Poly(lactic acid). Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Химия
Год издания: 0
isbn: 9781119767466
Скачать книгу
[51]. In the case of aluminum acetyl acetonate, the reactivity ratio of LLA was 44 and CL was 0.25. The microstructure depended on the temperature and the kind of initiator used. Also, magnesium complex with 4‐fluorophenol catalyst assisted as co‐catalyst system for sequential polymerization of LLA and CL to form poly(CL‐b‐LA) [120]. In the ligand‐assisted copolymerization, complexes with bulky substituents on the ligands yield a random arrangement, while less bulky substituents favor the formation of gradient copolymers, PLA‐gradual‐PCL. Alternating aluminum complexes bearing ligands containing electronegative atoms tends to favor CL polymerization rate selectively over LA [121, 122].

      A series of copolymers of DLA and CL were synthesized by ROP using zinc lactate as a catalyst and carrying out the reaction at 145°C for 8 days. The formation of copolymers was confirmed by gel permeation chromatography (GPC), DSC and NMR. Interestingly, Kister et al. [53] used vibrational spectroscopy, particularly Raman spectroscopy, for determination of morphology, conformation, configuration, and composition of the copolymers. Raman spectroscopy thus appeared to be a suitable method for the identification of PDLA‐co‐PCL samples directly from solid samples without any special preparation.

      4.2.5 1,5‐Dioxepan‐2‐One

      Albertsson and coworkers [59, 60,62–64] reported extensive studies on copolymers of LA and 1,5‐dioxepan‐2‐one (DXO). Poly(1,5‐dioxepan‐2‐one) (PDXO) is a completely amorphous and hydrophilic wax‐like polymer with a T g of −39°C. When DXO is used as a comonomer with LA, it increases the hydrophilicity and rate of degradation of the copolymers as compared with PLA. The copolymers show characteristics of thermoplastic elastomers that are suitable for biomedical applications such as slowly degrading sutures, temporary implants, and drug vehicles. Although the synthesis of DXO was reported as early as 1972, Mathisen et al. [59] in 1989 described an improved reaction scheme with a high yield. The DXO block forms a soft amorphous block, while LLA block forms a hard‐semicrystalline segment in the triblock copolymer of poly(LLA‐b‐DXO‐b‐LLA) [60, 62, 63]. Blocks of controlled lengths were synthesized using a tin oxide initiator. Degradable polyesters with strictly defined structure, unique mechanical properties, and tuned degradation profiles were prepared and characterized [64]. The morphology of spin‐coated films of triblock copolymers poly(LLA‐b‐DXOb‐LLA) was characterized by AFM. These studies revealed the absence of nanoscale morphology in copolymer films [64].

      4.2.6 Trimethylene Carbonate

Schematic illustration of representative structure of multiblock copolymers based on LLA and TMC [71].

      4.2.7 Poly(N‐Isopropylacrylamide)

Schematic illustration of reaction sequence for preparation of PNIPAAm-b-PLA [72].

      4.2.8 Alkylthiophene (P3AT)

      A semicrystalline block copolymer PLA‐b‐P3AT‐b‐PLA showed a tendency to phase segregate due to a self‐assembly process. Alkaline etching of LA blocks from the polythiophene matrix led to the formation of nanoporous templates, which is useful to generate ordered nanostructures. Other block copolymers can be prepared by combining 3‐(2′‐ethyl)hexylthiophene (3EHT) and LA using a change‐of‐mechanism polymerization technique that utilizes two controlled polymerization techniques. A Grignard metathesis (GRIM) reaction is used to polymerize 3EHT to form bromine‐terminated P3EHT, which is then end‐functionalized with a hydroxyl group through a Suzuki coupling reaction to form the P3EHT–OH macroinitiator. Subsequent controlled ROP of D,L‐LA using triethylaluminum results in the synthesis of P3EHT–PLA block copolymers [128]. It is unlikely to obtain complete functionalization of the P3EHT–CH2OH parent homopolymer with PLA, residual P3EHT in the reaction mixture was removed from P3EHT–PLA by selective precipitation in petroleum ether. Other block copolymers containing PEG and thiophene unit as 3‐hexylthiophene (3HT), 3‐dodecylthiophene (3DDT), and 3‐(2′‐ethyl)hexylthiophene (3EHT) blocks are also reported via azide‐alkyne coupling reaction [129]. Thienyl‐difluoroboron‐PLA have been synthesized and used to explore oxygen‐sensing capability based on phosphorescence [130].

Скачать книгу