymers has been prepared by this method . Using group transfer
polymerization and atom transfer radical polymerization conversion of synthesis
of PMMA-PS block copolymers. The specific method will be used for the PMMA
group transfer polymerization activity chain termination legitimate prepared
with bromine , bromine end groups obtained PMMA. Then using CuBr / bpy system
as initiator , styrene monomer by atom transfer radical polymerization , the
obtained PMMA-PS block copolymer.



By group transfer polymerization activity in combination with an
anionic polymerization , and PMMA can also be synthesized polydimethylsiloxane
block copolymer .

When the block copolymer prepared by sequential addition , if the same non-
selected monomer (e.g., acrylate / methacrylic acid ester ) , and the activity
difference between the larger , less active monomer should be polymerized ,
generating activity chain before adding higher activity monomers.



On the industrial production , the first group transfer
polymerization for the preparation of acrylate-based automotive coatings .
Group transfer polymerization with the resulting polymer molecular weight
distribution, a solids content of up to 60% , while the general radical
polymerization is typically a solid content of only 20% ; while the absence of
unreacted monomer is present , the volatile content in the coating less ,
reducing environmental pollution. Furthermore, the transfer radical
polymerization of the acrylate-based synthetic automotive coatings can be cured
at the mask 82
, and then cured at room
temperature is possible , but usually need automotive washcoat cured at 116 ~
127
.



Group
transfer polymerization has great advantages in the control of molecular
structure , using this method to obtain a specific molecular weight polymer ,
there may prepare biopolymer material, which has very important theoretical and
practical value.

DuPont group transfer polymerization has been used to develop an automotive
topcoat coating process , plans a photosensitive resin produced by this method
, a semiconductor wafer , with the production of optical fiber coatings,
thermoplastic elastomer, and may be a composite material instead of metal .



1.1.4.5 Controlled/“Living” Radical PolymerizationCRP



Radical PolymerizationRP has a single wide
variety of synthetic technology , simple operation , low cost of
industrialization , as well as various functional groups can be allowed to
carry on a monomer containing a proton solvent and water as a polymerization
medium , most of the monomer can be copolymer characteristics. Currently about
70 % of the polymeric material derived from free radical polymerization.
However , the presence of a radical polymerization reaction or sub- group
element is reacted with an active polymerization contradictory , such as the
coupling reaction was terminated by radical disproportionation termination
reactions, chain transfer reaction , the polymerization reaction is difficult
to control . Living / controlled radical polymerization using a radical
polymerization occurs so synthesized controllable structure , polymers having a
narrow molecular weight distribution becomes possible.



Recently developed living / controlled radical polymerization
methods are Iniferter radical polymerization (Initiator-transfer-terminator),
atom transfer radical polymerization (Atom Transfer Radical Polymerization,
ATRP), stable free radical polymerization (Stable Free Radical Polymerization,
SFRP) and reversible addition-fragmentation chain transfer polymerization
(Reversible Addition and Fragmentation Chain Transfer Radical Polymerization,
RAFT) , etc. [ 18-25 ] . These types of polymerization mechanism seemingly
different, careful analysis, but their basic principle is the same . By
introducing the dormant species , establish a balance between it and the rapid
growth of free radicals , reducing the instantaneous radical concentration .
This dynamic and fast balancing not only reduces the possibility of free
radicals is terminated, and by frequent switching between the active center and
the dormant species , so that all of the active or dormant polymer chain has an
equal chance of growing polymer thus obtained food chain length close to equal.



Establish a rapid and balanced growth of
free radicals and various dormant species is that all living / controlled
radical polymerization system core . Can be used to establish the balance of
free radicals in the deactivation / activation process reversible capture , see
diagram 1.2a, can also be used in the process of degeneration exchange
reversible transfer , see diagram 1.2b [26].



