Block Copolymers of PLA

Block copolymers of PLA with other comonomers have been foimd as very attractive biomaterials. It is known that block copoljnnerization allows combination 

As widely usable biodegradable materials, PLA is found to exhibit too hard and too brittle characters. The weakness might be improved when a polymer with lower glass transition temperature is used together [18]. Polyfe-caprolactone) (PCL) and poly(glycolic acid) (PGA), also known as polyglycolide, are two other 

Poly(p-dioxanone) (PPDO), one of the biodegradable and biocompatible aliphatic polyesters, has high flexibility, good tensile strength and mechanical properties. Due to its good biocompatibility and physical properties, PPDO has been considered as a candidate not only for medical uses but also for universal uses such as films, molded products, laminates, foams, non-woven materials, 

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Several other biodegradable, biocompatible, injectable polymers are being investigated for drug delivery systems. They include polyvinyl alcohol, block copolymer of PLA-PEG, polycyanoacrylate, polyanhydrides, cellulose, alginate, collagen, gelatin, albumin, starches, dextrans, hyaluronic acid and its derivatives, and hydroxyapatite. ... 

The morphology and structural architecture of polymers has significant effect on drug release. The release rates of 5-FU and paclitaxel, widely used chemotherapeutics, have been investigated in di-, tri-, and four-arm (star-branched) block copolymers of PLA and PEO. Micellar aggregates were prepared from these block copolymers and release rates were studied over 3 weeks. More complete dmg release was observed in star-shaped polymers [154]. 

Some other degradable (i.e., nonvinyl-type) polymers have been reported as components for amphiphilic block copolymers. For example, Hsiue reported the synthesis of a block copolymer of poly(2-ethyl oxazoline) and PLA by ROP. They reported the use of ABA-type triblock copolymers as pH-responsive polymer... 

Many kinds of nonbiodegradable vinyl-type hydrophilic polymers were also used in combination with aliphatic polyesters to prepare amphiphilic block copolymers. Two typical examples of the vinyl-polymers used are poly(/V-isopropylacrylamide) (PNIPAAm) [149-152] and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) [153]. PNIPAAm is well known as a temperature-responsive polymer and has been used in biomedicine to provide smart materials. Temperature-responsive nanoparticles or polymer micelles could be prepared using PNIPAAm-6-PLA block copolymers [149-152]. PMPC is also a well-known biocompatible polymer that suppresses protein adsorption and platelet adhesion, and has been used as the hydrophilic outer shell of polymer micelles consisting of a block copolymer of PMPC -co-PLA [153]. Many other vinyl-type polymers used for PLA-based amphiphilic block copolymers were also introduced in a recent review [16]. 

By introducing DXO units into the backbone of PLA, the Tg of the lactides may be lowered to or below body temperature, and the crystallinity may be lowered. Thus, more flexible materials are obtained where the properties and degradation rate can be varied over a broad range by means of the composition. This is interesting for biomedical applications. Blends of PLA and PDXO provide additional opportunities to vary the properties. There is a large difference in reactivity ratio of LA and DXO [154]. As a result, a more block-like copolymer structure than expected for a totally random copolymer is obtained, even though the structure is somewhat randomized by transesterification reactions. In contrast, DXO and e-CL form a truly random copolymer [156]. Tri-block copolymers of LA and DXO have recently been described [303]. 

Another type of stereo complex, i.e., a tightly wound 3/1-helix, is formed when polymers of L-lactic acid (PLLA) and D-lactic acid (PDLA) are mixed in the blend.247-248 Also in solution aggregation of block copolymers containing PLA segments has been found 249-250 however, no studies toward the morphology of the aggregates have been conducted. 

PEO-PPO-PEO triblock copolymers (Pluronics or Poloxamers) form reversible physically cross-linked hydrogels under certain concentration range and temperature. The use of this system in tissue engineering is scarce because of its inability to degrade. Di- or tri-block copolymers of PEG with PLA have been developed to overcome this problem. Multiple blocks of PEG and PLA, synthesized by condensation reaction of L-lactic acid in the presence of succinic acid. 

Methods of improving thermal resistance are to prepare a stereo complex (sc PLA) or a stereo-block copolymer (sbc PLA). Melting point and heat distortion temperature (HDT) will increase significantly. 

PHA chemical modification can be done via block copolymerizadon and grafting reactions, chlorination, cross-linking, epoxidation, hydroxyl and carboxylic acid functionalization, etc. (Chen et al. 2009 Wu et al. 2008 Li et al. 2003 Loh et al. 2007). A common approach to confer toughness to PLA is the use of a flexible monomer or macromolecules for copolymerization with lactide to form PLA-based random or block copolymers. Reported PLA-based block copolymers include diblock, triblock, and multiblock copolymers, such as poly(L-lactic acid) (PLLA)-polycaprolactone (Jeon et al. 2003), poly(ethylene glycol)-PLLA (Chen et al. 2003), poly(trimethylene carbonate)-PLLA (Tohru et al. 2003), and PLA-PBS-PLA. 

Figure 82 Proposed assembly and the corresponding SAXS curves of symmetric brush (a) random copolymers g(-[PLA-r-P/iBA] and (b) block copolymers g(-[PLA-b-P/iBA]. (inset in b) Photograph of slowly dried 0-[PLA2oo-i-PftBA2oo] showing green color due to reflectance from the large self-assembled domains. Reprinted from Xia, Y. Olsen, B. D. Kornfield, J. A. Grubbs, R. H. J. Am. Chem. Soc. 2009, 131 (51), 18525-18532, with permission from ACS. ...

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