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FIGURE 1.8 Ring‐opening polymerization of lactide to PLA initiated by an alcohol.
FIGURE 1.9 Equilibrium reaction of tin octoate with alcohol initiator or impurities to form catalytically active tin alkoxide bonds Sn─O─R [76].
In a nutshell, the total hydroxyl content, including R—OH initiator and lactic acid impurities, determines the maximum attainable M n (number‐average molecular weight) [4, 6]. The rate of polymerization is controlled by factors such as temperature and catalyst content, with the remark that a tin(II) octoate catalyst requires traces of the initiator to become active.
1.3.4.2 Alcohols
If water is the initiator, R equals H and hydrolysis of lactide produces lactoyl lactic acid (HL2). Propagation with lactide in the presence of a polymerization catalyst produces PLA with a hydroxyl and one carboxylic acid end group, as if the PLA was obtained by polycondensation of lactic acid.
If the hydroxyl group of lactic acid acts as an initiator, PLA with one hydroxyl end group and a lactic acid end group (HOOC─CH(CH3)─O─C(O)─) is obtained.
If the initiator itself is polymeric in nature, for example, polyethylene glycol (PEG), lactide can polymerize from the hydroxyl end group(s) of PEG resulting in PEG–PLLA diblock or triblock copolymers.
The molar ratio of monomer to initiator (M/I)—where initiator can also be read as total hydroxyl content—basically controls the final, average molecular weight (M n) of the PLA. A high amount of initiator produces short polymer chains, and a low amount of initiator produces high‐molecular‐weight polymer. The lower the amount of potentially initiating hydroxyls in the lactide monomer, the higher the maximum attainable degree of polymerization [69]. Since water and lactic acid can both cause ring scission of the lactide and initiate polymerization, their amounts in the lactide must be low and should be specified.
1.3.4.3 Carboxylic Acids
Carboxylic acids are poor initiators, but they are believed to interfere with the commonly used Sn(II) polymerization catalyst. According to Kowalski, carboxylic acids may suppress the rate of polymerization by shifting the equilibrium between ROH and Sn(Oct)2 to the inactive Sn(Oct)2 side [83, 84]. Consequently, longer polymerization times are needed to achieve the desired molecular weight, accompanied by unavoidable degradation caused by the extra residence time at high temperature in the presence of a catalyst [84].
The effect of carboxylic acids on lactide polymerization rate was published in 1993 in patents by Ford and O'Brien [78, 85]. The results clearly show the dramatic rate‐decreasing effect of organic acids: according to O'Brien, melt polymerization slows down by a factor of 2 upon increasing free acidity from less than 2 to between 2 and 4 meq/kg [85].
Witzke, however, states that the presence of lactic acid did not negatively influence polymerization rate [4, 6]. Lactic acid is therefore a practically used initiator that is already present in lactide as an impurity.
Lactic acid and its oligomers have a hydroxyl group and a carboxylic acid group. Consequently, a free acidity of 10 meq/kg—that is, 900 ppm expressed as lactic acid equivalents— in lactide corresponds to a hydroxyl concentration that limits M n to 100 kg/mol. Free acidity of 4 meq/kg sets a theoretical limit of 250 kg/mol to M n.
Free acid and water content specifications are essential for any lactide grade; the lower the amount of hydroxyl impurities, the better the storage stability and product properties of the lactide.
1.3.4.4 Metals
Metal cations such as Sn, Zn, Fe, Al, and Ti not only accelerate polymerization, but can also affect hydrolysis, oxidation, racemization, or other degradation mechanisms of PLA and lactides [4, 6]. Consequently, the lactic acid used for lactide preparation should be very low (ppm) in metal cations in order to avoid considerable racemization during lactide synthesis.
O'Brien has shown that the formation of dark color of lactide was a direct function of the iron content of the material in which the lactide was in contact [86]. Other examples in the patent (Examples 7 and 8) demonstrate the desirability of having low alkali (e.g., sodium) content and minimizing the depolymerization temperature.
Cationic impurities such as sodium ions have no direct effect on lactide production rate, but the sodium content has a direct correlation with the meso‐lactide content in the crude lactide [67, 87].
1.3.4.5 Stereochemical Purity
The higher the stereochemical purity of the lactide monomer, the higher the stereochemical purity of the obtained PLA, which controls material properties such as melting point, crystallinity and crystallization rate, and mechanical strength [8, 9, 88].
The strong dependence on D‐isomer content presents an opportunity to control polymer properties. NatureWorks Ingeo PLA is easily processable and suitable as amorphous biopackaging material as a result of its relatively high meso‐lactide content. The downside is the poor resistance to elevated temperatures (low heat distortion temperature, HDT) during transportation, storage, and use of articles produced from this bioplastic. meso‐Lactide—which contains an L‐ and a D‐isomer—is an unavoidable side product of lactide production and must be separated from L‐ and D‐lactides of high stereochemical purity.
Kolstad [9] investigated the crystallization behavior of copolymers of L‐lactide and meso‐lactide. He found that every 1% of meso‐lactide comonomer—or D‐isomer—causes a 3°C reduction in the melting point of the PLA copolymer. With 3% meso‐lactide in PLA, crystallization is more than two times slower than PLLA under the same conditions. With 6% meso‐lactide incorporation, the difference can be up to 10 times!
This underlines the need for a low meso‐lactide content in the monomer mixture for semicrystalline PLA, because meso‐lactide formation by racemization cannot be avoided during melt polymerization of lactides. According to Gruber and coworkers, racemization, which lowers the stereochemical purity of the PLA, is believed to be driven by factors such as temperature, pressure, time at a given temperature or pressure, the presence of catalysts or impurities, and relative concentrations of the two enantiomers at any given time during the polymerization process [88].
PLA grades for more demanding applications that require better heat resistance are achievable by stereocomplexation with PDLA [89]. This is only effective with PLA grades of high stereochemical purity. To prepare high‐quality PLA, it is necessary to start with lactide monomers with the highest possible stereochemical purity, that is, the lowest meso‐lactide content that is technically and economically achievable by purification.
D‐Lactide can be obtained if one has the appropriate biochemistry to produce the D‐enantiomer of lactic acid by fermentation of carbohydrates. Copolymerization of controlled mixtures of L‐ and D‐lactides subsequently offers the advantage of precise control over PLA properties. Moreover, D‐lactide is the monomer for the production of poly(D‐lactide), which is able to form high‐melting stereocomplex PLA via 1 : 1 racemic cocrystallization with P(L)LA, as will be discussed in Chapter 5 [89].
1.3.5 Concluding Remarks on Polymer‐Grade