1.2a









1.2b



1.2
The growth of free radicals
and dormant species to establish a balance schematic



The method relies on the reversible capture radicals constant
effect (Persistent Radical Effect, PRE) [27-30]. PRE is a special dynamic
characteristics of the system in a particular PRE CRP can provide
self-regulating effect . X is a radical increase rapidly captured inactivation
, X is typically a stable free radical such as a nitro group , or an organic
metal such as cobalt porphyrin type , inactivation rate constant . Activating a
dormant species in the spontaneous light conditions , may also be a catalyst
under appropriate conditions ( such as ATRP) a thermal initiator , so that once
again the growth center , the activation rate constant . Radicals can grow can
also be terminated. However, the constant radicals react with each other X can
not be terminated , and the kind of growth can only occur with the reversible
cross-coupling reaction was terminated . Each radical - radical termination
reactions are accompanied X irreversible agglomeration . X concentration
gradually increased over time , to comply with the unique 1 /3 of the energy
laws. Therefore , the possibility of concentration of free radicals and a
radical terminated with time is reduced. When the concentration 1000 times
higher than the X radical concentration increase , and no longer grow their own
radical reactions, the main reaction with X.



In compliance with the PRE system , the growth of free radicals in
the steady state by activating - deactivation process is established, but in
the traditional radical polymerization , it is by raising - termination process
established. PRE compliance stable free radical polymerization system
comprising polymerization (Stable Free Radical Polymerization, SFRP), more
precisely, is a stable nitroxide free radical polymerization of the
polymerizable (Nitroxide Mediated Polymerization, NMP) and a radical
polymerization regulator Co (Cobalt Mediated Radical Polymerization, CMRP).
These technical requirements stoichiometric telomerization media type , so that
all dormant chains are capped capture agents . PRE also act by ATRP , is
different, and the catalytic reduction process using the growing chain atom or
group transfer between the activity of the catalyst , the transition metal
catalyst is typically less than the stoichiometric amount .



In contrast , some of the degenerative transfer system instead of
using PRE, such as RAFT polymerization. These systems follow the slow kinetics
typical RP initiation and termination features fast . Transfer agent
concentration greater than the radical initiator used. Therefore , the transfer
agent 's role as a dormant species . Monomer concentration of free radicals is
a very small amount consumed , it may also be possible to stop the reaction
degeneration exchange (Degenerative transfer) and dormant species occur.



In all of the CRP system, the rapid exchange between the active
species and dormant species is necessary , which can be well controlled
molecular weight and polydispersity of the polymer chain structures . Ideally,
the colors of growth before it becomes deactivated dormant species (which may
be for a few seconds ) and a small amount of monomer should be the reaction (
complete within a few milliseconds ) only . CRP process, the active life of the
conventional state of the RP in the chain increase considerably the life of the
chain . However, since the increase in the CRP process may continue until one
day long , which makes it possible to implement the synthesis process , include
chain or a side chain functional group length increased [ 31 ] .



Living / controlled radical polymerization synthesis of block
copolymers can be used in two ways . The first method is a sequential addition
method. With living / controlled radical polymerization of a monomer is first
prepared the first homopolymer until after the completion of the first reaction
of a monomer , a second monomer is added directly , you can get two block
copolymers , the same The method of triblock and multiblock copolymers can be
prepared . The second method is to use a macroinitiator prepared block
copolymer. Macroinitiator is a synthetic polymer prepared from a functional
living polymerization methods , or other commercialization , then used ATRP,
NMP and RAFT , such as living / controlled radical polymerization, controlled
structure can be obtained , insert a narrow molecular weight distribution block
copolymers .



Following Szwarc has raised living polymerization , the
polymerization activity has become one of the most academic and industrial
application of research in the field of polymer chemistry . The most important
activity of the polymerization of the polymer is synthetic chemist controlled
structure and molecular weight polymers provide no means of conventional
polymerization methods . After decades of efforts, it has successfully
developed a series of activities for different monomer polymerization reaction
system , such as anionic polymerization , living cationic polymerization ,
ring-opening polymerization activity , the activity of the ring-opening
polymerization disproportionation , group transfer polymerization ,
coordination anionic polymerization, anionic polymerization of metal , so that
the majority of workers in polymer chemistry molecular design of polymer
materials for many years carried the dream become a reality. However, practice
shows , although these have been developed for preparing high living
polymerization can control some structural polymers , but really are not many
large-scale industrial production . The main problem is that they are generally
more stringent reaction conditions , the reaction process is also more complex,
resulting in the high cost of the product industrialization . Meanwhile , the
existing coverage monomer polymerization activity is narrow, mainly styrene, (
meth ) acrylate monomer , the molecular structure can be designed such that
resistance is small, thus greatly limiting the activity of the polymerization
the polymer applied Materials sector.



