Minoru Kumakura

Synthesis of copoly(d,l-lactic acid) with relatively low molecular weight and in vitro degradation

Abstract Copoly( d -lactic acid/ l -lactic acid; d -LA/ l -LA) with relatively low molecular weight ( M n ) of 300–3800, was synthesized by direct copolycondensation without catalyst in an atmosphere of nitrogen at 200°C, to evaluate the difference in degradation between crystalline and amorphous copolymers. The non-enzymatic degradation of crystalline copolymer, e.g. a copoly( d -LA/ l -LA, 2/98 mol%), showed a parabolic-type degradation pattern in the M n 900 to 2700 range; degradation preferentially occurs in the amorphous regions because the matrix remains highly crystalline in the initial stages. In contrast, the non-enzymatic degradation of amorphous copolymer, e.g. a copoly( d -LA/ l -LA, 50/50 mol %), can be divided into three types according to M n , viz. parabolic-type for M n = 1500 , linear-type for M n = 2200 , S-type for M n = 3500 . For comparison, the copolymers were degraded in the presence of enzymes; the process was markedly accelerated by the action of esterase-type enzymes, of which Rhizopus delemer lipase gave the highest degradation activity.

Synthesis of low-molecular-weight copoly(l-lactic acid/ɛ-caprolactone) by direct copolycondensation in the absence of catalysts, and enzymatic degradation of the polymers

Low-molecular weight copolymers of l -lactic acid (LA) and ɛ-caprolactone (CL) were synthesized by direct copolycondensation without catalysts at 200°C under a nitrogen atmosphere. The reaction proceeds by direct condensation between linear LA and linear 6-hydroxycaproic acid produced by hydrolysis of the cyclic CL. These copolymers were characterized with respect to molecular weight by gel permeation chromatography, composition by 1 H nuclear magnetic resonance (n.m.r.) spectroscopy, sequence by 13 C n.m.r. spectroscopy, and crystallinities by both differential scanning calorimetry and X-ray measurement. The morphology of copoly (LA/CL) can be subdivided into three states as a function of monomer composition according to the differences in T g and crystallinity, e.g. solid (0–15 mol% CL composition ranges), paste (30–70 mol% CL composition ranges) and wax (85–100 mol% CL composition ranges). The in vitro degradation of the copolymer was examined by treatment in buffer solutions with and without enzymes and, as a result, Rhizopus delemer lipase showed the strongest degradation activity. In this case, the degree of enzymatic degradation is strongly dependent on the morphology of copoly(LA/CL), in which the pasty copolymer is much more subject to hydrolysis than the solid and waxy copolymers. For comparison the in vivo degradation of the copolymer was investigated by implanting it subcutaneously in the back of rats.

A new temperature-sensitive hydrogel with α-amino acid group as side chain of polymer

Abstract New thermo-responsive hydrogels with α-amino acid groups as side chains, e.g. methacryloylglycine methyl ester, methacryloyl- l -proline [MA-( l )Pro] and methacryloyl- l -phenylalanine methyl ester, were synthesized by copolymerizing a hydrophobic monomer viz. 2-hydroxypropyl methacrylate (HPMA) and a crosslinking monomer viz. polyethylene glycol No. 600 dimethacrylate (14G), using γ-rays from a 60 Co source. Of these hydrogels, the MA-( l )Pro-containing copolymer gels gave the strongest reversible thermo-response; the swelling-deswelling mechanisms can be classified into two types according to the kind of copolymer, viz. the surface regulating system for the MA-( l )Pro/HPMA copolymer and matrix pumping system for the MA-( l )Pro/14G copolymer. Thus, when raising the temperature, the copoly[MA-( l )Pro/HPMA] gel in the swollen state undergoes initial rapid shrinkage of the outer layer of the sample in contact with water, followed by a gradual deswelling. From microscopic observation, it was found that there are many micropores in the swollen gel but they disappear immediately after treating at high temperature, especially at the surface of the gel. In contrast, the swollen copoly[MA-( l )Pro/14G] gel undergoes rapid shrinkage of the whole sample without the disappearance of micropores. In addition, the control of drug release from such thermo-responsive copolymer formulations was investigated.

Effect of radiation pretreatment of bagasse on enzymatic and acid hydrolysis

Abstract The effect of radiation pretreatment of bagasse on enzymatic and acid hydrolysis was studied. At irradiation doses above 10 MR, bagasse was extensively degraded and became very fragile; at the same time the fraction of powder (below 250 mesh) and the water-soluble components increased. Irradiation increased the glucose yield from both enzymatic and acid hydrolysis. Bagasse irradiated with 100 MR yielded two or four times as much glucose as the untreated bagasse in enzymatic and acid hydrolysis, respectively. Most of the cellulose components in bagasse irradiated with 100 MR could be hydrolysed by subsequent acid hydrolysis at 180°C for 10 min. The irradiation effect on enhancing acid hydrolysis appeared to be greater than that on enzymatic hydrolysis.

In vivo characteristics of low molecular weight copoly (D,L-lactic acid) formulations with controlled release of LH-RH agonist.

