Polymerization of Lactide Monomers

PEO monomethyl ether was used to initiate the ringopening polymerization of lactide monomers on its free side chain hydroxyl. The polymerization route was thoroughly discussed in a previous study by our group [13]. 

Polylactide is a kind of biodegradable polyester that possesses many desirable properties such as non-toxicity, hydrolyzability and biocompatibility for use for varied biomedical purposes such as sutures, fracture fixation, oral implant and drug delivery microspheres.It is popularly synthesized by the ringopening polymerization of lactide monomers which are the cyclic dimers of lactic acid. Polymerization of racemic D,L-lactide typically results in atactic, amorphous polymers named poly(D,L-lactide) (rg 60 °C), whereas polymerization of L-lactide or D-lactide results in isotactic, semicrystalline polymers called poly(L-lactide) or poly(D-lactide) (7" 180 PLA fractures... 

Degee, P., Dubois, PH., Jerome, R., Jacobsen, S., and Fritzh, G. (1999). New Catalysis for Fast Bulk Ring-Opening Polymerization of Lactide Monomers, Macromol Symp., 144,289—302. 

Polyester syntheses have been achieved by enzymatic ring-opening polymerization of lactide and lactones with various ring-sizes. Here, we focus not only on these cyclic esters but also other cyclic monomers for lipase-catalyzed ringopening polymerizations. Figure 8 summarizes cyclic monomers for providing polyesters via lipase catalysis. 

Fig. 1 Synthesis of lactide monomer from natural resources, lactide polymerization in the presence of a metal catalysts, and biodegradation of PLA. MOR metal alkoxide...

While using 19 and 20 as catalysts, a linear relationship between M , conversion of monomer to polymer (%), and PDI (1.10-1.25) was recorded, which support a controlled polymerization of L-lactide. The kinetic studies revealed a first order polymerization of lactide and metal complex in CD2CI2 and CDCI3 solutions. The results indicated that magnesium complex 19 is more active than its Zn(II) analogs (6 days, [M]o/[I]o = 500, CH2CI2, 25 °C, 90% conversion) due to the expected higher polarity of the Mg-OR bond relative to that of the zinc complexes. 

Many works on the synthesis of cyclic polymers and block copolymers using kinetically controlled ring-expansion polymerizations of cyclic monomers, such as lactones and lactides with various types of cyclic tin initiators, were reviewed by Kricheldorf [147,148]. Kricheldorfs group continued the synthesis of cyclic polymers, and their recent works have focused on the following. Polycondensations of 4,4/-difluorodiphenylsulfone with tris(4-hydroxy phenyl)ethane were performed in DMSO to give multi-cyclic poly(ether sulfone)s derived from tris(4-hydroxyphenyl)ethane [149]. 

Lactide is the cycUc dimer of lactic acid, which exists as two optical active isomers d and l. L-Lactide is the naturally occurring isomer, while OL-lactide is the synthetic blend of D-lactide and L-lactide. The polymerization of these monomers leads to either a semicrystalline polymer or an amorphous polymer. Poly(L-lactide) (PLLA), for example, is a semicrystaUine polymer with a degree of crystallinity around 37%. It has a glass transition temperature of 60-65 C and a melting temperature of approximately 175 °C. Conversely, poly(DL-lactide) (DLPLA) is an amorphous polymer with random distribution of both isomeric forms of... 

The range of polymers prepared using lanthanides and earth alkaline alkoxides can be expected to increase, especially bearing in mind that the polymerization activity of these alkoxides is not restricted to the polymerization of lactone-type monomers and diat monomers such as cyclic anhydrides, carbonates, oxides and others can be (co)polymerized satisfactorily. Furthermore, a broader range of ligand environments of the metal elements must be evaluated for a full appreciation of the potential of these initiators. Recent examples on the stereoselective polymerization of lactides reveal the possibilities of these new catalyst-initiators in macromolecular engineering. Eventually, it may be expected that for every specific polymer an optimal initiator will be available. 

Aluminum porphyrins are versatile initiators which are applicable to controlled ring-opening polymerization of various heterocyclic monomers (Table 1) such as epoxides (11)," oxetanes (12) lactones with four-, six-, and seven-membered rings (13-15), lactide (16), six-membered cyclic carbonates (17) and cyclic siloxanes (19).-" They are also excellent initiators for the controlled addition polymerization of unsaturated monomers such as acrylates (20), methacrylates (21)-- and methacrylonitrile (22).- - ... 

As one-pot reactions by the simultaneous initiation of both polymerizations always affect one another, control of the overall process is often very difficult to achieve. Another fairly new strategy, the AROP of lactones [210-213], lactides [214] or benzyl-L-glutamate [215] and the controlled radical polymerization of vinyl monomer, which take place in one-pot but in consecutive fashion, has been introduced by several groups. In this strategy, the AROP of lactones, lactides, or benzyl-L-glutamate can st be initiated by either an enzymatic or a metal catalyst at low temperature. In a second step, ATRP of MMA [210-213], tBMA [212], or 2-hydroxyethyl MA [214] and NMRP of styrene [211, 215] can be activated by increasing the reaction temperature and injecting the ATRP catalyst, respectively (Scheme 11.48). The reaction was conducted in one-pot, without any intermediate work-up and purification. 

PLA can be synthesized by two routes polycondensation of lactic acid or ringopening polymerization of its cyclic dimer, lactide [12]. PLA prepared from polycondensation has low molar mass and poor mechanical properties and Is therefore not suitable for many applications [13]. High-molar-mass PLA Is most commonly made by ring-opening polymerization of lactide. In both cases, lactic acid is the feedstock for PLA production. Lactic acid has an asymmetric carbon atom, which leads to two optically active forms called L-lactic acid and D-lactic acid. When producing PLA from lactide, polymerization can start from three types of monomers LL-lactide made from two L-lactic acid molecules, DD-lactide from dimerization of D-lactic acid, and LD or wieso-lactide made from a combination of one L- and one D-lactic acid molecules [14,15]. The chemical structures of lactic acid and lactide molecules are illustrated in Figure 5.1. 

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