Page 54 - Biodegradable Polyesters

32 2 Functional (Bio)degradable Polyesters by Radical Ring-Opening Polymerization

biodegradable ester linkages onto the vinyl polymer backbone as shown for the
first time by Bailey et al. [11]. In the last few years, many studies have been devoted
to the copolymerization behavior of CKAs with various vinyl monomers such as
St, MMA, vinyl acetate (VAc), and glycidyl methacrylate (GMA) and has led to a
new class of vinyl polymers with varied physical and chemical properties besides
degradability. Each new vinyl monomer pair with CKA provided a different set
of functionalities and property profiles. Similar to the homopolymerization of
CKAs, copolymerizations with vinyl monomers are also possible using both the
conventional and controlled radical polymerization initiators. The copolymer-
ization tendency of a particular CKA with vinyl monomers and the resulting
microstructure of the copolymers are dependent upon the reactivity and stability
of growing radicals. Different microstructures such as statistical, alternate,
gradient, and blocky structures were generated using different combinations of
CKAs and vinyl monomers. For example, during copolymerization of MDO with
St, MMA, and methyl acrylate (MA), big reactivity differences were seen (reac-
tivity ratios: r = 0.021 and r = 22.6; r = 0.057 and r = 34.12;
MDO st MDO MMA
r MDO = 0.0235 and r MA = 26.535) [34–36]. Therefore, the result of copolymer-
ization was polymers with only low amounts of the CKAs and long blocks of vinyl
polymers separated by ester units. The hydrolysis of the resulting polymers would
provide vinyl polymer telechelics with functional groups like OH and COOH at
the chain ends. The t-radical formed from CKA (1, Scheme 2.3) is highly unstable
owing to the presence of two electron-donating oxygen atoms and therefore
makes the copolymerization difficult as the attack of the growing radical on vinyl
monomer would be more prominent. The less reactive monomer VAc formed
statistical copolymers with MDO and showed r = 1∶53 and r = 0∶47
VAc MDO
[37]. The copolymerization of pentafluorostyrene (PFS), a vinyl monomer with
an electron-deficient double bond with BMDO led to the formation of gradi-
ent copolymers with the first block having statistically distributed ester units
onto the poly(pentafluorostyrene) backbone followed by a block of polyester
(reactivity ratios were r BMDO = 0.35 and r PFS = 9.9) [38]. In contrast, the
copolymers of MDO with fluoroalkenes such as 3,3,4,4,5,5,6,6,6-nonafluoro-1-
hexene, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octene, and 3,3,4,4,5,5,6,6,7,7,8,
8,9,9,10,10,10-heptadecafluoro-1-decene were alternate copolymers with
ester units alternating with the corresponding vinyl monomer units. The
films made from these copolymers showed highly hydrophobic surfaces
[39]. The CKA double bond is highly electron rich owing to the presence of
two oxygen atoms directly attached to it. It can make charge transfer com-
plex with highly electron-deficient double bonds and could lead to either
alternate copolymers or even polymerization without initiators as observed
for reaction with a vinyl bio-based monomer β-methyl-α-methylene-γ-
butyrolactone (Tulipalin-A). The simple mixing of Tulipalin-A and MDO at
∘
high temperatures (70 C and above) in the entire composition range with-
out any initiator provides copolymers having ester units in the backbone
(Scheme 2.7) [40].