Polysiloxanes, also known as silicones, are synthesized via ring-opening polymerization of cyclosiloxanes. They can be prepared through cationic polymerization using initiators like Lewis acids, or anionic polymerization using bases. Cationic polymerization may proceed by acidolysis/condensation or generation of an active propagating center. Anionic polymerization yields higher molecular weight polymers through chain propagation and termination with end-capping. Copolymers and crosslinked polysiloxanes can also be synthesized. Hybrid polymers combining polysiloxane segments with other backbones can be prepared through step-growth condensation polymerization.

1. POLYSILOXANES
Introduction and synthesis
Mr. Himanshu dubey
M.Sc. Polymer science
2 nd sem (2014-2016)
Enroll. NO.-201402301157

2. CONTENT
 Introduction
 Preparation of monomer
 Synthesis of polysiloxanes
 Ring opening polymerizations
 Ring-opening Polymerization of Cyclosiloxanes by Ionic Initiators
 Cationic Polymerization
 Anionic Polymerization
 Preparation of Copolymers by Ring-Opening Polymerization
 Crosslinking of Polysiloxanes
 Hybrid polymer
 Bibliography

3.  Polysiloxanes, also called silicones, are characterized by
combinations of chemical, mechanical, and electrical properties
which taken together are not common to any other commercially
available class of polymers.
 Polysiloxanes are unique among inorganic and semi-inorganic
polymers.
 The most important organometalloid polymers are the
polysiloxanes based on the siloxane Si–O linkage found in glass
and quartz.

4. These have the general formula (R2SiO)n .
These may be linear ,cyclic or cross linked polymers .They
have high thermal stability and are also called high
Temperature polymers.

5. The elemental silicon on which the entire technology is based
is typically obtained by reduction of the mineral silica with
carbon at high temperatures.
(1)
The silicon is then converted directly to tetrachlorosilane by
the reaction
(2)
As already mentioned, this can be used to form an
organosilane by the Grignard reaction
(3)

6. This relatively complicated reaction has been replaced by the so-called
“Direct Process” or “Rochow Process, which starts from elemental silicon. It
is illustrated by the reaction
(4)
But the process also yields RSiCl3 and R3SiCl, which can be removed by
distillation. 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.

7.  The polysiloxanes were also prepared by condensation reactions. In
fact, Friedel and Crafts reported the condensation of Si(C2H5)2(OR)2,
where R=alkyl, with water producing [(C2H5)2SiO]n and the alcohol in
1866:
 At present, polysiloxane polymers are usually prepared by ring-opening
polymerizations of small cyclic oligomers that have been prepared by
hydrolysis (a condensation reaction) of the appropriate
dihalodialkylsilane. But even today some special siloxanes use
condensation reactions for the polymerization step.

8. Silphenylene-siloxane polymers can be prepared by a low
temperature step condensation polymerization of a
bis(ureido)silane with 1,4bis(hydroxydimethylsilyl)benzene at 20 c.
Example-
The synthesis of silphenylene-siloxane polymers

9. Example-
The condensation of trichlorophenylsilane with water is thought to
produce the ladder-type sesquisiloxane polymer.
Reaction for producing a sesquisiloxane polymer.

10. The Cyclotrisiloxane [{Si(CH=CH2)2{SiMe2}O3] Can Be Prepared
ByThe Condensation Of Divinyldichlorosilane With
Tetramethyldisiloxane-1,3-diol .
Example-

11. The compound [{SiPh2}{SiMe2}2O3] can be prepared by
the condensation of Me2SiCl2 with tetraphenyldisiloxane-
1,3-diol .
Example-

12. 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 hexamethylcyclotrisiloxaneor the tetramer
octamethylcyclotetrasiloxane
where R can be alkyl or aryl
x is the degree of polymerization

13. Cyclosiloxanes, [R2SiO]n, (n = 3,4) can be polymerized by ROP
methods to the corresponding linear polymer [R2SiO]n (R = alkyl,
aryl).Two different types of initiators are used for this reaction-
(1) Cationic initiators
(2) Anionic initiators.

14. Several cationic initiators such as Lewis acids or protic acids can be
employed to effect the polymerization of cyclosiloxanes.Among
protic acids, H2SO4, HClO4, CF3SO3H etc. have been widely used.
Many Lewis acids have been used including AlCl3 and SnCl4.The
mechanism of polymerization initiated by CF3SO3H.
Two broad types of mechanisms are envisaged –
1- Acidolysis/condensation
2- Generation of an active propagating center.

15. In the acidolysis/condensation mechanism the initiation of
the polymerization is believed to generate the linear chain
which contains a hydroxyl terminal group as well as a CF3SO2
end-group.
The propagation reaction proceeds in two ways.The first of these
involves condensation of two molecules involving their hydroxyl end
groups.This leaves terminal triflate groups .

16. The second route of propagation involves condensation of a
molecule containing one hydroxyl end-group with another
molecule containing a CF3SO2 end-group . Further reactions lead
to the formation of the high polymer.