CRP appeared
, inherited the tradition of radical polymerization simple operation, wide
range of monomers , process varied advantages. CRP technology enables the
design and control of molecular weight and molecular structure of the
distribution , so CRP became the most industrialized living polymerization
promising technologies .



1.2 CRP Summarize



1.2.1 CRP Classification



Exchange
described above , between the propagating radicals and the dormant species is
the most important feature of CRP system . According to the structure and
chemical mechanism of exchange of dormant species , living radical
polymerization can be divided into Iniferter radical polymerization, atom
transfer radical polymerization (ATRP), stable free radical polymerization
(SFRP) and reversible addition-fragmentation chain transfer polymerization
(RAFT ) .



1.2.1.1 Iniferter
Radical Polymerization



Iniferter
1982 by the Ostu , who discovered and successfully applied it to free radical
polymerization system , Iniferter meaning iniferter , refers to the radical
polymerization process, and played lead , transfer and termination of the role
of a compounds [ 18,19 ] . Common reagents added Iniferter radical
polymerization system , and the system will show the characteristics of living
polymerization . The agents are generally used Iniferter six symmetrical
substituted ethane compound or compounds diethyldithiocarbamate carbamoyloxy
group -containing can be a photoinitiator or a thermal initiator . This technique
can be applied to the polymerization of styrene , methyl methacrylate , methyl
acrylate , acrylonitrile monomer. By the design of the reagent may be prepared
Iniferter block and star polymers . However, precise control of molecular
weight and distribution lacking .



1.2.1.2
Atom transfer radical
polymerization





In 1995 the success of the transition
metal-catalyzed atom transfer radical addition of Matyjiaszewski et al
introduced into the polymer science , proposed ATRP method [ 20-22 ] . The
mechanism shown in Figure 1.3 .









1.3 ATRPPolymerization mechanism



Ligand
transition metal oxide through a process of extracting from the electronic
dormant species (R-X) of the radical , to form a reactive species, a free
radical polymerization process to achieve this balance is controlled by . ATRP
can be successfully used for molecular design and the design of the polymer
material . ATRP polymerization under mild conditions , broad adaptability
monomers , metal ions and ligands , but the reaction difficult to handle ,
difficult to achieve some of the reactive monomer polymerization active end
group containing a halogen atom may affect stability of the polymer



1.2.1.3
Stable free radical
polymerization





1993 Georges et al found that , under conditions
of stable free radical nitroxide TEMPO exist, BPO caused by the bulk
polymerization of styrene polymerization activity [23,24 ] . Stable free
radical polymerization mechanism shown in Figure 1.4 . During the
polymerization , TEMPO stable free radical , the radical formation and growth
only covalent coupling reaction , the covalent bond can be decomposed at high
temperature to generate free radicals . After capturing growth thus TEMPO
radical , not a real active chains of death , but only temporarily inactivated
, become dormant species .







Pic1.4 Stable free radical polymerization mechanism



 



The use of SFRP, can be studied copolymerizable
monomers , the molecular weight control , narrow distribution of the random
copolymer . SFRP can also be used for synthesis of block copolymers and graft
copolymers , i.e., the first synthesis of the nitroxide end-capped
macroinitiator then raised above 120 ℃ polymerization of the second monomer to
obtain a block copolymer. Lokaj other side with this fat synthesis of
polystyrene and poly methyl methacrylate, 2 - dimethylaminoethyl methacrylate
(PDAMA) block polymer of PSt-b-PDAMA.



1.2.1.4 Reversible addition-fragmentation
radical polymerization



RAFT Institute of
Rizzardo CSIRO found that in 1998 [ 25 ] . By introducing an efficient chain
transfer agent in the conventional radical polymerization system , and growth
radical reversible addition - fragmentation chain transfer , to obtain a
controlled molecular weight , polydispersity index smaller polymer. And other
technologies, NMP and ATRP controlled radical technology is based on a
reversible termination mechanism , and RAFT based transfer mechanism is reversible
. RAFT polymerization mechanism is shown in Figure 1.5 .







