Chemistry Reference
In-Depth Information
due to the lack of entanglements film forming properties are poor. Multicyclic
polymers may play the role of host molecules for ions and small organic molecules
such as drugs or dyes. The ring and pore size may be varied and fine-tuned by the
length of the a
2
monomers. Multicycles are more stable than linear or branched
polymers against thermal degradation of the molar mass, because one, two or three
chain scissions do not automatically reduce the molar mass. Multicyclic polymers
are particularly well suited as thermosets and high temperature adhesives. These
properties are inherent in their architecture, whereas the chemical structure is only
relevant for the minimum (or optimum) temperature of such an application. These
applications result from the fact that the thermal cleavage of a bond (regardless
which one) generates two radicals which may attack a neighbouring molecule and
thus, induce a three-dimensional chain growth and crosslinking process. The
thermally generated radicals may also attack surfaces of other materials, thereby
generating stable covalent bonds with a neighbouring material. Certainly research
for more than one decade will be necessary to explore these and other applications
in detail.
12.7 Multicyclic Polymers and the Theory of Gelation
The classical network theory is based on the experimental and theoretical studies
characterized by the relationship of between gel point and conversion according to
Eq. (
12.3
) which is a more general version of the combined Eqs. (
12.2
) and (
12.3
).
The Flory-Stockmayer theory is based on two assumptions. First, the reactivities
of the functional groups is independent of the conversion and independent of the
extent of branching and crosslinking. Steric hindrance and reduced mobility of
functional groups in the neighbourhood of branching points is not considered.
Second, cyclization plays a negligible role (see
Sect. 12.1
) and only occurs close to
the gel point in large hyperbranched molecules containing numerous ''a'' and ''b''
functionalities. This second assumption was revised by Stepto [
38
,
39
], who,
together with Gordon and Temple [
40
,
41
], developed a new theory of KC step-
permanent competition of cyclization and chain growth, so that cyclization begins
at the first percent conversion (i.e. formation and cyclization of dimers) and ends
up at 100 % conversion with the consequence that all reaction products are cycles.
Stepto computed [
38
,
39
] that in the case of three-dimensional polycondensations.
The fraction of cyclic structures increases more rapidly than in the case of anal-
ogous two-dimensional polycondensations. However, Stepto did not say anything
about the exact structure of the cyclic species, and thus the formation of bicyclic
and multicyclic oligomers and polymers was not mentioned or discussed. Yet, the
structure of the low molar mass reaction products is decisive for the structure of
the networks and for the composition of the sol-gel phase.
