Types of bioresorbable polymers for medical applications - Durability and Reliability of Medical Polymers - page 19

This approach makes it possible to obtain tyrosine-based materials. The
protecting groups have a role in determining the properties of the final
product; the challenge is identifying suitable groups that will lead to a
non-toxic and bioresorbable material. However, this was not achieved, and
tyrosine dipeptide was substituted with a dimer of tyrosine and l -tyrosine, or
desaminotyrosine [3-(4'-hydroxyphenyl)propionic acid], finally obtaining a
fully biocompatible substitute for diphenols (Fig. 1.21). However, diphenolic
monomers used in this field are obtained from desaminotyrosine and alkyl
esters of tyrosine (sen Gupta and lopina, 2002). The resulting pendent chain
protects carboxylic group.
There are four main categories of tyrosine-based polymers (Bourke and
Kohn, 2003):

∑ Tyrosine-derived polycarbonates (Fig. 1.22): they are a group of

carbonate-amide copolymers which differ in the length of their alkyl ester
pendent chains. Monomers are subjected to polymerization using phosgene
or bis(chloromethyl) carbonate triphosgene. Weight-average molecular
weight in the range of 100 000-400 000 can be reached. Chemical and
physical properties can be easily modulated by changing the length of
the alkyl ester pendent chain, even if this does not influence degradation
rate. Properties of such polymers have already been investigated (Bourke
and Kohn, 2003).

∑ Tyrosine-derived polyarylates (Fig. 1.23): these materials are obtained

starting from the tyrosine-derived monomers described above and alkyl

R

O

O

O

￿ ￿ ￿ ￿ ￿ ￿

OH

HO N

H

1.21 Tyrosine-derived monomer.

R

O

O

O

O

O

O *

* N

H

n

1.22 Tyrosine-derived polycarbonates.