Lactide, cationic polymerization

Due to the lack of vinyl monomers giving rise to crystalline segment by cationic polymerization, amorphous/crystalline block copolymers have not been prepared by living cationic sequential block copolymerization. Although site-transformation has been utilized extensively for the synthesis of block copolymers, only a few PIB/crystalline block copolymers such as poly(L-lactide-fc-IB-fc-L-lactide) [92], poly(IB-fr- -caprolactone( -CL)) [93] diblock and poly( -CL-fr-IB-fr- -CL) [94] triblock copolymers with relatively short PIB block segment (Mn< 10,000 g/mol) were reported. This is most likely due to difficulties in quantitative end-functionalization of high molecular weight PIB. 

Monomers listed above polymerize by the cationic mechanism. For some groups of monomers (lactones, carbonates) anionic or coordinate mechanism also operates and, from a synthetic point of view, this is the preferred method of converting cyclic esters into linear polyesters. The cationic polymerization of lactones, glycolide and it substituted analog, lactide, as well as spiroorthoesters and bicyclic orthoesters has been studied in some detail. 

Hydrolysis of L-lactide gives lactic acid with the same optical purity as hydrolysis of the polymer of L-lactide. This indicates that cationic polymerization of L-lactide proceeds with retention of configuration on both C atoms, i.e. by O-acyl cleavage 21,22) ... 

The ring opening polymerization of lactones, lactides, and glycolides to polyesters can be carried out by both anionic and cationic means using a variety of initiators similar to those used for cyclic ethers. However, cationic polymerization is... 

In the cationic ROP, only triflic acid and trifluoromethanesulfonic acid have shown potential as initiators for the polymerization of lactide. The polymerization starts by protonation or alkylation of carbonyl oxygen, which results in positively charged alkyl-oxygen bond and the propagation step involves cleavage of this bond. The PLA obtained by this route is optically active if the appropriate reaction temperature is chosen (< 50 °C) however, the product obtained at such temperatures is of low molecular weight. 

Studies of cationic polymerization, carried out in parallel, solved an initial controversy concerning the nature of artive species, that is, acylium versus tertiary oxonium cations, in favor of the latter ones. Attempts of several laboratories to prepare high-molar-mass aliphatic polyesters in the controlled cationic process eventually failed. The first example of a controlled cationic process involving cyclic ester is the activated monomer (AM) polymerization of CL, conducted in the presence of an alcohol. More recendy, Basko and Kubisa " published a series of papers that confirm applicability of the AM mechanism to the controlled synthesis of aliphatic polyesters. Interestingly, it has been also shown for the first time that triflic acid initiation of L,L-lactide (l,l-1A) polymerization, without a purposely introduced alcohol, leads to the living process proceeding in agreement with the AM mechanism. ... 

This review aims at reporting on the synthesis of aliphatic polyesters by ROP of lactones. It is worth noting that lactones include cyclic mono- and diesters. Typical cyclic diesters are lactide and glycolide, whose polymerizations provide aliphatic polyesters widely used in the frame of biomedical applications. Nevertheless, this review will focus on the polymerization of cyclic monoesters. It will be shown that the ROP of lactones can take place by various mechanisms. The polymerization can be initiated by anions, organometallic species, cations, and nucleophiles. It can also be catalyzed by Bronsted acids, Lewis acids, enzymes, organic nucleophiles, and bases. The number of processes reported for the ROP of lactones is so huge that it is almost impossible to describe aU of them. In this review, we will focus on the more... 

Apart from the development of lithium initiators for the facile polymerization of L-lactide, mainly by our group, there are only a limited number of Na and K initiators known in the literature. Sodium and potassium cations are nontoxic and are essential to life, and we reported the first EDBP-Na complex as efficient initiator for the preparation of PLA [41]. Similarly, potassium EDBP complex [EDBPH-K-(THE)2] 18 has been demonstrated to be an efficient catalyst for the ROP of L-lactide in a controlled fashion, yielding PLAs with expected molecular weights and moderate PDIs (1.29-1.58) [42],... 

Early-on it was discovered that these Salen compounds, and the related six-coordinate cations [6], were useful as catalysts for the polymerization of oxiranes. These applications were anticipated in the efforts of Spassky [7] and in the substantial work of Inoue [8]. Subsequently, applications of these compounds in organic synthesis have been developed [9]. Additional applications include their use in catalytic lactide polymerization [10], lactone oligomerization [11], the phospho-aldol reaction [12], and as an initiator in methyl methacrylate polymerization [13]. 

Abstract. This paper reviews ring-opening polymerization of lactones and lactides with different types of initiators and catalysts as well as their use in the synthesis of macromolecules with advanced architecture. The purpose of this paper is to review the latest developments within the coordination-insertion mechanism, and to describe the mechanisms and typical kinetic features. Cationic and anionic ring-opening polymerizations are mentioned only briefly. 

Poly(e-caprolactone) (PCL) is synthesized by anionic, cationic or coordination polymerization of e-caprolactone. Degradable block copolymers with polyethylene glycol, diglycolide, substituted caprolactones and /-valerolactone can also be synthesized. Like the lactide polymers, PCL and its copolymers degrade both in vitro and in vivo by bulk hydrolysis, with the degradation rate affected by the size and shape of the device and additives. 

Resulting poly(a-hydroxyacids) are important biomaterials used as resorbable sutures and prostheses [196]. The mechanism of polymerization is not well established. Polymerization may be initiated with Lewis acids (SbF3, ZnCl2, SnCl4) however, other typical cationic initiators (e.g, triethyloxonium or triphenylcarbenium salts) fail to initiate polymerization [197]. Thus, it is not clear whether polymerization proceeds by typical cationic mechanism or rather involves the coordination mechanism. The chain transfer to polymer resulting in transesterification was postulated [198,199] and confirmed later by detailed, 3C NMR studies of lactide copolymers [200]. 

Lactones, i.e. esters of hydroxyacids and their dimers, like glycolide and lactide, are two major groups of cyclic esters used in polymerization. These compounds are used on the large scale and polymerized mostly by anionic or coordinative mechanisms. Four, six-, and seven-membered lactones polymerize by both cationic and anionic mechanisms. Polyesters prepared in this way are however only a small fraction of the polyesters prepared by polycondensation. 

The cationic and coordinative polymerizations are the only routes for polymerization of the six-membered dimers of the glycolic and lactic acids, i.e., glycolide and lactide, leading to polymers used in medicine. 

The mechanism for the polymerization of lactide can be cationic, anionic, coordination or free radical polymerization (Auras, Rafael) as discussed below ... 

The NHC complex (l-ethyl-3-methylimidazol-2-ylidene)silver(i) chloride is an ionic liquid, and was foimd to catalyze the ring-opening polymerization of lactide at elevated temperatures to give narrowly dispersed polylactide of predictable molecular weight [94]. Here, the ionic liquid is acting as a source of the NHC by the thermal decomposition of the silver imidazolylidene complex cation. 

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