Pic1.5 RAFTSchematic polymerization mechanism





1.2.2 Difference
between
CRP
&
RP



CRP
and RP by the same radical mechanism works , showing the chemical selectivity,
selectivity and stereoselectivity configuration similar to , but also the
approximate range of polymerization of the monomers [26 ] .



The
main difference between the RP and CRP by the following points:

1 ) In the RP in the life of the growing chain of about one second , but due to
the intermittent participation of CRP in reversible activation of dormant
species , growing chain of life extended to more than one hour .

2 ) In the conventional RP , the initiator slow rate , not the complete
consumption of the free radical initiator is typically the last remaining in
the system . In most of the CRP system, the initiation rate quickly, and can
achieve almost the same moment all the chain grew , eventually making chain
structure controllable .



3)
RP almost all of the resulting polymer chains dead links , and the proportion
of dead chains generated in CRP usually < 10% .

4) CRP polymerization rate is usually slower than RP . However, in some cases ,
such as when the target molecular weight of CRP is relatively low, the two
rates are almost equivalent .

5) based on the RP , the steady state concentration of free radicals , similar
to the initiation and termination rate . In the CRP system , based on the PRE,
the steady-state concentration of radicals , and the rate of activation by the
balance between the rate of inactivation achieved.



6) RP in to terminate the reaction generally
occurs between the long polymer chains , lead to the formation of new chains.
PRE -based system in the CRP , the initial reaction stage all chains are short
chains , but gradually became longer , and therefore , the termination rate
decreased significantly over time. In the RAFT process, the new chains are
usually produced by a small conventional initiators , is likely to terminate
the reaction throughout the entire polymerization process.



1.2.3 Summary and Comparison
for
SFRP,ATRP
&
RAFT



All elements of a unified system is a dynamic
equilibrium growth CRP radicals and between various types of dormant species .
Growth radical reaction may occur , and a dormant species exchange reaction ,
to terminate the reaction will occur , but also participate in various other
types of free radical reactions , such as the shift reaction , rearrangement
reaction and cleavage reaction and the like. RP selectivity and chemical
polymerization in CRP , selectivity and stereoselectivity configuration is
similar. CRP technology has the ability to control the molecular weight and
degree of dispersion , to provide the specific molecular structure of synthetic
polymers, in particular those features from the fast initiator and the level of
restriction to the growth of the chain , the chain termination reaction of the
process can be ignored .



There are generally two ways to create a balance
dormant species and active species. A method based on reversible termination of
the first , the second method using the reversible transfer . In both
conditions, the free radical before it becomes a dormant species , to be
subjected to intermittent activation, several complete addition. The
polydispersity of the polymer chain depends on the efficiency of initiation ,
chain termination reactions and the exchange contribution to the process
kinetics. The faster switching , the lower the polydispersity , which means
that each time the activation step , the addition of a few monomer units .



The
CRP PRE -based systems, such as ATRP and SFRP, and the dynamics of RAFT system
is significantly different. In the RAFT system, the rate depends on the
continued generation of free radical initiators , which constantly generate new
chain. In the system of SFRP and RAFT end group functionalized radical
substituted and usually includes the addition chemistry. In ATRP and iodine
transfer radical polymerization, nucleophilic substitution and electrophilic
addition is possible. In all systems , each chain of a dormant species are
actually covered by a protective group . SFRP in dormant species is metastable
at RAFT dormant species may also be sensitive type . The most readily available
as a dormant species , stable , low group is a halogenated alkane used in ATRP
. However , ATRP transition metal complexes still required as a catalyst to be
removed after completion of the polymerization .


Dormant
species reactivity determines the order of the block copolymer synthesis block
sequence. The resulting stability of this sequence is typically used to measure
free radicals , methacrylates > Styrene > acrylate. However , this
sequence also depends on the structure of the capture agent and the mechanism
with different CRP changes .



