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　The investigations of biomedical polymers in Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences in recent years are summarized:

A. Polymerization of aliphatic ring esters by novel catalysts
　Novel catalysts were developed in our lab to polymerize lactide, glycolide and ε-caprolactone. They
are Ca ammoniate, organic Ca ammoniate, organic Sr ammoniate, alkyl Zn, Sn benzoate and Al/Schiff
base complexes. They all exhibit very high activity and possess a feature of quasi-living polymerization.
Among them, Al/Schiff base complexes can initiate stero-selective polymerization of racemic lactides.
Single crystals of these catalysts were prepared. They exhibit asymmetric structures of five-coordination
whereas they do not show chiral property in solution. When they are combined with a lactide monomer,
they gain chirality, leading to stero-selective polymerization of racemic lactides. By this way, a polymer
that has melting temperature of as high as 201°C was prepared from recamic lactides.
B. Modification of aliphatic polyesters
　Aliphatic polyesters have many excellent advantages. But their disadvantages are obvious. They are
hydrophobic. They do not have active functional groups. And they are electrically neutral. These
shortcomings limit their application, especially in medical fields. Therefore, we tried to chemically
modify polylactides by copolymerizing with proper monomers, such as ethylene oxide, amino acids and
others. In this direction, we have prepared diblock and triblock copolymers of lactide or ε-caprolactone
with ethylene oxide, four-armed copolymer PCL-PEO, diblock copolymers of lactide with glutamate or
triblock copolymers of the both with polyethylene glycol (PEG), and diblock copolymer of sterospecific
PLA with PEG. Some monomers with functional groups were synthesized, such as 2,2-bis(methylol)
propionate, 2-methyl-2-(2-carboxyethyl)-1,3-propylene carbonate. By incorporating such monomers into
PLA, side-chain carboxy and other groups can be introduced. They are further linked to related moieties
to render required biological activity.
C. Electrospun ultrafine fibers for drug carriers
　PLA, PCL and PLGA are electrospun into ultrafine fibers and typical drugs are successfully
capsulated in the fibers. The medicated fiber mats release the drugs in the kinetics of nearly zero order in
the presence of proper enzymes, such as lipase PL for PCL and proteinase K for PLA or PLGA. These
results show that such medicated fiber mats would be promising formulation for postoperative local
chemotherapy.
D. Poly(ε-caprolactone) polyurethane shape memory materials
　PCL with two terminal OH groups are prepared and are reacted with diisocyanates and ethylene
glycol to obtain PCL polyurethanes (PCLU) with various PCL lengths and hard-to-soft segment ratios.Their shape memory behaviors are examined as a function of PCL length and hard-to-soft ratio. It is
demonstrated that the characteristic temperatures for shape memory of PCLU are near room temperature
and near body temperature. That means that it may find important medical applications, such as
esophagus stent.
E. Diblock copolymer conjugate of antitumor drugs
　Paclitaxel is a well known antitumor drug. Because it is highly hydrophobic, a mixture of polyepoxy
(caster oil), Chremophor EL and ethanol is used as its medium. Many patients exhibit hypersensitivity to
this formulation. To develop a new formulation for paclitaxel, we synthesized a diblock copolymer
PEG-PLA, converted its hydroxyl end-group into carboxyl group, and finally reacted it with paclitaxel to
achieve a covalent linkage between the polymer and the drug. Because PEG-PLA is amphiphilic and
paclitaxel is hydrophobic, PEG-PLA- paclitaxel can spontaneously form micelles in an aqueous system
with paclitaxel in the core region and PEG blocks in the shell region. In this way, a stable suspension may
be obtained. Because paclitaxel is combined to PLA through an ester linkage, it may be released via
hydrolysis of the ester group and the biodegradation of PLA blocks would speed up its release. In fact,
PEG-PLA-paclitaxel showed quite high antitumor activity against human liver cancer H7402 cells.
F. Animal experiments on the feasibilities of PLGA CBD stents
　After copolymerization of lactide with other monomers, the mechanical property and degradation rate
of the products can be adjusted to fulfill the requirement of definite medical applications. Take common
bile duct (CBD) stent as an example. A random copolymer PLGA was chosen as the stent material. It
lasted 24 days in in-vitro experiment. Small tubes of ~1mm diameter were implanted into the CBD’s of
60 rats and 8 dogs. By monitoring the change in the tubes and in the liver function of the animals, it was
shown that the tubes degraded gradually and disappeared in 4-5 weeks from CBD, and the liver function
entirely recovered in 8 weeks.