Environmental Engineering Reference
In-Depth Information
[66-68], zinc (Zn) [66, 69-73], aluminum (Al) (74-83), tin (Sn) [64, 65, 73, 80, 82, 84-94],
and yittrium (Y) [95, 96] effectively catalyze the ring-opening polymerization of lactide. In
general, the most useful catalysts possess highly covalent metal-ligand bonds [33].
The general catalytic mechanism involves coordination of the lactide carbonyl group to
the catalyst metal, followed by cleavage of the ring acyl-oxygen bond and attachment of the
growing, catalyst-attached polymer (Figure 11). The terminally bound catalyst then promotes
the addition of successive monomers. Because each catalyst or initiator molecule facilitates
the extension of a single polymer chain, molecular weights of PLA are controlled by varying
the proportion of catalyst to lactide.
Among the coordination insertion catalysts, tin 2-ethylhexanoate (tin octoate, or
Sn[Oct]2) is the most widely used and studied due to its ability to produce highly crystalline
PLA in relatively short periods of time with high conversion and low racemization up to
180°C. It has also been approved by the U.S. Food and Drug Administration for food contact
[16], making it ideal for many packaging applications Both kinetics and mechanisms of tin
octoate polymerizations have been well characterized at both low [64] and high (≥1 80°C)
temperatures [65]. Water, lactic acid, and lactyl lactate can also form other species with tin
octoate, including tin oxide, tin hydroxides, tin lactate, all of which have been shown to
catalyze the ring opening of lactic acid [65, 97, 98]. Lactide polymerized with tin octoate is
best described by a second-order insertion mechanism [64] that is first order in monomer
concentration [73].
Recently, single-site catalysts prepared by the addition of bulky side groups to metals
have produced PLA with stereochemically-controlled structures [99]. Highly syndiotactic
PLA has been formed from meso-lactide using bulky racemic aluminum catalysts [100].
Using the same catalyst, D,L-lactide was polymerized to form “stereoblocks” of PLA,
possessing blocks of isotactic D- and L-PLA [101]. The stereoblock copolymer had a Tg
slightly higher than isotactic L- or D-PLA, but lower than the stereocomplex formed between
D- and L-PLA. Similar results were observed for the polymerization of D,L-lactide [102]
using the same catalyst. Single-site catalyst work thus demonstrates a method to produce PLA
with good control of the polymer backbone stereochemistry. This is a promising
development, now opening the possibility of forming stereocomplexes between copolymer
blocks that can withstand higher temperatures before heat distortion occurs at the interfaces
between blocks.
3.4. Architectural Variations
Using techniques developed for polymerization of linear PLA, it has been possible to
synthesize random copolymers, block copolymers, and branched polymers to modify the
properties of the materials produced.
Random copolymers are produced by polymerization of two or more species together
using a catalyst functional with both. Monomers that fit the criteria for ring-opening
copolymerizations with lactide are glycolide [103-105], glycolide derivatives [106-109],
lactones [30, 110-112], cyclic amide ethers [113], cyclic amide esters [114, 115], cyclic ether
esters [31, 116, 117], cyclic phosphates [118] cyclic anhydrides (119) or cyclic carbonates
[120-122]. Additional copolymers have been made through transesterification reactions of
PLA with other polyesters, including PET [123]. By adjusting the proportions of each
monomer, properties such as Tg, ease of biodegradation, and backbone flexibility can be
altered [31, 117].
