The paper describes a new method of copolymerization of glycolide with l-lactide with the
use of a low toxic compoundZr(acac)4as initiator at a temperature of 100 up to 150 °C. Copolymerization
at 150 °C is very fast and reached nearly 100% yield. The values of the copolymerization coefficients
were estimated as r
G = 3.3 and r
L = 0.5. The process of chain propagation was also examined, and a
significant influence of transesterification on the final structure of copolymer was observed. The
microstructure of the chain was determined by NMR spectroscopy. It was found that more segmental
structure had been formed as compared with the structure of the copolymer obtained in copolymerization
initiated by tin compounds. This structure influences thermomechanical properties of the copolymers.
The crystallinity of copolymers obtained is higher than that of the formed in the presence of Sn(oct)2.
One of their most characteristic features concerned their good mechanical properties, in many cases better
than those of adequate copolymers obtained with the use of tin compounds as initiators.
A series of copolymers with various compositions were synthesized by bulk ring-opening polymerization of glycolide and epsilon-caprolactone, using stannous (II) octoate or zirconium (IV) acetylacetonate as initiator. Reaction time and temperature were varied so as to induce different chain microstructures. The resulting copolymers were characterized by (1)H NMR, SEC, DSC, and X-ray diffraction. The average lengths of glycolyl (L(G)) and caproyl sequences (L(C)) and the degree of randomness (R) were calculated and compared to the values of completely random chains. The concentration of CGC sequences was also obtained which resulted from transesterification reactions. Data showed that stannous (II) octoate leads to less transesterification than zirconium (IV) acetylacetonate, and lower temperatures lead to less transesterification than higher ones. The copolymers exhibited a more or less blocky chain structure because of the reactivity difference between glycolide and epsilon-caprolactone. The crystalline structure and thermal properties depend on both the composition and the chain microstructure. PGA- and PCL-type crystallites were obtained for copolymers with intermediate compositions.
Bioresorbable new terpolymers of l-lactide, glycolide, and trimethylene carbonate with different compositions
were synthesized via ring-opening polymerization reaction of the cyclic monomers using low-toxicity zirconium(IV) acetylacetonate as initiator. The thermal and mechanical properties were investigated by means of
thermogravimetry, differential scanning calorimetry, stress−strain measurements, and dynamical mechanical
analysis. The glass transition temperature of the terpolymers changes with composition from 12 to 42 °C in a
predictable manner. All terpolymers display shape memory properties and, after undergoing 100% deformation,
they recover the permanent shape in a time frame of seconds. Terpolymers with high l-lactide content show a
glass transition in the range of 38−42 °C, recovery temperature close to body temperature, and good recovery
ratio (>0.89). Low-toxicity bioresorbable terpolymers with shape memory properties are promising new materials
for biomedical applications.
A series of glycolide/epsilon-caprolactone copolymers were compression molded and allowed to degrade in a pH 7.4 phosphate buffer at 37 degrees C. Degradation was monitored by various analytical techniques such as (1)H NMR, X-ray diffraction, DSC, CZE, ESI-MS, and inherent viscosity measurements. The results show that the degradation rate depends not only on the copolymer composition but also on its chain microstructure. Generally, copolymers with a higher C-G bond content or a higher degree of randomness exhibit higher degradation rates. Sequences with odd numbers of glycolyl units such as -CGC- and -CGGGC-, which result from the second mode transesterification, appear more resistant to hydrolysis. As a consequence, degradation residues obtained at the later stages of degradation are mainly composed of long glycolyl and caproyl sequences linked by -CGC- and -CGGGC- ones. The degradation rate of the copolymers depends also on the degree of crystallinity of each component which is related to the block length. The caproyl component can be preferentially degraded if it is in the amorphous state and the glycolyl component is semicrystalline.
Poly(l-lactide-co-glycolide) (PLLAGA) copolymers and poly(dl-lactide) (PDLLA) were
synthesized using the low toxicity compound zirconium(IV) acetylacetonate. Blends with synthetic atactic
poly(3-hydroxybutyrate) (a-PHB) were prepared in film form by solvent casting. The solid-state properties
of the plain polymers and of the blends were investigated by thermogravimetric analysis coupled with
mass spectrometry (TGA−MS), differential scanning calorimetry (DSC), wide-angle X-ray diffraction
(WAXD) and stress−strain measurements. The a-PHB/PDLLA blends are miscible over the whole
composition range. On the other hand, blends of a-PHB with PLLAGA copolymers containing 5 and 16
mol % of GA units show partial miscibility. The solubility limit of a-PHB in PLLAGA is around 20 wt %
in both cases. In the blends, the a-PHB in excess to this value segregates as a pure phase. The mechanical
properties of blend films of either PDLLA or PLLAGA containing up to 50 wt % of a-PHB can be modulated
by changing blend composition. The tenacity progressively increases by the addition of a-PHB not only
in the miscible amorphous a-PHB/PDLLA system but also in the partially miscible a-PHB/PLLAGA blends.
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