Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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Macromolecular synthesis background
1 – Basic definitions
polymer
: substance composed of
macromolecules
which
have long sequences of one or more species of atoms or
groups of atoms linked to each other by primary, usually
covalent, bonds.
polymer and macromolecules are used interchangeably,
but the latter strictly defines the molecules of which the former is
composed.
macromolecules : formed by linking together
monomer
molecules
through chemical reactions, the process by which
this is achieved being known as
polymerization
.
a) skeletal structure
linear
: a chain with two ends
non-linear
:
!
branched polymers
: with side
chains or
branches
of significant length
which are bonded to the main chain at
branch points
(or
junction points
)
!
network polymers
: with three-
dimensional structures in which each
chain is connected to all others by a
sequence of junction points and other
chains :
" said to be
crosslinked
"
characterized by their
crosslink density
or
degree
of crosslinking
(number of junction points per unit
volume)
"
formed by polymerization or by linking together
pre-existing linear chains (i.e.
crosslinking
)
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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b) homopolymer and copolymer
homopolymer
: polymer derived from one species of
monomer :
-A-A-A-A-A-A-
or
-[A]
n
-
where n is the number of
repeat units
(or
monomer units
)
linked together
copolymer
: polymer derived from more than one species
of monomer :
!
statistical copolymers
: in which the sequential
distribution of the repeat units obeys known statistical laws
!
random copolymers
: a special type of statistical
copolymer in which the distribution of repeat units is truly
random
!
alternating copolymers
: only two different types of
repeat unit arranged alternately along the polymer chain
!
block copolymers
: linear copolymers in which the
repeat units exist only in long sequences, or blocks, of the
same type :
-A-A-A-A-A-A-A-A
-B-B-B-B-B-B-B-B-B-
: AB di-block copolymer
-A-A-A-A-A
-B-B-B-B-B-B-B-
A-A-A-A-
: ABA tri-block copolymer
!
graft copolymers
: branched polymers in which the
branches have a different chemical structure to that of the main
chain
"
statistical, random and alternating copolymers :
properties intermediate to those of the corresponding
homopolymers
"
block and graft copolymers : properties characteristic of
each of the constituent homopolymers
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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c) thermoplastics, elastomers and thermosets
Thermoplastics
(or "plastics") : linear or branched polymers
which can be melted upon the application of heat :
"
can be molded (and remolded) into virtually any shape
using processing techniques (injection molding or
extrusion)
"
constitute by far the largest proportion of the polymers
used in industry
"
semi-crystalline
materials with both crystalline and
amorphous regions, characterized by their
degree of
crystallinity
"
crystalline phase with
melting temperature T
m
"
amorphous phase (bowl of spaghetti) with
glass
transition temperature T
g
: abrupt transformation from
glassy state (hard) to rubbery state (soft)
corresponding to the onset of chain motion
Elastomers
: rubbery polymers of low crosslink density
"
easily stretched to high extension (e.g. 3x to 10x their
original dimensions)
"
rapidly recover their original dimensions
Thermosets
: rigid network polymers in which chain motion is
greatly restricted by a high degree of crosslinking
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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d) molar mass and degree of polymerization
molar mass M
: mass of 1 mole of the polymer (g.mol
-1
)
" for network polymers the molar mass is infinite
degree of polymerization x
: number of repeat units in the
polymer chain
"M = x M
0
where M
0
is the molar mass of the repeat unit
polydispersity
: polymers consist of macromolecules with
a range of molar masses and for long chains, the molar mass
distribution can be assumed to be continuous and is
characterized in terms of
molar mass averages
M
i
/ g.mol
-1
M
n
M
w
weight
fraction
"
number-average molar mass
:
∑
∑
=
i
i
i
i
i
n
n
M
n
M
"
weight-average molar mass
:
∑
∑
=
i
i
i
i
2
i
i
w
M
n
M
n
M
by considering the discontinuous nature of the distribution in
which the macromolecules exist in discrete fractions i (in
intervals of M
0
) containing n
i
molecules of molar mass M
i
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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"
polydispersity index
:
1
M
M
I
n
w
≥
=
and I = 1.00 in the case of a perfectly monodisperse polymer
"
number-average degree of polymerization
:
0
n
n
M
M
x
=
"
weight-average degree of polymerization
:
0
w
w
M
M
x
=
2 – Classification of polymerization reactions
the most basic requirement : each monomer must be
capable of being linked to two (or more) other monomer by
chemical reaction, i.e. monomers must have a
functionality
of
two (or higher)
" a multitude of chemical reactions and associated
monomer types can be used to effect polymerization
classification : based on the polymerization mechanisms :
!