Each CRP system has its own advantages and disadvantages. The
following will be discussed



1.2.3.1 SFRP



SFRP in the radical generated by the reversible dormant species
captured , after a period of inactive state by activation of the thermal
initiator and return spontaneously form . The most successful capture species
include nitrooxy (NMP) and the metal radical (SFRP usually paramagnetic CoII).
However , the compound need not necessarily be the capture of free radicals,
can be a non- paramagnetic metal , or an organic species such as a phosphite or
thione [ 32,33 ] . The activation process is typically a typical thermal
initiator , a photoinitiator may also (e.g., dithiocarbamate ) . For SFRP two
initiator system . Using a first pre-dormant species , such as alkoxy , or a
so-called single-molecule amine initiator . Another use of binary systems ,
comprising a radical scavenger, and free radicals can be produced by various
methods , such as peroxides, azide compounds , γ- rays. The second initiator
system used in the synthesis rather than a homopolymer block copolymer.



Advantage over ATRP, SFRP include : organic systems can be applied
to pure (NMP), for a variety of monomers ( including acidic monomers ) . The
disadvantage is that telogen usually more expensive, and a stoichiometric
amount of the polymer is usually related to the number of chains . Type telogen
required in large-scale use of these , and even toxic transition metal
compound. Substituted olefins , such as methyl acrylate monomer polymerization
, difficult control. Introduction of functional end groups in the polymer more
difficult . The polymerization is generally carried out at higher temperatures
needed .



1.2.3.2 ATRP



ATRP and SFRP comply with stable free radical effects. However,
since ATRP is a bimolecular reaction activation process , the dormant species
which is very stable , and only when there is a transition metal catalyst to
activation. According ATRP reaction rate constants and equilibrium LMnt and
LMXnt1 + - to determine the ratio of the concentration . The principle of the
selection of catalysts have also been developed to enable them to order the
matching system or aggregate , to obtain the best control . SFRP and ATRP ,
with the conversion rate , the chain length increases , the reaction was
terminated , the coefficient gradually decreases. Because after an initial
period , only the presence of long-chain , which terminates the reaction rate
is slower than a lot of short-chain and initiator radicals . The catalyst
should work through a single electron transfer process , and ideally, should
not extensively involved β-H elimination reaction or formation of metal organic
species.



The initial CRP system , including ordinary ATRP and reverse ATRP,
ordinary ATRP transition metal suitable halogenated alkanes and low oxidation
state complexes began to reverse ATRP using high oxidation state transition
metal complexes with radical initiator starts. Although the initial oxidation
of residual air and is not sensitive to contaminants , but the technology
behind such SFRP binary system can be used to synthesize a block copolymer.
Normal and reverse initiator used to prepare block copolymers , but will form
some homopolymer.



ATRP technical advantages are: the amount of the transition metal
complex catalyst level ; including multi-functional system including a variety
of mixed initiators are commercially available ; unprotected acid addition ,
there are a large number of monomers which can be polymerized ; functional end
group is very simple ; no Trommsdorf effect ; temperature over a wide range of
applications ; may be in any order the preparation of block copolymers , which
are other CRP can not do. Its main drawback is the need to remove the product
after polymerization transition metal complexes , in addition to the need to
protect the polymerization of acidic monomers.



1.2.3.3 RAFT



Compared with the mass RP, CRP DT change in the least. Depending
on the rate of polymerization initiator is usually the square root of the value
of the initial concentration . The degree of polymerization and concentration
of the available transfer consumed monomer concentration ratio is defined . DT
system uses the atom / group transfer or addition - fragmentation chemistry.
For the latter, RAFT system may be involved in some of the intermediate radical
side reactions , such as crosslinked termination. Select the appropriate
transfer agent for a particular monomer is very important. Weak transfer
agents, such as dithiocarbamates, vinyl acetate and the polymerization is
effective , but can not control the RAFT polymerization of styrene or MMA .
Strong transfer agents, such as dithiobenzoate is effective for polymerization
of MMA , but the delay effect of the polymerization of styrene and acrylic
monomers . Equally important is the structure of the leaving group R , has a
strong impact on the initiator efficiency, but also potentially affect the
polymerization cycle. Some of the monomer or initiator functional groups
incompatible with some of the reagents DT . For example, in iodide transfer
radical polymerization (iodide transfer radical polymerization, ITRP) dioxane
may be converted into quaternary amine , primary amine groups dithio compound
decomposes .



