Block polymer microspheres

It is well known that block copolymers and graft copolymers composed of incompatible sequences form the self-assemblies (the microphase separations). These morphologies of the microphase separation are governed by Molau s law [1] in the solid state. Nowadays, not only the three basic morphologies but also novel morphologies, such as ordered bicontinuous double diamond structure, are reported [2-6]. The applications of the microphase separation are also investigated [7-12]. As one of the applications of the microphase separation of AB diblock copolymers, it is possible to synthesize coreshell type polymer microspheres upon crosslinking the spherical microdomains [13-16]. 

The feature of the core-shell type polymer microspheres that differentiates them the most from the (AB)n type star block copolymers is size. The external diameters of the core-shell type polymer microspheres are generally from about 20-200 nm in the good solvents instead... 

The core-shell type polymer microspheres were synthesized upon the chemical crosslinking of the spherical microdomains in the microphase separated films. The block copolymers were dissolved in 1,1,2-trichloroeth-... 

In three dimensions, Ohta and Kurokawa [32] reported that a BCC arrangement was only slightly more favored than the FCC arrangement. In fact, many BCC structures have been reported for AB type block copolymers and the blends of homopolymer-block copolymer systems [27,33-35]. However, the lattice structure of the core-shell type polymer microspheres was FCC. This FCC formation resulted in the lower viscosity of... 

B. Alloy of the Core-Shell Type Polymer Microspheres and Block Copolymers... 

Microsphere Syntheses by Linear and Block Polymer Dispersants.303... 

Recently, we have found that acrylamide derivatives such as N-methylacrylamide-(NMAAm) and N-methylmethacrylamide (NMMAm) were polymerized by radical initiators in adequate solvents to form polymer microspheres, which contained the very stable propagating radicals of the amide monomers in high concentrations. Furthermore, the living polymer radicals were found to react readily with other vinyl monomers at room temperature, yielding block copolymers. We have also investigated these reactions by means of ESR. This article reviews our recent work on the formation of living propagating radicals, their reactions with vinyl monomers, and their use in block copolymer synthesis. 

As described above, when NMAAm or NMMAm is photo-polymerized in benzene at room temperature, the amide monomer is converted to polymer microspheres containing its living propagating radicals in high concentrations. These living radicals react easily with other vinyl monomers. These reactions can be applied to the synthesis of block copolymers. 

Generally, the number of the shell chains in a microsphere ranges from a few hundred to a few thousand. The range of the diameter of the core is from 10-100 nm. Such a core-shell structure is very similar to the (AB)n type star block copolymers, which have many arms and spherical polymer micelles of the block or graft copolymers formed in selective solvents that are good for the corona sequence and bad for the core sequence. In fact, many theoretical investigations of the chain con-... 

The block copolymer and the microsphere were cast from polymer-benzene solution on a Teflon sheet. The solution was gradually dried at room temperature. Film was microtomed vertically at 80 nm thick by the Ul-traCut-N (Reichert Nissei). In order to obtain enough contrast for TEM observation, the P4VP microdomains in the film were stained with OSO4. The film was observed by TEM (JEOL CX-100) at 100 kV. 

Figure 2 The transmission electron micrographs of samples cast from solution containing 1 wt% of polymer, (a) the block copolymer BCl, and (b) the microsphere, MCI [24].

Several attempts were made to influence the biodistribution of microspheres. The site of embolization depends on the site of administration and on the diameter of the microspheres [335]. Block copolymers have been used to coat the microspheres with the aim to prevent the recognition by the RES [9, 336-338] or to target them to the bone marrow [339], Interesting results in changing the body distribution of coated microspheres have been obtained. However, it appears that covalent attachment of hydrophilic polymer chains, e.g., PEG chains, is the better way [340]. 

Albertsson and coworkers [240-244] carried out extensive research to develop polymers in which the polymer properties are altered for different applications. The predominant procedure is ring-opening polymerization which provides a way to achieve pure and well defined structures. They have utilized cyclic monomers such as lactones, anhydrides, carbonates, ether-lactones. The work involved the synthesis of monomers not commercially available, studies of polymerization to form homopolymers, random and block copolymers, development of cross-linked polymers and polymer blends, surface modification in some cases, and characterization of the materials formed. The characterization is carried out with respect to the chemical composition and both chemical and physical structures, the degradation behavior in vitro and in vivo, and in some cases the ability to release drug components from microspheres prepared from the polymers. 

Pistel KF, Bittner B, Koll H, Winter G, Kissel T. Biodegradable recombinant human erythropoietin loaded microspheres prepared from linear and star-branched block copolymers Influence of encapsulation technique and polymer composition on particle characteristics. J Control Release 1999 59 309-325. 

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