step polymerization
(or step-growth polymerization) in
which the polymer chains grow step-wise by reactions that can
occur between any two molecular species
!
chain polymerization
(or chain-growth polymerization)
in which a polymer chain grows only by reaction of monomer
with a reactive end-group on the growing chain. They usually
require an initial reaction between the monomer and an
initiator
to start the growth of the chain
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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step polymerization
chain polymerization
first step
none
initiation
:
I + o
→
I-o*
initiator
active
centre
steadily throughout the polymerization
dimer
formation
o + o
→
o-o
I-o* + o
→
I-o-o*
trimer
formation
o + o-o
→
o-o-o
I-o-o* + o
→
I-o-o-o*
tetramer
formation
o + o-o-o
→
o-o-o-o
o-o + o-o
→
o-o-o-o
I-o-o-o* + o
→
I-o-o-o-o*
pentamer
formation
o + o-o-o-o
→
o-o-o-o-o
o-o-o + o-o
→
o-o-o-o-o
I-o-o-o-o* + o
→
I-o-o-o-o-o*
growing
chain
principle
reactions can occur between any
two molecular species
consequent upon every addition of
monomer, the active centre is transferred
to the newly-created chain end
monomer
consumption
rapidly in the early stages (e.g.
when
n
x = 10, less than 1% of the
monomer remains unreacted)
steadily throughout the reaction
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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step polymerization
chain polymerization
polymer
formation
the degree of polymerization
increases steadily throughout the
reaction, but large macromolecules
are obtained for very high extents of
reaction
high degrees of polymerization are
attained at low monomer conversion,
because at any moment the number of
growing macromolecules is low, but as
soon as the active centre is created, it
reacts in few minutes with several
thousands of monomer molecules before it
dies through a
termination
reaction.
polymer
molar mass
evolution as
a function of
reaction
extent
0
50
100
extent of reaction (%)
molar mass
0
50
100
extent of reaction (%)
molar mass
Table
:
Fundamental differences in reaction mechanism between step polymerization
and chain polymerization
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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3 – Step polymerization
" involve successive reactions between pairs of mutually-
reactive functional groups which initially are provided by
the monomer(s).
" consider the reaction between terephthalic acid and
ethylene glycol, both of which are
difunctional
:
+ H
2
O
+ HOCH
2
CH
2
OH
C
O
OH
C
O
HO
C
O
OCH
2
CH
2
OH
C
O
HO
dimer
and later :
C
O
OCH
2
CH
2
O
C
O
HO
H
C
O
OH
C
O
HO
+ n HOCH
2
CH
2
OH
+ (2n-1) H
2
O
n
n
poly(ethylene terephthalate) or PET
" polymerizations involving monomers of functionality
greater than two produce non-linear polymers : if a
trifunctional
monomer was included, reaction at each of
the three functional groups would lead initially to the
formation of a branched polymer but ultimately to the
formation of a network (three-dimensional
macromolecules) :
+ HOCH
2
CHOHCH
2
OH
C
O
OH
C
O
HO
terephthalic acid
glycerol
C
O
C
O
O
O
O
CH
2
CH
2
CH
O
C
O
C
O
C
C
O
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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a) polycondensation and polyaddition
Polycondensation
: step polymerization involving reactions in
which small molecules are eliminated (ex : polyamides,
polyesters…).
" 3 types of monomer system exist, where R and R' are
divalent groups and A and B represent the mutually-
reactive functional groups :
!
RA
2
+ R'B
2
step polymerization
:
A-R-A + B-R’-B
→
A-R-AB-R’-B
→
....
