By Jan H. Schut
A new two-stage catalytic process from NatureWorks LLC, Minnetonka, MN (www.natureworksllc.com), the world’s largest producer of polylactic acid (PLA) biopolymer, takes an unusual mixed lactic acid feed stream, invented at Nagoya University in Japan (www.agr.nagoya-u-ac.jp), and makes it into semi-crystalline PLA block copolymers. The new copolymers, which are still in R&D, could potentially offer substantially higher melt temperatures than PLA homopolymer.
All commercial PLA so far is based on bacterial fermentation to a single lactic acid isomer (L-HLA), which is condensed into L-lactide monomer, which is polymerized into PLLA homopolymer. PLLA homopolymer was introduced in the 1990s by Cargill Inc., Minnetonka, MN (www.cargill.com) and commercialized in 2003 by NatureWorks, which is 50% owned by Cargill.
Instead of L-lactide, the feedstock for NatureWorks’ developmental PLA block copolymers is a mixture of “racemic-lactide” and “meso-lactide.” Lactides are ring compounds made by dehydration and condensation of hydroxyl and carboxyl groups from lactic acid and come in three shapes. L-lactide is a ring made of two L lactic acids; D-lactide is a ring made of two D lactic acids; and meso-lactide is a ring made of one of each. A racemic mixture means a 50/50 mixture of left- and right-handed molecules, so rac-lactide has equal amounts of L-lactide and D-lactide.
NEW CHEMICAL ROUTE TO LACTIDE MONOMERS
The rac-lactide mixture is made by a process invented by Professor Nobuyoshi Nomura of the graduate bio-agricultural polymer chemistry department of Nagoya University and reported in a series of Japanese patents and technical papers in 2003-2011. Nagoya University’s patented process (JP Pat. Applic. # 2003-64174) uses a purely chemical reaction of glycerin, or glycol, a byproduct from bio diesel production, with sodium hydroxide, or caustic soda.
The process was interesting, but not commercially significant at the time because the racemic lactic acid mixture yielded mixed lactides that didn’t make PLA with useful properties. Conventional ring-opening catalyst polymerized mixed lactides into atactic amorphous PLA, which has no melt temperature. Rac-lactide (half LLA and half DLA) could be separated from meso-lactide because rac-lactide has a higher melt temperature, but then one-third of the feedstock would be wasted. So the cost of separation and waste would have made Nagoya University’s chemical route to PLLA copolymer too expensive vs. existing fermentation routes to PLLA homopolymer.
Test Properties of NatureWorks’ PLA Block Copolymers in R&D
rac-LA/ meso-LA Ratio
|Step 1||Step 2||Mol. wt.||Melt temp. °C||Tacticity type|
|Reference example 3||100/0||2 hrs||0||109,000||183||Semi- crystalline|
|Working example 6||90/10||1 hr||3 hrs||143,000||177||Semi- crystalline|
|Comparative example 7||90/10||3 hrs||0||136,000||N/A||Amorphous|
Source: NatureWorks/Nagoya Univ. JP Patent #50689; U.S. Pat. Applic. #20150087799)
Dozens of researchers since the mid-1990s tried to find a catalyst process to polymerize a mixture of racemic-LA and meso-LA. Nagoya University’s Nomura tested a wide range of site-selective catalysts, including chiral salen- and homosalen aluminum complexes and conventional ring-opening catalysts with chain-end polymerization trying to create isotactic PLA copolymers. But with both site selective and chain end catalysts, the meso-lactide in the mix created “errors” in the PLA chains that made the material amorphous and unusable with low isotacticity. The isotacticity level of a polymer is the average number of molecules strung together with regularly alternating “thumbs” without errors in the sequence.
TWO-STAGE POLYMERIZATION OF PLA COPOLYMERS
NatureWorks’ invented a two-stage polymerization process that converts Nagoya University’s glycerin-based mixture of lactides (33% L-LA, 33% D-LA, and 33% meso-LA) into stereo regular, semi-crystalline PLA copolymers without separation. The patent (JP# 5806890; U.S. Pat. Applic. #20150087799) names NatureWorks, Nagoya University, and two companies, Hitachi Zosen Corp., Tokyo, Japan (www.hitachizosen.co.jp), and Tohoku Electric Power Co., Sendai, Japan (www.tohoku-epco.co.jp), which were on the original Nagoya University patent.
In the first stage, the higher solubility of meso-LA at lower temperature allows it to melt and dissolve into a small amount of solvent and be polymerized with ring-opening catalyst into amorphous polymeso-LA blocks, while the lower solubility of rac-lactide leaves it unreacted in the solid phase. In one patent example, first stage polymerization of meso-LA is done at 90 degrees C for one hour. Since the first polymerization stage allows meso-LA to be separated from racemic-LA, NatureWorks could test several different ratios of racemic-LA to meso-LA including 80/20, 90/10, and 100% racemic-LA with no meso-LA.
In the second polymerization stage, rac-lactide (50% L-LA and 50% D-LA) is heated to a higher temperature, allowing it to dissolve and react in the presence of Nagoya University’s single-site salen aluminum catalyst. Second-stage polymerization is described in one patent example as taking three hours at 130 degrees C without solvent. In the second polymerization stage, rac-lactide is therefore grafted onto polymeso-lactide.
“The final architecture is a block copolymer with one block of meso-lactide followed by large blocks of L-lactide followed by large blocks of D-lactide and vice versa,” NatureWorks’ chief scientist, Joseph Schroeder explains. “The catalyst is capable of generating blocks of L and D that are long enough to form stereo complex crystals that melt at 220 °C.” NatureWorks’ patent describes PLLA and PDLA blocks of eight molecules or more. PLLA and PDLA homopolymers have lower melt temperatures of 162-180 °C.
NatureWorks’ R&D on glycerin-based PLA copolymers so far is only on a bench top scale, and no timing has been set for when it might be scaled up. In the meantime, NatureWorks more immediate R&D priority is PLA based on methane, which could come from a variety of sources. Methane, which is produced by natural decomposition of plant matter, is a component of natural gas and is also generated from landfills and waste water treatment.
PLA from methane is part of NatureWorks’ joint R&D program with Calysta Inc., Menlo Park, CA (www.calysta.com), supported by a U.S. Department of Energy grant, for bacterial fermentation of methane to lactic acid, which would feed NatureWorks’ existing PLA homopolymer. Both glycerin and methane based research programs are environmentally interesting because they create PLA from industrial bi-products, not from sugar.