Читать книгу Poly(lactic acid) - Группа авторов - Страница 97

PLAs having amino, carboxyl, or other functional (pendant or chain end) groups are well reported in the literature. These functional groups can be utilized for chemical modification or as binding sites for biomolecules to impart selective binding and adhesion. ROP of LLA or DLA using bis(hydroxymethyl) butyric acid (BHMBA) as an initiator and Sn(Oct)₂ as a catalyst at 130°C yielded PLA with pendant carboxyl groups. The chain extension of this polymer with diisocyanate yielded poly(ester–urethane) containing carboxyl groups as pendant functional groups [137].

Thiol‐functionalized PEG‐b‐PLA was prepared by ROP of DLA using PEG disulfide as the macroinitiator. The disulfide bond was cleaved using tributylphosphine to generate a block copolymer having a thiol unit at the PEG end [138]. Finne‐Wistrand and coworkers [139] utilized thiol chemistry to form redox responsive PLA‐b‐PEG nanoparticles. In addition, peptide‐functionalized porous scaffolds were prepared by disulfide exchange reaction of pendant thiol groups in poly(LLA‐co‐CL) [140].

Functionalization of PLA by grafting of maleic anhydride (MAn) has been carried out in the presence of free radical initiators (tert‐butyl peroxide and dicumyl peroxide) [141]. The presence of high succinic anhydride units in the grafts was confirmed by FTIR and NMR. Low percentage grafting was observed in PLA due to the presence of limited free radical sites [142].

Finne and Albertsson introduced a double bond in PLA by using 1,1‐di‐n‐butyl‐stanna‐2,7‐dixacyclo‐4‐heptene as initiator [143, 144]. The presence of a double bond in the LA macromonomer provided a variety of opportunities for further modification. For example, epoxidation was carried out with m‐chloroperoxybenzoic acid (mCPBA) and a quantitative conversion of the double bond to epoxide was observed.

PLA‐functionalized polyoxanorbornenes with one or two exo‐PLA chains, as well as two endo‐, exo‐chains were prepared using Sn(Oct)₂ as a catalyst in the presence of mono‐ or di‐alcohol derivatives of oxanorbornenes [145]. These macromonomers were then subjected to ring‐opening metathesis polymerization (ROMP) to yield graft copolymers as shown in Figure 4.12.

A sequential ROP and ROMP reaction was carried out in the same pot to yield well‐defined bottlebrush polymers. The process involved the synthesis of an LA‐based macromonomer via ROP of DLA initiated by an alcohol‐functionalized norbornene. This was followed by ROMP grafting‐through process in the same pot to produce the bottlebrush polymer architecture [146]. Simple one‐pot synthesis of block copolymers of norbornene and LA using a bifunctional initiator based on a ruthenium complex for the ROMP of norbornenes and an alcohol to initiate ROP of LLA using 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD) as a catalyst is also reported [147]. Low molar mass oligoLAs end capped with fumarate groups were used for in situ cross‐linkable scaffolds for tissue engineering [148]. Side‐chain functionalized diastereomeric LAs were synthesized from commercially available amino acids and their subsequent polymerization or copolymerization [108]. This approach allows the incorporation of any protected amino acid for the preparation of functionalized cyclic monomers. The quantitative deprotection of amino acids lead to the formation of new functionalized LA‐based polymers.

Protected functional LA copolymers can be synthesized by copolymerization of dibenzyloxy‐substituted monomers with LA. Deprotection followed by modification with succinic anhydride with carboxyl side chains was shown to be suitable for peptide coupling. Such a modification can control the attachment of cells in tissue engineering and other biomedical applications [109].

FIGURE 4.12 Synthesis of oxanorbornenes and LA‐based copolymers [145].