Designing Hydrogels by ATRP - In-Situ Gelling Polymers: For Biomedical Applications - page 76

In contrast to conventional RP, ATRP has several advantages that allow the
preparation of more homogeneous polymer networks, due to the fast initiation
and reversible deactivation reactions. Fast initiation reactions, relative to propa-
gation reactions, result in a quick conversion of all initiators into primary chains
and a nearly constant number of growing primary chains throughout the polym-
erization (Scheme 2 ). The dynamic equilibrium between a low concentration of
growing radicals and a significantly higher fraction of dormant species ensures
a slow but steady chain growth providing even incorporation of vinyl groups
(from monomers, crosslinkers and pendent vinyl groups) into the polymer chains.
Therefore, the branched sols and/or gels synthesized by an ATRP process have a
more homogeneous structure than the polymers synthesized by a RP method at
similar concentrations of monomers and crosslinkers [ 75 ]. In addition, the chain-
end functionalities are preserved in the branched polymers and/or gels synthesized
by ATRP, and can be further used for chain-end modification and chain extension
reactions [ 76 , 77 ].
Several research groups have exploited ATRP [ 78 - 82 ] as well as other CRP
[ 75 , 77 , 83 - 87 ] of monomers and crosslinkers to study the gelation kinetics
and synthesize soluble branched polymers. All these studies indicate a devia-
tion of experimental gel points from the theoretical values, which is generally
believed to be due to the unavoidable intramolecular cyclization reactions (both
primary and secondary cyclizations) that consume pendant vinyl groups, but
have no contribution to an increase in the molecular weight of the polymers.
For instance, in an ATRP system with high initiation efficiency and good control
over the dispersity of primary chains (low M w /M n ), no gelation was observed
when the initial molar ratio of crosslinker to initiator was less than 1, even under
bulk conditions with complete conversion [ 80 , 81 , 88 ]. This result indicates that
at least half of the vinyl groups in the crosslinker were consumed via intramo-
lecular cyclizations.
The extent of cyclization reactions occurring during the copolymeriza-
tion can be controlled by adjusting the initial concentration of reagents. For
instance, during the ATRP of methyl acrylate (MA) and ethylene glycol dia-
crylate (EGDA) at fixed molar ratios of monomer, crosslinker and initiator, sim-
ply diluting the system, via addition of more solvent, dramatically postponed, or
even prevented, the experimental gel point at higher monomer conversion, i.e.,
longer reaction time, than the value based on FS theory [ 89 ]. On the other hand,
enhanced intramolecular cyclization can also be a practical method to produce
soluble branched polymers when copolymerizing, or even homo-polymerizing,
multivinyl crosslinkers. Since low-cost multivinyl crosslinkers are readily avail-
able, radical polymerization of crosslinkers represents a facile method to pro-
duce soluble (hyper)branched polymers. Common methods employed to avoid
macroscopic gelation, include the use of (a) dilution, (b) high concentration
of initiators, and (c) high concentration of transfer agents. Recently, a novel
method named deactivation-enhanced ATRP was reported [ 90 - 92 ] that produced
soluble cyclized polymers through homopolymerization of divinyl crosslinker by
forming and keeping a high ratio of Cu(II) to Cu(I) in the reaction medium to