17. This type of polymerization reaction can also be used to generate short
chain linear siloxanes.Thus, carrying out the acid-catalyzed ROP of
[Me2SiO]4 in the presence of the disiloxane Me3Si-O-SiMe3 allows the
formation of (predominantly) Me3SiO(SiMe2O)4Me3Si.

18. In the case of the short chain generation, the reactions involved are
summarized.Thus, the initiation is envisaged as the ring-opening of
the eight-membered ring.This generates the difunctional linear
siloxane (containing Si-OH and Si-OSO2CF3 end-groups).

19. Simultaneously, the disiloxane Me3SiOSiMe3 is also opened up by
the protic acid.
Subsequent combination of either Me3Si0H or Me3SiOSO2CF3
with the linear siloxane affords Me3Si0(SiMe20)4Me3Si .

20. Linear short-chain polysiloxanes Me3Si(OSiMe2)nOSiMe3 are
used as silicone oils. Such oils are characterized by their high
thermal stability and low viscosity temperature coefficient
(VTC).

21. The second mechanism of cationic polymerization involves the
generation of an active propagation center.This could be an oxonium
ion (oxygen centered cation) or a siliconium ion (silicon-centered
cation).This type of ion has been detected in small molecule
experiments.Thus, the reaction of Me3SiH and Me3Si0SiMe3 with
[Ph3C]+[B(C6F5)4]~ leads to the formation of [(Me3Si)3O]+ which has
been detected by NMR spectroscopic methods.

22. Another example of an oxonium ion generation involves the reaction
of Me3SiH with [Me2Si0]3 with [Ph3C]+[B(C6F5)4]~ to generate the
cyclotrisiloxane-centered oxonium ion.
Generation of a cyclotrisiloxane-based oxonium ion

23. Anionic polymerization is more effective than the cationic
polymerization in generating high-molecular-weight polysiloxanes.
Generally, the molecular weights obtained in this method are in the
order of 2-5 x 106.
The initiators could be metal hydroxides such as KOH or even
lithium salts of silanediols such as Ph2SiO2Li2.

24. The anionic polymerization is initiated by the reaction of the base
with a cyclosiloxane such as [Me2Si0]4 to generate the open-chain
compound with a Si-O"K+end-group .

25. The propagation reaction is similar to that encountered in many
conventional anionic polymerizations using organic monomers.
Thus, the incoming monomer can insert between the ion-pair.This
leads to chain propagation.
Propagation reaction in the anionic polymerization of Me2SiO4

26. The termination of the reaction can be accomplished by the use of
Me3Si end-capping reactions.
Termination reactions of the anionic polymerization

27. Copolymers of the type [(SiR2O)n(SiR'2O)m] can be prepared by the
polymerization of mixtures of cyclosiloxanes.Thus, for example, heating a
mixture of [Me2SiO]4 and [Ph2SiO]4 in the presence of KOH as the catalyst
affords the random copolymer [{Me2SiO}n{Ph2SiO}m]
Copolymerization of a mixture of [Me2SiO]4 and [Ph2SiO]4

28. Crosslinking or curing of polysiloxanes is the most important procedure
that allows these polymers to be used as advanced elastomers.
High-molecular-weight polysiloxanes, along with reinforcing fillers such
as fumed high-surface-area silica (as opposed to silica obtained from
hydrolysis) are heated together with organic peroxides (along with some
amount of cyclosiloxanes and some coloring pigments) to afford
crosslinked products via -CH2CH2- links.
This crosslinking is different from that applicable in organic rubber
materials where sulfur cross-links are quite effective.

29. The siloxane polymers can be tailored to contain a few percent of vinyl
groups and these can be crosslinked with polymers that contain Si-H
units.

30. Polymers containing polysiloxane segments and other types of backbones
are of interest in generating new hybrid polymers.These polymers are
expected to possess new properties which could be an increase in glass
transition temperatures, increase in the thermal stability etc.These
polymers are synthesized by using a step-growth condensation
polymerization method.
The silarylene unit is introduced by the monomerOH-Si(Me2)-C6H4:p-
Si(Me2)-C)H .while the siloxane is introduced by the diueridosilane,
Me2Si(Ureido)2. Condensation of these reagents in a solvent such as
chlorobenzene leads to the elimination of urea as an insoluble by-product
leaving the exactly alternating copolymer in solution .

31. Preparation of the monomer OHSiMe2-C6H4-/7-SiMe2OH
Condensation of OHSiMe2-C6H4-p-SiMe2OH with RR'Si(ureido)2 to afford exactly
alternating hybrid copolymers
R = R' = CH3
R = Me,R' =
CH=CH2

32. Another example for the synthesis of hybrid polymers…
Block copolymers containing polysiloxane segments and
polyphosphazene segments have been synthesized by the reaction of
hydride-terminated polysiloxane H-[Si(Me2)-O]n-SiMe2H with the
telechelic polyphosphazene containing an amino end-group.
Poly(phosphazene-siloxane) block copolymers

33. [Ronald_D._Archer]_Inorganic_and_Organo
metallic
Mark J., Allcock H.,West R. Inorganic
Polymers
[Vadapalli_Chandrasekhar]_Inorganic_and_
organometallic