DT initiator system includes a transfer agent and a free radical .
In some cases , the precursor can be used as a transfer agent, such as an
iodide ( for reverse iodide radical polymerization ) or dioxane, di tellurium
compounds ( for telomer radical polymerization , tellurium mediated radical
polymerization, TERP) [ 34-36 ] . DT new continuous process requires a radical
initiator , to produce a new polymer chain . However, the DT system , it is
impossible to obtain pure block copolymers. Compared with SFRP and ATRP, when
generating a new free radical initiator or a short chain , long chain polymer
chain growth occurs more easily crosslinked terminated. DT , the polydispersity
of the concentration is not affected by the transfer agent , depending on the
ratio between the simple exchange reaction and the reaction rate constant
between the growth , however, the polydispersity of the polymer can be reduced
by adding to the monomer mixture was added slowly approach.



It is not clear the extent Trommsdorf effects occur in DT system.
In conventional free radical polymerization with a similar system , with the
termination of the conversion coefficient decreases, increasing the reaction
temperature , to accelerate the decomposition of the radical -induced , will
accelerate the polymerization . However, the small amount of initiator radical
, may be small number Trommsdorf effect . In DT system, a steady-state
concentration of free radicals caused by / termination is established, and
follow the system PRE contrary , as SFRP, ATRP, these systems through
activation / deactivation establish balance.



DT system is suitable for a wide range of monomers , varying the
kinetics of the RP technique is minimal , the system is usually an organic
reagent with pure poly- tune DT which is an important advantage of the system ,
but many commercially easily transfer agent and a poor stability due to its
color, taste and toxicity , and ultimately want two thioester removed.
Functional end groups have a certain degree of difficulty .



1.2.3.4 Contrast Among SFRPATRP
&
DT



Difficult to provide an absolute method to assess these three
techniques , and explain what is the most effective. From the economic viewpoint
, the three substantially no difference . In a suitable size, cost three
techniques mainly by lower product cost impact corresponding special monomers.



The
view from the particularity of the structure and process of the target
compounds to compare these three systems . 2002 compared to the version shown
on the left in Figure 1.5 . The figure trying to compare an integrated area of NMP,
ATRP and RAFT. The area associated with the synthesis of the following types of
polymer: high molecular weight polymer (HMW), low molecular weight polymer
(LMW), end -functional polymers (End Funct), the block copolymer (Blocks), and
also with range polymerizable monomer (Mon Range), the synthesis of various
composites (Hybrids), the environment (Env) , and the possibility of
polymerization in water (water) , and other related .
Pic1.5(right) 2006 is an upgraded
version . Since each of the relevant phenomena and mechanisms of in-depth
understanding of polymerization technology , compared with the old version of
2002 , the area of the three technologies have made progress [ 37 ] .



Pic
1.5 ATRPNMPRAFT
Schematic
comparison



High molecular weight polymer : only a few monomers using
conventional ATRP , such as DMAEMA, can be used for the preparation of high
molecular weight linear polymer [ 38 ] , the majority of high molecular weight
polymer is difficult to prepare a monomer ATRP method , because outer electron
transfer processes , including the oxidation or reduction of free radicals and
β-H elimination reaction . But with a low concentration of Cu catalyst ARGET
(Activators Regenerated by Electron Transfer) and ICAR ATRP (Initiators for
Continuous Activator Regeneration ATRP) system, you can be prepared over a
linear polymer molecular weight 200000g/mol [39 ] . Further , in NMP and
bifunctional ATRP initiator , either by free radical coupling reaction can be
increased to maintain the functionality and molecular weight [ 40 ] . Using the
brush type polymer backbone chain synthetic polymer having a molecular weight
of a plurality of raised positions is a very effective method . In fact , with
a multifunctional ATRP initiators can be prepared over 10000000g/mol has
molecular weight polymers [ 41,42 ] .



Low molecular weight polymer : low molecular weight polymer was
prepared using the RAFT was very difficult because there is a strong delayed
response RAFT system. But now by selecting the appropriate RAFT agents have to
solve this problem . For all CRP technology, low molecular weight polymers synthesis
, the cost of the end groups remains a major problem.



End group functionalized polymers : You have to try some new ways
to promote the use of end group functionalized NMP and RAFT system. For
example, in RAFT system can use two thioester substitution reaction and
reduction reaction of a thiol [ 43-45 ] . In NMP by controlling maleic
anhydride and maleic imide derivatives of mono -substituted alkoxy side chain
amines may be introduced to a wide variety of functional groups [ 46 ] .



