polysiloxanes, synthesis and properties

1. Lecture No. 04
Course title:
Inorganic Polymers
Topic: Polysiloxanes
Course instructor: Dr. Salma Amir
GFCW Peshawar

2. Polysiloxanes (silicones)
 Polysiloxanes are the most important class of inorganic polymers.
Unlike most other polymers, silicones possess a fully inorganic
backbone of -(Si-O)- repeat units.
 The name silicone usually refers to linear polymers with a silicon-
oxygen backbone (-Si-O-) and with alkyl side groups.
 The most important polyalkylsiloxane possesses only methyl side
groups, called polydimethyl siloxane and often abbreviated as PDMS.
 General structure -[Si(R2)-O]- where R = -CH3 is called poly(dimethyl
siloxane)

3. Preparation
Commercial silicones are often prepared by hydrolysis of chlorosilanes
in the presence of water. This reaction is exothermic and yields in a
first step silanols, which are condensed to linear and cyclic oligomers by
inter- and intramolecular condensation reactions.
 Preparation of Monomers
 Hydrolysis
 Polymerization

4. 1. Preparation of monomers
To prepare alkyl or aryl derivatives of silicon tetrachloride:
Examples of such derivatives are RSiCl3, R2SiCI2 and R3SiCl2 where R is an alkyl (e.g.,
CH3, C2H5 etc.) or aryl (e.g., C6H5) group.
 The elemental silicon on which the entire technology is based is typically obtained
by reduction of the mineral silica with carbon at high temperatures
SiO2 + C Si + 2CO
 The silicon is then converted directly to tetrachlorosilane by the reaction
Si + 2Cl2 SiCl4
 Alkyl chlorosilanes can also be obtained by the action of Grignard reagent on SiCl4
RMgCl + SiCl4 → RSiCl3 + MgCl2
2RMgCl + SiCl4 → R2SiCl2 + 2MgCl2
3RMgCI + SiCl4 → R3SiCI + 3MgCl2

5.  Method 2
“Direct Process” or “Rochow Process,”which starts from elemental silicon. It is
illustrated by the reaction
Si + 2RCl → R2 SiCl2
Methyl chlorosilanes like (CH3)SiCl3, (CH3)2SiCl2 and (CH3)3SiCl are prepared by
heating methyl chloride, CH3Cl with Si, catalyzed by Cu, at 300°C. This reaction
gives a mixture of methyl chlorosilanes.
2CH3Cl + Si Cu (CH3)SiCl3 + (CH3)2SiCl2 + (CH3)3SiCl
300 C
The yield of (CH3)2SiCl2 (b.p. = 69.6°C) is over 50%. Careful fractionation is used to
separate (CH3)2SiCI2, from (CH3)SiCl3 (b.p. = 66.9°C) and (CH3)3SiCl (b.p. = 87.7°C).
Compounds of formula R2SiCl2 are extremely important, because they provide
access to the preparation of a wide variety of substances having both organic and
inorganic character. Their hydrolysis gives dihydroxy structures which condense to
give the basic [-SiR2O-] repeat unit.

6. 2. Hydrolysis
To prepare alkyl or aryl hydroxy derivatives of silicon tetrachloride (called
silanols or silandiols):
Examples of such silanols are R Si (OH)3, R2Si(OH)2 and R3Si(OH). These silanols
are obtained by the hydrolysis of RSiCl3, R2SiCl2 and R3SiCl respectively.
RSiCl3 + 3HOH RSi(OH)3 + 3HCl
R2SiCl2 + 2HOH R2Si(OH)2 + 2HCl
R3SiCl + HOH R3Si(OH) + HCl
The general equation representing the hydrolysis reaction can be written as:
Chlorosilane + Water hydrolysis Silanols + HCl

7. 2. Polymerization
(condensation polymerization)
 To allow the alkyl or aryl hydroxy derivatives to undergo
polymerization: Polymerisation process involves removal of some H2O
molecules and leads to the formation of different types of silicones.
The type of silicone obtained depends on the nature of alkyl or aryl
hydroxyderivative and the way in which the hydroxy-derivative
undergoes polymerisation. For example:

8.  (a) When many molecules of alkyl trihydroxy-silane, RSi(OH)3 undergoes
polymerisation, a cross-linked two dimensional silicone is obtained.

