Page 53 - Biodegradable Polyesters

2.2 Radical Ring-Opening Polymerization (RROP) of Cyclic Ketene Acetals 31

Also, cross-linked polyesters can be easily made by reaction of CKAs with
vinyl cross-linkers with more than one double bond. In one of the studies,
porous monolithic supports for tissue engineering were made by electron-beam-
initiated free-radical polymerization of 2-methylene-4-phenyl-1,3-dioxolane
with trimethylolpropane triacrylate as a cross linker [24].
Since CKAs are vinyl monomers, a controlled radical polymerization using
chain end capping and de-capping mechanism for reducing termination or
transfer reactions is also possible. The aim of carrying out controlled radical
polymerizations is to provide either polyesters of defined molar mass and
low polydispersity or various macromolecular architectures such as block and
graft copolymers. Various known controlled radical polymerization methods
such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), atom transfer radical
polymerization (ATRP), and reversible addition fragmentation transfer (RAFT)
polymerization have also been tried on various CKAs [25–30]. The RAFT method
appears to be more promising as it provided polyesters with low polydispersities.
Coordination of the CKAs with the metal catalysts used in the ATRP method
could affect the degree of ring opening and also the polydispersities making it
less attractive. Controlled radical polymerization made possible the formation
of block copolymers of vinyl polymers such as PMMA and poly(styrene) (PSt)
with polyesters (Scheme 2.6) and could also be utilized for the covalent fixing of
degradable polyester chains on a substrate using grafting from technique shown
for copolymerization of BMDO and poly(ethylene glycol) methacrylate from
ATRP initiator mobilized on silicon wafers [31–33].

Controlled radical Controlled radical
R′ polymerization; ATRP R′ R′ polymerization; ATRP R′ R′ O O
Br O O
R R n R R n R m
R′ = H; R = C 6 H 5
O O
R′ = H; R = C(O)OCH 3
R′ = CH 3 ; R = C(O)OCH 3
Poly(vinyl polymer-b-polyester)
Scheme 2.6 Use of controlled radical polymerization in making block copolymers of vinyl
polymers and polyesters.

2.2.4
Functional Polyesters by Copolymerization of CKAs and Vinyl Monomers

The simultaneous copolymerization of CKAs with other vinyl monomers pro-
vides the opportunity of not only the property tuning of polyesters but also giving
functional polymers with ester linkages in the backbone. Therefore, the RROP of
CKAs is a very advantageous addition to the existing conventional methods of
making polyesters such as condensation and ROP of cyclic esters. The copoly-
merization of CKAs with vinyl monomers is one of the simplest ways to bring