Block copolymer : a block copolymer by adding
a small amount of NMP in the copolymer can be improved. For ATRP system ,
although a deeper understanding of halogen exchange , but still can not be used
for new ARGET and ICAR system, although these systems do not need to use too
much Cu catalyst.



 



Polymerizable monomers range : CRP in all techniques , the
polymerizable monomer ranges are significantly broadened. In NMP system when
added in a small amount of styrene , the polymerization of MMA can be
controlled very well [ 47 ] . In ATRP , the vinyl chloride , vinyl acetate, and
some acidic monomers can now control the polymerization . Use of new ligands
that ATRP polymerizable vinyl ketone, maleic anhydride or a diene monomer such
as [ 48-50 ] .



Composites: In recent years, organic - inorganic composite
materials and composite materials in the field of biological research hot. In
many cases the ATRP method is more commonly used as surface active alkyl halide
groups , it is easy functionalization . But more inorganic base and start using
biomolecules alkoxy amines and di -functional thioester , respectively, thus
opening a method for preparing RAFT NMP and composites . The inorganic surface
, generated by a strong molecular brush branched chain , the resulting
composite material to prevent corrosion, having a novel lubrication properties
, and in certain applications when they are not compressed, but if the branched
low density when the polymer material , it will be squashed. CRP Technology use
synthetic materials can be used for efficient drug delivery and biological
tissue and bone bionics engineering components [ 51-53 ] preparation .



Environmental issues : For ATRP, the application of new and ICAR
ARGET initiator system can greatly reduce the required amount of catalyst , with
obvious environmental friendliness . Removing dithio esters and xanthates from
RAFT / MADIX system products is feasible. However, should the system is clean
alkoxy amine stabilizer NMP as a polymerization system.



Environmental
problem is not related to the material and manufacturing process , and also has
a relationship with the material itself, the impact on the environment. CRP all
current techniques can be used to prepare non-ionic surfactants and dispersing
agents, to increase the efficiency and effectiveness of the use of these
materials can also be used for the preparation does not require a plasticizer
from plasticized molding material , so as to reduce environmental burden [
54-57 ] .



Aqueous polymerization : Water polymerization has
been successfully implemented [ 58,59 ] under homogeneous and heterogeneous
conditions . Initially, only the dispersion and the miniemulsion process is
valid , genuine emulsion will not work , because the presence of a
telomerization emulsion through the aqueous phase species transmission problems
. However, RAFT, NMP may be reacted with a surfactant and the microemulsion as
a seed emulsion polymerization , can alleviate the problems [ 60 ] . Further ,
the aqueous emulsion polymerization of the monomers in the continuous organic
phase of the reverse is also successful, the gel can be used to prepare nano-
reversible , and is expected to be used as drug delivery carriers of biological
macromolecules [ 61 ] .



1.2.3.5 CRP
Future
research directions



Use
of controlled / living polymerization of the molecular structure requires
precise control of chain termination suppression . These CRP newly developed
technologies have been able to synthesize new materials with special purposes
and to help establish a linear relationship between molecular structure and
macroscopic properties of the micro- between . CRP is currently the fastest
growing polymer synthetic chemistry , the following reasons: there are plenty
of polymerizable monomer , the reaction device is simple , less demanding on
the reaction conditions , so radical polymerization controllable , CRP
technical development of products has a huge market potential. However, more
research should reach its maximum potential, still needs various fields of CRP
[ 26 ] .



CRP
on the mechanism and kinetics of the process of basic research is still
necessary . NMP -depth understanding of the system , the structure - activity
relationships of the reaction can be controlled so that the polymerization of
methyl acrylate to obtain satisfactory results. Dithio esters may be suitably
selected reducing agent RAFT polymerization delayed effect . Further
understanding of the impact of the ATRP catalyst structure contribute to the
development of more effective complexes, less the amount of it , which can make
the environmental impact is minimized ATRP chemistry , but also to extend the
range of the polymerizable monomers to acrylic acid and α- methyl- olefins.
ATRP , and SFRP applied several transition metal complexes , may also be used
for coordination polymerization . This will be developed into an effective
polar monomer to the polyolefin backbone route .



