Page 63 - Biodegradable Polyesters

References 41

2.3
Conclusions

RROP of CKAs gives quantitative formation of polyesters depending upon the ring
size of the starting monomers (CKAs) and polymerization conditions. Many dif-
ferent CKAs with varied chemical structures are known till date to introduce extra
functionalities to the resulting polyesters on homopolymerization, that is, (super)
hydrophobicity, cross-linking sites, and so on. Clearly, RROP of CKAs cannot
substitute conventional methods such as condensation and ionic/metal-catalyzed
polymerizations of cyclic esters for the synthesis of homopolyesters because of
limited molecular weights and transfer reactions occurring during polymeriza-
tion but it is unique in providing many of the functional polyesters which cannot
be made by other conventional methods.
One of the CKAs (MDO) is a well-studied example in terms of reaction mech-
anism and gives a polyester on RROP with the same repeat units as that of a
very well-known biodegradable polymer, PCL. Owing to the H-transfer reactions
during radical polymerization, the resulting PCL is branched unlike the linear
PCL obtained by ROP of ε-CL. The branches lead to poor crystallization and
a complete amorphous PCL, depending on the amount of branches. RROP is
the only method making possible the formation of amorphous PCL. The method
of making polyesters by RROP of CKAs also provided opportunities of making
polyesters with controlled chain lengths and predictable chain ends using con-
trolled radical polymerization techniques. This would make the synthesis of var-
ious macromolecular architectures such as block and star polymers, based on
polyester repeat units simple. Furthermore, the chemistry of RROP opened ways
to a new class of degradable functional materials called poly(vinyl-co-ester)s made
by simple copolymerization of CKAs with vinyl monomers. RROP is the only
method that made possible the combination of C–C backbone of vinyl polymers
with ester units in a random way, thereby generating many new materials and
properties. Each new vinyl comonomer pair with CKAs provided a new functional
material with novel properties.

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