9.  Since an active OH group is present at each end of the chain,
polymerisation continues on both the ends and hence the length of
the chain increases. The increase in the length of the chain produces
cross-linked silicone as shown below:

11.  (b) When many molecules of dialkyl dihydroxy-silane, R2Si(OH)2
undergo polymerisation, a straight chain (linear) or cyclic (ring)
silicone is obtained.

12.  Since an active OH group is present at each end of the chain,
polymerisation continues and hence the length of the chain increases
and gives rise to the formation of long chain silicon, as shown below:

13.  (c) When two molecules of trialkyl monohydroxy-silane, R3Si(OH)
undergo polymerisation, a straight chain silicone (dimer) is obtained.

14. Ring opening polymerization
 The hydrolysis approach to polysiloxane synthesis has now been largely
replaced by ring-opening polymerizations of organosilicon cyclic trimers
and tetramers, with the use of ionic initiation. These cyclic monomers
are produced by the hydrolysis of dimethyldichlorosilane. Under the
right conditions, at least 50 wt % of the products are cyclic oligomers.
The desired cyclic species are separated from the mixture for use in
ring-opening polymerizations such as those described below.

15.  Cyclic siloxanes can undergo a ring-opening polymerization that is a
chain-growth process. Free radicals are not useful as initiator species,
because of the nature of the siloxane bond, but anionic and cationic
initiators are very effective. The reaction is illustrated using the most
common cyclic oligomers, the trimer (hexamethylcyclotrisiloxane) or
the tetramer (octamethylcyclotetrasiloxane)
(SiR2O)3,4 [-SiR2O-]x
where R can be alkyl or aryl and x is the degree of polymerization
Ring-Opening Polymerizations

16. Properties of Polysiloxanes
1. Tailoring property
The methyl groups along the chain can be substituted by many other groups such
as ethyl, phenyl, or vinyl, which allows for tailoring the chemical, mechanical and
thermophysical properties for a wide variety of applications.
2. Surface tension
Polydimethysiloxanes have a low surface tension in the range of 20 to 25 N/m2
and consequently can wet most surfaces. With the methyl groups located on the
outside, silicones produce very hydrophobic films.
Unlike most other polymers, silicones possess an inorganic backbone of -(Si-O)-
repeat units. The Si-O bonds are strongly polarized and without side groups,
should lead to strong intermolecular interactions. However, the nonpolar methyl
groups shield the polar backbone. For this reason, silicone polymers have a very
low critical surface tension despite a very polar backbone. In fact, PDMS has one of
the lowest critical surface tension of all polymers which is comparable to that of
Teflon.

17. 3. Flexibility
Due to the low rotation barriers, most siloxanes are very flexible. For
example, the rotation energy around a CH2-CH2 bond in polyethylene is
about 12.1 kJ/mol but only 3.8 kJ/mol around a Me2Si-O bond,
corresponding to a nearly free rotation. This has far-reaching effects on
the thermal and mechanical properties of silicones. Furthermore, chain-
to-chain interaction is rather week due to the low cohesive energy, and
the distance between adjacent chains is noticeably larger in silicones
than in alkanes which also contributes to the greater flexibility of PDMS
4. Viscosity
Due to great flexibility of the chain backbone, the activation energy of
viscous flow is rather low and they do not become too viscous on
cooling, and the viscosity is less dependent on temperature compared to
hydrocarbon polymers.

18. 5. Stability
Due to the strong Si-O and Si-C bonds, silicone polymers have a very
high heat and oxidative stability and outstanding chemical resistance.
6. Diffusibility of gases
Due to the large free volume, most gases have a high solubility and high
diffusion coefficient in silicones. That is, silicones have a high
permeability to oxygen, nitrogen and water vapor, even if in this case
liquid water is not capable of wetting the silicone surface! As expected,
silicone compressibility is also high.
7. Solubility
Many of low molecular weight silicones dissolve in solvents like C6H6,
ether and CCI4.