→
A-(R-AB-R’)
n
-B
H
2
N (CH
2
)
6
NH
2
n
+ n
HOOC (CH
2
)
4
COOH
hexamethylene diamine
adipic acid
(CH
2
)
6
NH
NH
H
C
O
(CH
2
)
4
C
O
OH
n
+ (2n-1) H
2
O
polyamide-6,6 or nylon-6,6
ex :
" drawback : very slight excesses of one monomer
significantly reduce the attainable degree of
polymerization because the polymer chains become
terminated with functional groups derived from the
monomer present in excess (e.g. both end-groups are
ultimately of type B if RB
2
is in excess). Since these
functional groups are unreactive towards each other,
further growth of the chains is not possible.
!
ARB step polymerization
:
A-R-B + A-R-B
→
A-R-BA-R-B
→
....
→
A-(R-BA)
n
-R-B
ex :
polyamide-6 or nylon-6
+ (n-1) H
2
O
n
(CH
2
)
5
NH
H
OH
O
C
6-amino hexanoic acid
n
H
2
N (CH
2
)
5
COOH
" advantage : with each condensation reaction the
polymer chain grows but remains an
ω
-amino carboxylic
acid and so can react further. So an exact stoichiometric
equivalence of the two functional groups is guaranteed.
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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!
RA
2
step polymerization
:
A-R-A + A-R-A
→
A-R-A-R-A
→
....
→
A-(R-A)
n
-R-A
" few examples :
n HO-R-OH
→
H-[O-R]
n
-OH + (n-1) H
2
O
n Cl-Si(CH
3
)
2
-Cl + (n+1) H
2
O
→
H-[O-Si(CH
3
)
2
]
n
-OH + 2n HCl
Polyadditions
: step polymerizations in which the monomers
react together without the elimination of other molecules
" few important examples (RA
2
+ R'B
2
reaction) :
n O=C=N-R-N=C=O + n HO-R'-OH
→
-[C(O)-NH-R-NH-C(O)-O-R'-O]
n
-
polyurethane
n O=C=N-R-N=C=O + n H
2
N-R'-NH
2
→
-[C(O)-NH-R-NH-C(O)-NH-R'-NH]
n
-
polyurea
b) molar mass control for linear step polymerization
Theory based on :
principle of equal reactivity of functional
groups
whatever the size of the reacting molecules
" prediction of the molar mass of macromolecules at time t
as a function of the
extent of reaction p
in the case of
an exact stoichiometric balance in the numbers of
mutually-reactive functional groups :
p
1
1
x
n
−
=
and
p
1
M
M
I
n
w
+
=
=
where
initially
present
groups
functional
of
number
reacted
have
that
groups
functional
of
number
p
=
" very high extents are needed for producing high
polymers
" necessity for using monomers of high purity and
reactions which are either highly efficient or can be
forced towards completion
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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" in practice, slight stoichiometric imbalances are used to
control
n
x
" another way for limiting molar masses : the addition of
very low quantities of a monofunctional monomer, which
will prematurely stop the growth of macromolecules by
creating some unreacting chain ends
Important drawback : the intramolecular reaction of terminal
functional groups on the same molecule leading to
ring
formation
(i.e. cyclic molecules)
" disturbs the form of the molar mass distribution and
reduces the ultimate molar mass attainable
" polymerization
in bulk
(i.e. using only monomer(s) plus
catalysts)
c) gelation during non-linear step polymerization
The inclusion of a monomer with a functionality greater than
2 has a dramatic effect upon the structure and molar mass of
the polymer formed
" in the early stages : branched macromolecules and
much more rapid increase of molar mass
" as the reaction proceeds, further branching reactions
lead to the first network molecule : the
gel-point
manifested by
gelation
"
critical extent of reaction p
C
for gelation always in the
case of a stoichiometric balance in the numbers of
mutually-reactive functional groups :
f
2
p
C
=
with
∑
∑
=
i
i
i
i
i
n
f
n
f
where f is the number-average functionality (n
i
is the
initial number of molecules of monomer i which has
functionality f
i
)
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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4 – Chain polymerization : free-radical polymerization
Free-radicals
: independently-existing species which
possess an unpaired electron and normally are highly reactive
with short life times
Free-radical polymerization
: chain polymerization in
which each polymer molecule grows by addition of monomer to
a terminal free-radical reactive site (
active centre
).
