Biomedical Engineering Reference
In-Depth Information
Spectroscopic analysis revealed that the thermally initiated [3 + 2] poly-
cycloaddition produced 1,4- and 1,5-substituted triazole isomers in an ap-
proximately 1 : 1 ratio. This ratio appears to be statistic and dependant on the
bulkiness of the organic moieties. For example,
hb
-
r
-P[
30
(4)-
20
]withbutyl
spacers contained slightly more 1,4-triazole isomers than did
hb
-
r
-P[
30
(6)-
20
] with hexyl spacers. This becomes clearer if we look at the proposed
transition states
a
and
b
of the [3 + 2]-dipolar cycloaddition (Scheme 16). Be-
cause of their molecular orbital symmetry, the acetylene and azide functional
groups arrange in two parallel planes, a so-called “two-plane orientation
complex” [48], which facilitates a concerted ring formation. If the monomer
fragment or the polymer branch (
) attached to the functional groups
are bulky, steric repulsion will come into play and transition state
a
will be
∼∼∼
Scheme 16
Proposed mechanisms for thermal and catalytic cycloaddition reactions
