Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1

2 - 1

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

2 - 2

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

2 - 5

"

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

2 - 8

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

2 - 9

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

2 - 10

!

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

2 - 11

" 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

2 - 12

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

2 - 13

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

2 - 15

" 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

2 - 16

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 :

Introduction to Hybrid Organic-Inorganic Materials / Etienne Duguet / university Bordeaux-1

2 - 17

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)