a) initiation, propagation and termination
Initiation stage
:
! the formation of free radicals from an initiator (e.g. by
homolysis of a single bond) :
ex :
C
O
O O C
O
∆
2
C
O
O
benzoyl peroxide
benzoyloxy radicals
! the addition of one of these free radicals to a molecule of
monomer. For vinyl monomers, the active centre is created
when the free radical attacks the
π
-bond :
H
2
C CH
Y
R +
A
R
CH
2
CH
Y
R
CH CH
2
R
Y
or
k
d
Propagation stage
: growth of the polymer chain by rapid
sequential addition of monomer to the active centre (1
millisecond for each monomer addition)
+
CH
2
CH
Y
R
H
2
C CH
Y
or
CH
2
CH
Y
R
CH
2
CH
Y
CH
2
CH
Y
R
CH CH
2
Y
head-to-tail addition
head-to-head addition
k
p
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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Termination stage
: two common mechanisms taking place
simultaneously, but to different extents depending upon the
monomer and the polymerization conditions :
!
combination
:
CH
2
CH
Y
CH
2
HC
Y
+
CH
2
CH
Y
Y
CH CH
2
k
tc
!
disproportionation
:
+
CH
HC
Y H
CH
2
CH
Y
k
td
+
CH
HC
Y
CH
2
CH
2
Y
b) rate and degree of polymerization
!
!
!
!
steady-state conditions
: rapidly, the rate of radical loss
exactly equals the rate of radical formation
!
!
!
!
rate of polymerization R
p
:
½
[M][I]
k
fk
k
R
t
d
p
p
=
where k
p
, k
d
and k
t
are the rate constants for propagation,
initiator dissociation and termination respectively and f is the
initiator efficiency (i.e. the fraction of primary free radicals that
successfully initiate polymerization)
!
!
!
! number-average degree of polymerization of the
polymer produced at time t :
½
[I]
k
fk
q)
(1
[M]
k
time
unit
in
formed
polymer
of
moles
time
unit
in
consumed
monomer
of
moles
x
t
d
p
n
+
=
=
where q is the fraction of termination reactions proceeding by
disproportionation
" by increasing [M] :
n
x and R
p
increase
" increasing [I] gives rise to a reduction in
n
x
whilst
causing an increase in R
p
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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c) chain transfer reactions
-T + A
ktr
+ T-A
where T and A are fragments linked in a single bond in a
hypothetical molecule T-A (monomer, initiator, solvent and/or
macromolecules)
" if re-initiation is rapid the rate of polymerization is not
affected and under steady-state conditions :
[M]
k
A]
-
[T
k
[M]
k
]
I
[
k
fk
q)
(1
x
1
p
tr
p
2
1
t
d
n
+
+
=
"
chain transfer agents
: compounds with high chain
transfer constants (e.g. carbon tetrabromide, dodecyl
mercaptan…) may be employed at low concentrations to
control (reduce) molar mass
chain transfer to polymer : no effect upon
n
x
but results in the
formation of branched polymer molecules
" intramolecular reactions : short-chain branches :
CH
H
H
2
C
CH
2
CH
2
CH
2
CH
H
2
C
CH
2
CH
2
CH
3
+ n H
2
C=CH
2
branch
n-butyl
+ H
2
C=CH
2
CH
CH
2
CH
2
CH
2
CH CH
2
CH
3
H
CH
CH
2
CH
3
CH
2
CH CH
2
CH
3
+ n H
2
C=CH
2
branches
ethyl
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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" intermolecular reactions : long-chain branches :
CH
2
CH
COOCH
3
+
C
CH
2
COOCH
3
H
C
CH
2
COOCH
3
+
CH
2
CH
2
COOCH
3
+ n
CH
2
=CHCOOCH
3
COOCH
3
CH
2
C
COOCH
3
CH
CH
2
d) bulk polymerization and its alternatives
Bulk polymerization
is the simplest and involves only the
monomer and a monomer-soluble initiator
" advantages : high rates of polymerization, high degrees
of polymerization and polymer of high purity
" drawbacks : rapid increase of viscosity, inefficient
stirring, difficult removing of the heat evolved upon
polymerization
"
autoacceleration
or
Tromsdorf-Norrish effect
:
increasing of R
p
and molar mass due to the reduction of
the long-chain radical mobility and therefore to their
termination probability ; the initiation and propagation
reactions are not affected because the monomer
molecules are small and more mobile
Solution polymerization
in a solvent
" advantages : low viscosity, good heat transfer and low
likelihood of autoacceleration
" drawbacks : laborious isolation of the polymer
(evaporation or precipitation), solvent toxicity and hazard
" commercial use restricted for applications which require
the polymer to be used in solution
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
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Suspension polymerization
: the reaction mixture is
suspended as droplets in water (where it is insoluble)
"
necessitates (i) vigorous agitation throughout the
reaction and (ii)
dispersion stabilizers
dissolved in the
aqueous phase [typically low molar polymers such as
poly(vinyl alcohol)].
