Читать книгу Poly(lactic acid) - Группа авторов - Страница 98

Studies on copolymers with LA having a core of LA (or another comonomer) and branches of another monomer (or LA comonomer) have been extensively reported in the literature. Graft copolymers having different architectures (linear branches, hyperbranched, star‐like, brush‐like, and comb‐like) were synthesized to modify the properties of PLA. The hydrophilicity or crystallinity of these copolymers can be varied and controlled by preparing such architectures. A general reaction for the preparation of such copolymers is depicted in Figure 4.13.

Branched PLA is different from linear PLA in physical, thermal, and mechanical properties. Such polymers were prepared by using multifunctional alcohols, for example, inositol, pentaerythritol, glycerol, and so on [149–154]. Finne and Albertsson prepared four‐arm star‐shaped PLLA using novel spirocyclic tin initiators [149]. Kricheldorf et al. [150] polymerized LLA using bismuth triacetate and pentaerythritol as initiator and co‐initiator, respectively. Kim et al. [151] and Arvanitoyannis et al. [152] used Sn(Oct)₂, tetraphenyl tin and pentaerythritol, respectively, as the initiator and co‐initiator system for LLA polymerization. Similarly, Korhonen et al. [153] reported star‐shaped polymers using co‐initiators containing multiple hydroxyl groups.

As can be seen, many types of branched PLA are prepared by using organometallic catalysts and multifunctional alcohols. Figure 4.14 shows the synthesis of branched PLA using lipase PS (Pseudomonas fluorescens) catalyzed ROP of LA monomers (LLA, DLA, DLLA) [154].

Branched copolymers were also synthesized by the preparation of macromonomers. Various types of methacrylate‐functionalized macromonomers are reported in the literature for the preparation of graft and star copolyesters. The reaction scheme used for the preparation of the macromonomers is depicted in Figure 4.15.

FIGURE 4.13 Synthetic route for the preparation of branched PLLA.

FIGURE 4.14 Reaction scheme of enzymatic polymerization [154].

FIGURE 4.15 Synthesis of macromonomers.

Segmented terpolymers of poly(alkylmethacrylate‐g‐DLA/dimethylsiloxane) were prepared by combination of a “grafting through” technique (macromonomer method) and controlled/living radical polymerization such as ATRP or RAFT. Different synthetic approaches for the ATRP synthesis of graft terpolymers can be adopted by either one‐step copolymerization or two‐step sequential approach. In a single‐step approach, the low‐molecular‐weight methacrylate monomer [methyl methacrylate (MMA), butyl methacrylate (BuMA)] (Figure 4.16) was polymerized onto a LA or dimethylsiloxane (DMS) macromonomer. The second strategy was a two‐step approach in which a graft copolymer containing one macromonomer is chain‐extended with the copolymerization of the second macromonomer and the low‐molecular‐weight monomer, forming a second block–graft copolymer [155].

FIGURE 4.16 Structure of LA‐ and DMS‐based macromonomers and macroinitiators (M: methacrylate, A: acrylate) [155].