A typical low
molecular weight and oligomeric reaction to quantify the impact of chain
termination reactions . To accelerate the polymerization can evaluate the
microstructure and provides enhanced ( stereoregularity and sequence ) control
additives. They can increase the selectivity and chemical stereoselective ,
which radical polymerization is difficult.



1.2.4 Industrial applications CRP



Development of controlled radical polymerization initial impetus
comes from academic research and application of new structural and functional
polymers. Its applications include coatings, adhesives , non-ionic surface
active agent , a dispersing agent , a thermoplastic rubber , bulk material
properties , film , personal care products , detergents, amphiphilic block
copolymers , gels , lubricants, agents , surface modifiers , composite
materials, biological and electronic materials.



CRP has a very bright future , many new products based on
estimates of the technology will enter the market in the future years , the
average annual value of these materials is expected to reach $ 20 billion ,
roughly equivalent to the material prepared by conventional free radical polymerization
of 10% [ 26 ] . Of course, to achieve this is expected , there is still a long
way to go , but also polymer synthesis, the joint efforts of polymer physics,
polymer engineering and marketing personnel.



DuPont is one of the earliest use of special coatings sector CRP
technology company, currently has a variety of commercial products based on CRP
technology can be used in paints , coatings and inks in the field [ 62 ] . Its
use of technology to create a variety of CRP commercial products totaling several
million euros in the market , and is still growing year by year . DuPont and
other companies in the most important commercial products are block copolymers,
diblock and triblock has been successfully applied in many fields. DuPont has
devoted substantial efforts in research and development of CRP , expect to see
more in the next few years based on this synthesis technology.



IBM
CRP synthesized using the block copolymer , and the ability to study the
self-assembling block copolymers , manufactured by nano- technology to
manufacture a silicon memory chips, this idea can be extended to the surface
modified nano- template and brought the new method [ 63 ] .



Ciba
technology makes use of NMP and ATRP and other monomers copolymerizable
macromonomer amphiphilic graft copolymers , comb copolymer can be a specific
structure . CRP first product based on technology is acrylic block copolymer as
in EFCA Release 2004 , the product can be used in coatings provide excellent
rheological properties and improve stability of the paste [ 64 ] .

RohMax Oil Additives is a subsidiary of Degussa , the long chain poly alkyl
methacrylate is the main ingredient used in lubricants , the company producing
the product use ATRP technique and studied the commercial viability and
economically acceptable preparation conditions . Degussa also studied the
preparation of block copolymers , and the possibility of removing trace amounts
of the commercial catalyst [ 65,66 ] .



PPG
after research indicated the material prepared by ATRP polymerization manner
other than by having more benefits , polymer molecular weight can be controlled
to obtain a narrow molecular weight distribution. PPG also noted that ATRP can
control the composition , function and structure of the polymer. It can form
complex structures, such as block , gradient, comb and star polymers , and
evaluated as a component of their various properties of the coating material [
67 ] . PPG introduced TiO2 treated glass products have similar features and
pollution prevention , self- cleaning properties, can be used for office and
hotel buildings. Heat oil ATRP manufactured material is also an ideal material
for the manufacture of various projects against liquid seal .



BYK use of controlled polymerization
techniques developed coatings and plastics additives, is the first technology
company to aid the controlled products allegedly have special plants in Europe
, the device can be adapted to a variety of commercial production of CRP
Technology , DISPERBYK-2000 series has the advantage of narrow molecular weight
distribution , has a plurality of brands of products available [ 68 ] .



Kaneka
announced that it has owned a large leading device manufacturers to
commercialize the product , and is building a full-scale plant to produce
reactive use of ATRP telechelic materials. Its products include a range of
acrylic sealants and adhesives used directly in the market , the main advantage
of the product is high temperature resistant , anti- oil and anti- UV . The
environmental advantages of stable performance of the material used for the
pollution they have. When the sealant prepared by ATRP for the exterior walls
of marble , with obvious dirt capacity [ 69 ] .



Arkema
is Atofina and Dionex 's predecessor , is currently considering commercial CRP
technology. Arkema has developed a new freedom of tone stable polymerization
inhibitor for CRP polymerization of acrylic monomers for the preparation of
high -solids coating resins , the company also plans to launch a block
copolymer based on acrylic and methacrylic monomers used as a plasticizer .



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