Amorphous and crystalline copolymers with a relatively low molecular weight of 1800 were synthesized by direct copolycondensation of D-lactic acid and L-lactic acid in the absence of a catalyst, to evaluate their in vivo capabilities as biodegradable carriers for drug delivery systems. A luteinizing hormone-releasing hormone agonist, des-Gly10-(D-Leu6)-LH-RH ethylamide, was incorporated in a fine cylindrical copolymer formulation, under melt-pressing technique, a mild heat-pressure condition. This formulation was implanted subcutaneously in the back of male rats. The rate of in vivo degradation of amorphous copolymer was much faster than that of crystalline copolymer. Contrary to this tendency, the in vivo release of the drug from this amorphous formulation was held constant over a longer period, compared with the crystalline formulation. This can be closely related to the difference in dispersion of the drug in the formulation.

Immobilization of enzymes by radiation-induced polymerization of glass-forming monomers: 1. Immobilization of some enzymes by poly(2-hydroxyethyl methacrylate)

Abstract The immobilization of some enzymes has been studied by radiation-induced polymerization using 2-hydroxyethyl methacrylate as a glass-forming monomer. Radiation damage of enzymes was slight after irradiation at low temperatures. Moreover, the activity yield of immobilized enzymes increased markedly at polymerization temperatures below −24°C. The polymer formed was characterized by its porous structure which was studied in detail in relation to the activity yield and its activity retention with repeated use. It was deduced that enzymes were partly trapped at the pore surface within the polymer matrix and partly within the pores from which they were able to leak out with repeated use. Hence, the use of low temperature and super-cooled monomer was necessary for effective enzyme immobilization.

Radiation-induced degradation and subsequent hydrolysis of waste cellulose materials

Abstract The effect of γ-pre-irradiation of cellulose in cellulose containing waste plants was investigated through enzymatic and acidic hydrolysis reaction. Pre-irradiation of waste rice straw, chaff and saw dust accelerated the enzymatic hydrolysis by cellulase. Reducing sugar and glucose yields were higher with an increasing radiation dose in these materials. The required dose for effective acceleration of enzymatic hydrolysis was much reduced by the addition of chlorine during radiation. On the other hand, the reducing sugar and glucose yields in the subsequent acidic hydrolysis of waste products rather decreased through pre-irradiation treatment. This was attributed to an acceleration effect of a secondary acidic decomposition of sugar to lower molecular weight-products through pre-irradiation.

In vivo controlled release of a luteinizing hormone-releasing hormone agonist from poly(dl-lactic acid) formulations of varying degradation pattern

Abstract Biodegradable formulations with a desired lag time were prepared from blends of low molecular weight poly( dl -lactic acid) (low MW-PLA, number-average molecular weight: M n = 1400 , parabolic-type degradation pattern, 100% in vivo degradation at 5th week of implantation) and high molecular weight poly( dl -lactide) (high MW-PLA, M n = 11 500 , S-type degradation pattern with a lag time of 10 weeks). A luteinizing hormone-releasing hormone agonist (LH-RH agonist), des-Gly10-[Leu6]LH-RH ethylamide monoacetate, was incorporated into the small cylinders of PLA blends. The initial burst of drug release from cylindrical formulation, which was implanted subcutaneously in the back of male rats, could be controlled by adjusting the amount of high MW-PLA in the blend and, as a result, it was found by measuring the pharmacological influence on rat prostate and serum drug levels that the best efficacy and the most constant release over a long period are obtained with a 25/75% low MW-PLA/high MW-PLA blend.

Pretreatment of lignocellulosic wastes by combination of irradiation and mechanical crushing

Abstract Pretreatment of lignocellulosic wastes such as sawdust, chaff and paper was studied using a combination of irradiation and mechanical crushing. An acceleration effect of irradiation on crushing of these wastes was observed. The time taken to crush the irradiated samples to a size below 250 mesh decreased markedly with increasing enzymatic hydrolysis owing to the increased susceptibility of the irradiated samples to cellulase.

Artificially intelligent hydrogels responding to external stimuli such as temperature and pH

Abstract Methacryloyl-D-alanine methyl ester (MA-D-AlaOMe) and methacryloyl-L-alanine methyl ester (MA-L-AlaOMe) were synthesized and polymerized by irradiation using γ-rays from a 60Co source, in order to investigate the effect of optical isomerism on the swelling of homopolymer hydrogels which changes in temperature in water. These hydrogels showed a typical thermo-response such as low-temperature-swelling and high-temperature-deswelling, but no difference in swelling between the two hydrogels could be regarded as significant. The stable swelling-deswelling kinetics on repeated use are characterized by very limited temperature ranges (0–40°C), and above this range (for example, when cycled between 10 and 70°C) the reversible thermo-response perfectly disappeared because of gel collapse. This defect could be overcome by copolymerization with other functional monomers and the resulting copolymers exhibited a reversible thermostability superior to those of homopolymers of MA-D-AlaOMe and MA-L-AlaOMe, depending strongly upon the type of copolymer and its composition. The hydrolysis products of these hydrogels gave a reversible-response not only for temperature but also for pH. The swelling-deswelling kinetics of non-hydrolyzed and hydrolyzed hydrogels were characterized with regard to porous structure formed in the hydrogel.