" advantages : low viscosity and good heat transfer
" each droplets acts as a small bulk polymerization reactor
for which the normal kinetics apply
" polymer in the form of beads (typically 0.1-2 mm
diameter) easily isolated by filtration
" widely used on an industrial scale
e) emulsion polymerization
Second heterogeneous process where surfactants are used
and initiator must not be soluble in monomer but soluble only in
the aqueous dispersion medium
"
reaction product : colloidally-stable dispersion of
particulate polymer (0.05-1 µm) in water known as
latex
"
anionic surfactants
: molecules with hydrophobic
hydrocarbon chains at one end of which is a hydrophilic
anionic head group and its associated counter-ion (e.g.
sodium lauryl sulfate : CH
3
-(CH
2
)-SO
4
-
,Na
+
)
"
critical micelle concentration CMC
: above which
surfactant molecules form into spherical aggregates (5
nm) known as
micelles
which contain of the order of 100
molecules
" when a water-insoluble monomer is added to an
aqueous solution containing a surfactant well above its
CMC three phases are established :
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dissolved
free radical
surfactant
molecule
monomer
molecularly
dissolved
monomer-
swollen
micelle
R
large droplet
of monomer
as reservoir
" initiation : in the aqueous phase, the initiator molecules
(e.g. persulfate :
•
∆
−
→
4
2
8
2
O
S
2
O
S
) react with monomer to
produce oligomeric radical species which then diffuse
into monomer-swollen micelles to initiate polymerization
" propagation within the micelles is supported by
absorption of monomer from the aqueous phase, there
being concurrent diffusion of monomer droplets into the
aqueous phase to maintain equilibrium
" interval $ : particle nucleation with the consumption of
micelles ; the number N
p
of latex particles per unit
volume of latex then remains constant
time
conversion (%)
0
100
$
$
$
$
%
%
%
%
&
&
&
&
" interval % : the rate of monomer diffusion exceeds the
rate of polymerization so that the concentration [M]
p
of
Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1
2 - 18
monomer within a particle remains constant. Since N
p
is
constant, the rate of polymerization also is constant.
Because the particles are very small, termination can be
considered to occur immediately upon entry of a second
radical species into a particle containing a single
propagating chain radical. The particle then remains
dormant until entry of another radical initiates the
propagation of a new chain radical :
a
p
p
p
p
N
2
N
]
M
[
k
R
=
and
a
i
p
p
p
n
N
N
]
M
[
k
x
ρ
=
where N
a
is the Avogadro constant and
ρ
i
is the molar
rate of formation of radical species from the initiator
" interval & : [M]
p
and the rate of polymerization decrease
continuously as the remaining monomer present in the
particles is polymerized
" advantages : good heat transfer, low viscosity of the
product latexes at high polymer concentrations and the
ability to control particle morphology (e.g. formation of
core-shell particle
structures by successive additions of
different monomers)
" drawback : contamination by the surfactant
" polymers used either directly in the latex form (e.g.
emulsion paints, adhesives, foamed carpet-backings) or
after isolation by
coagulation
or
spray-drying
of the
latex (e.g. synthetic rubber and thermoplastics)
! bibliography
' Introduction to Polymers by R.J. Young and P.A. Lovell –
Chapman & Hall (1991)