Page 52 - Biodegradable Polyesters

30 2 Functional (Bio)degradable Polyesters by Radical Ring-Opening Polymerization

Scheme 2.4) leading to the branched polymers. The extent of H-abstraction
reaction occurring is dependent upon the temperature of polymerization, type of
initiator, and the chemical structure of the CKA. For MDO polymerization, the
situation regarding transfer reactions is very similar to the formation of branched
low-density polyethylene by radical polymerization. The presence of branches
affects the density, crystallinity, and thermal properties of the end polyester [17,
19, 21].

2.2.3
Functional Polyesters by Conventional and Controlled Radical Homopolymerization
of CKAs

A wide variety of structurally different CKAs were synthesized and studied for
RROP for the formation of functional polyesters such as phenyl-, alkyl-, and
alkene-substituted and fluorinated polyesters (Chart 2.1) [22]. In addition, special
CKAs are known that undergo either double radical isomerization or double
ring opening (Scheme 2.5) during RROP, making unsaturated polyesters (USPs)
[23]. USP resins have double bonds in the polymer backbone providing sites for
further cross-linking and polymer-analogous reactions and are highly important
as adhesives, paints, matrix resins, and so on. ROPs are of special interest for
making functional polyesters with low volume shrinkage for applications such
as coatings, adhesives, dental fillings, and matrix resins for precision castings.
Although much data is not available regarding volume shrinkage of CKAs during
polyester formation, values like 7.2% volume shrinkage for MDO on photopoly-
merization and 8.8% and 3.1% for 1-vinyl-5-phenyl-4,7-dioxaspiro[2.4]heptane
and 1-vinyl-6,7-benzo-4,9-dioxaspiro[2.6]nonane, respectively on radical thermal
polymerization are highly encouraging and these values are much less than the
21% volume shrinkage of poly(methyl methacrylate) (PMMA) [23b].

R′ R′ R′
I . I
O I. O . O
R R R
O O O
I .
O O R
R n
O O
R′
R′
Unsaturated polyester
R′ = R = Ph: l-viny1-5-phenyl-4,7-dioxaspiro[2.4]heptane
R′ = H, R = : l-vinyl-6,7-benzo-4,9-dioxaspiro[2,6]nonane

Scheme 2.5 Unsaturated polyester formation with low volume shrinkage by radical initi-
ated double ring-opening polymerization [23].