NOVELTY - Biodegradable, semicrystalline, phase separated thermoplastic multi-block copolymer comprising at least one hydrolyzable pre-polymer (A1) segment and at least one hydrolyzable pre-polymer (B1) segment, where the multi-block copolymer has a glass transition temperature of less than or equal to 37 degrees C and a melting temperature of 110-250 degrees C under physiological conditions, the segments are linked by a multifunctional chain extender, the segments are randomly distributed over the polymer chain, and at least part of the pre-polymer (A1) segment is derived from a water-soluble polymer, is claimed.    USE - The biodegradable, semicrystalline, phase separated thermoplastic multi-block copolymer is useful: for drug delivery; and in a composition for delivering a biologically active compound (a non-peptide, non-protein small sized drug molecule or a biologically active polypeptide) to a host, and for treating rheumatoid arthritis, hepatitis, diabetes, metabolic syndromes, osteoarthritis, renal disease, inflammation, local pain processes, local infections, local skin diseases, tumors (or their sites after surgical removal as a postoperative treatment to destroy any tumor cells possibly remaining), prostate or breast cancer, acromegaly, ocular diseases (e.g. age related macular degeneration), local brain diseases (e.g. Parkinson's disease), and cardiovascular diseases (e.g. acute myocardial infarction, chronic heart failure and atherosclerosis), where the non-peptide, non-protein small sized drug comprises at least one antitumor agent, anti-microbial agent, cephalosporin, aminoglycoside, macrolide, tetracycline, chemotherapeutic agent, urinary tract antiseptic, drug for anaerobic infections, drug for tuberculosis, drug for leprosy, antifungal agent, antiviral agent, anti-helminthiasis agent, antiinflammatory agent, antigout agent, centrally acting (opioid) analgesic, local anesthetic, drug for Parkinson's disease, centrally acting muscle relaxant, hormone or hormone antagonist, corticosteroid, glucocorticosteroid, androgen, androgenic steroid, anabolic steroid, antiandrogen, estrogen, estrogenic steroid, antiestrogen, progestin, thyroid drug or antithyroid drug, and the biologically active polypeptide comprises at least one protein/peptide drug, enzyme, receptor ligand, neurotransmitter, inhibitory peptide, regulatory peptide, activator peptide, cytokine, growth factor, monoclonal antibody, monoclonal antibody fragment, antitumor peptide, antibiotic, antigen, vaccine, or hormone (all claimed).    ADVANTAGE - The biodegradable, semicrystalline, phase separated thermoplastic multi-block copolymer: exhibits degradation and swelling characteristics; provides sustained release/diffusion controlled release of the encapsulated therapeutic agent either locally or into the systemic circulation; provides a natural environment for biologically active compounds, and avoids accumulation of acidic degradation products (formed during hydrolysis of the polymer chains) in the polymer matrix, due to the swellability of the phase separated multi-block copolymers; and exhibits good phase separated morphology, and good mechanical and thermal properties. The controlled release ability of copolymer containing encapsulated therapeutic agent was tested in vitro release kinetics of goserelin from microspheres. The result showed that goserelin-loaded 20(D,L-lactide-co-(polyethylene glycol)1000-co-D,L-lactide)2000-80(L-lactide)4000 microspheres had an encapsulation efficiency of 88 %.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a composition for the delivery of at least one biologically active compound to a host, comprising at least one biologically active compound encapsulated in a matrix, where the matrix comprises at least one biodegradable, semicrystalline, phase separated thermoplastic multi-block copolymer;    (2) manufacturing the composition, comprising the successive steps of either: (a) emulsifying an aqueous solution of a water-soluble biologically active compound in a solution of the biodegradable, semicrystalline, phase separated thermoplastic multi-block copolymer in an organic solvent e.g. dichloromethane and ethyl acetate, (b) subsequently emulsifying the resultant emulsion in an aqueous solution comprising a surfactant e.g. polyvinyl alcohol, thus yielding a water-in-oil-in-water emulsion, and (c) extracting the organic solvent to solidify microspheres; or (a1) dispersing the biologically active compound as a solid powder in a solution of the biodegradable, semicrystalline, phase separated thermoplastic multi-block copolymer in the organic solvent, (b1) emulsifying the resultant dispersion in an aqueous solution comprising the surfactant, thus yielding a solid-in-oil-in-water emulsion, and step (c) as above per se; or step (a) as above per se, (b2) adding a polymer precipitant e.g. silicon oil, to the resultant emulsion to form embryonic microparticles, and (c2) extracting the polymer precipitant and the organic solvent to solidify the microspheres; or step (a1) as above per se; (b3) adding the polymer precipitant to the resultant dispersion, to form embryonic microparticles, and step (c2) as above per se; and    (3) preparing the biodegradable, semicrystalline, phase separated thermoplastic multi-block copolymer, comprising either performing a chain extension reaction of pre-polymer (A1) and pre-polymer (B1) in the presence of a multifunctional chain extender, or performing a chain extension reaction using a coupling agent, where the pre-polymers (A1) and (B1) are both diol or diacid terminated, the chain extender is dicarboxylic acid or diol terminated, and the coupling agent is preferably dicyclohexyl carbodiimide.