Chemistry Reference
In-Depth Information
polymers. Acidic catalysts tend to produce cyclic small molecules or low
molecular weight polymers.
The hydrolysis approach to polysiloxane synthesis has been largely re-
placed by ring-opening polymerization 2, 8 -14 of organosilicon cyclic tri-
mers and tetramers, with 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 de-
sired cyclic species are separated from the mixture for use in ring-opening
polymerizations such as those described in the following section.
In addition, “click” chemistry has been developed for new synthesis tech-
niques in general, 15 -18 and polymerizations in particular. 19 hese approaches
have been used to prepare polysiloxane elastomers 20 and polydimethylsilox-
ane (PDMS) copolymers that can function as thermoplastic elastomers. 21
New synthetic strategies for structured silicones, based on B(C 6 F 5 ) 3 have
also been developed. 22 Another new approach involves enzymes, 23 such as
the lipase enzymatically catalyzed synthesis of silicone aromatic polyesters
and silicone aromatic polyamides. 24
2.2 RING-OPENING POLYMERIZATIONS
Cyclic siloxanes can undergo a ring-opening chain-growth polymeriza-
tion. Free radicals are not useful as initiator species, because of the nature
of the siloxane bond, but cationic initiators are very effective. The reaction
is illustrated using the most common cyclic oligomers, the timer hexa-
methylcyclotrisiloxane, or the tetramer octamethylcyclotetrasiloxane 9-10 :
(
)
[
]
SiRO   
 -SiRO- x
(2.5)
2
2
3,4
where R can be alkyl or aryl and x is the degree of polymerization. In prin-
ciple, the reaction is reversible, but in practice it is made essentially irre-
versible by the choice of monomer, initiator, and polymerization conditions.
Because of this potential reversibility however it is important to remove all
initiators from the finished product, typically done by neutralization of
the acidic or basic terminal chain residues. Alternatively, some initiators
can be removed by volatilization or thermolysis. End blocking is frequently
used to place groups on chain ends that increase thermal stability and pre-
vent re-equilibration.
Because it is frequently impossible to remove all traces of active spe-
cies, some reorganization is almost inevitable. In these processes, siloxane
bonds interchange to bring about variation in both molecular weight and
Search WWH ::




Custom Search