Types of bioresorbable polymers for medical applications - Durability and Reliability of Medical Polymers - page 21

Tyrosine-derived polycarbonates have been tested for orthopedic implants,
because of their high mechanical strength. Polyacrylates, instead, are used
where a more flexible material is needed, for example for 'thrombo-resistant'
coating for blood-contacting devices.

1.7 Polyalkylcyanoacrylates
Polyalkylcyanoacrylates (PaCa) are considered interesting polymers because
of the high reactivity of the corresponding monomer, which can polymerize in
various media, and also in water. in particular, monomers are cyanoacrylate
esters where alkyl chain length varies from methyl to decyl (Fig. 1.26).
Cyanoacrylate monomers, in pure form, are quite unstable; they must
be treated with acid stabilizers or radical scavengers in order to avoid
spontaneous polymerization. However, polymerization follows both a free
radical mechanism or an anionic/zwitterionic mechanism. Free radical
mechanism is characterized by a high value of activation energy (125 kJ
mol -1 ) (Domb et al. , 1997); the process is slow and strongly dependent
on temperature and radical amount. Anionic/zwitterionic polymerization
is more interesting because it is more rapid and easier to perform; it also
takes place in weak bases. Classical initiators are ionic (i - , Br - , OH - , etc.)
and nucleophilic compounds, such as tertiary bases (phosphine, pyridine).
The propagation reaction occurs because of the formation of carbanions that
react with other monomers; termination is realized with a cation, adding a
strong mineral acid. However, in the case of zwitterionic polymerization,
termination occurs with the formation of intra- or intermolecular bonds (Fig.
1.27). in the former, polymer cyclization takes place, whereas in the latter,
chain length doubling is obtained (Domb and Jain, 2011).
Polymerization of polyalkylcyanoacrylates can be carried out in various
ways:

￿ ￿ ￿ ￿ ￿ ￿

CN

CH 2 C

COOR

1.26 Cyanoacrylate monomer.

CN CN CN CN

*

*
n

COOR COOR COOR COOR

1.27 Polyalkylcyanoacrylate structure.