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Dissertation:
„Studies of poly(L-lactide) and L-lactide alcoholysis reactions
catalysed by metal alkoxides”
mgr Dominik Jerzy Bykowski
Supervisor: prof. dr hab. Piotr Sobota
The subject of this PhD dissertation regarding the investigation of the reaction of poly(L-lactide) and L-lactide alcoholysis which is catalysed by specific metal alkoxides. The effect
of the catalyst, amount and type of alcohol, conditions concerning to both solvent used
and temperature upon the reaction of PLLA and L-LA alcoholysis. Furthermore,
the selectivity of the products formation was discussed. Recently, a great interest in the field
of synthesis and usage of the chemicals has been paid to environmentally friendly substances
derived from renewable sources. Ecological approach in this aspect is intensively studied
by so-called „green chemistry”.
Poly(lactide) (PLA) is a aliphatic polyester obtained from renewable sources, and
is intensively used in the pharmaceutical and medical industries and in the production
of packages devoted to food products. The performed alcoholysis reactions of PLLA
and L-LA showed significant catalytic activity of metal alkoxides. The compounds based on
lithium, sodium, potassium, magnesium, calcium, zinc and aluminum metal centers were used
as catalysts for the alcoholysis reactions of PLLA and L-LA. Ethanol, methanol, and other
alcohols such as n-propanol, isopropanol, n-butanol and allyl alcohol were used in alcoholysis
reactions, whereas dichloromethane, toluene and ethyl lactate served as solvents. In the study
a commercially available high-weight PLLA in the form of granules was used. Applying these
catalysts and alcohols, the alkyl lactates and lactyllactates were obtained in high yields. For
obtained ethyl (S)-lactate (EtlacH) optical purity was determined, what confirmed the lack
of adverse racemization reaction in the PLLA alcoholysis process.
Performed studies clearly showed that both the amount and ratio of reagents [catalyst]:
[polymer]: [alcohol] used in the reaction have a significant impact on rate and selectivity
of the alcoholysis process and on the formation of lactic acid esters. It is worth noting that
using an excess of alcohol significantly reduces the step of the polyester chain fragmentation
and allows for determination of the thermodynamic equilibrium in the reaction mixture
between lactic and lactyllactic acid esters.
In the reaction with 10 equivs of ethanol (in relation to lactic acid units in the polymer
[-CH(CH3)COO-]) after 1 h reaction resulting the mixture which contains 18% of ethyl
lactate, 63% of ethyl lactyllactate and 19% of oligomeric products of PLLA fragmentation.
Extending the reaction time to 3 h allow to complete consumption of polyester and obtain
ethyl lactate with 96% overall yield. The reaction carried out with the ratio of the reagents
[Mg(OEt)2] : [PLLA] : [EtOH] = 1 : 100 : 500 proceeds much more slowly. The reaction
mixture contained 31% of lactic acid oligomers after 1 h. Increased reaction time to 3 h
caused the reduction of oligomers amount to 4%, and the quantity of the esters of lactic and
lactyllactic acids equalled 78 and 18%, respectively. A similar course of alcoholysis reaction
was also observed for other alcohols.
In the case of PLLA alcoholysis process that was proceeded rapidly in the presence of metal
alkoxides such as LiOEt and KOEt, the complete conversion of PLLA into ethyl esters
of lactic and lactyllactic acid was observed after 1 h. Here, no catalytic activity of zinc,
aluminum and sodium alkoxides was observed, and the lower reactivity of Ca(OEt)2 was
affected due to aggregation of these compounds in the presence of ethyl alcohol. Further
investigations revealed that prepared in situ magnesium alkoxides Mg(OR)2 have an optimum
activity in PLLA and L-LA alcoholysis reactions and allow to obtain alkyl lactates under mild
conditions, in a shorter period of time and with higher yield. Studies of alcoholysis reaction
in which ethyl lactate (hydroxyester) was used both as a solvent and reactant were also carried
out and allowed to eliminate toxic substances from the reaction system.
Furthermore, the alcoholysis reactions of L-LA with different alcohols were performed.
Compounds such as hydroxyesters: methyl and ethyl lactate, and alcohols containing chlorine
and fluorine (2-chloroethanol and 2,2,2-trifluoroethanol), allyl alcohol having a carbon-carbon double bonds (C=C) and long-chain aliphatic alcohols (C10-C16) were used. It has
been shown that in the presence of hydroxyesters: ethyl lactate and methyl lactate alkyl esters
of lactic acid trimer R(lac)3H are formed. These studies have additionally shown that during
L-LA
alcoholysis reaction, the self-condensation of RlacH with formation of ROH and
R(lac)2H occurs. Alcoholysis reactions of L-LA carried out in the presence of 2-chloroethane,
2,2,2-trifluoroethanol and allyl alcohol allowed to obtain 2-chloroethyl and 2,2,2-trifluoroethyl lactyllactates, and allyl lactate. Reactions of L-LA conducted in the presence of
long chain alcohols (C10-C16) were similar and allowed to yield the corresponding -esters
of lactic acid. In the last part of this work, the reactions of L-LA with fatty acids (C10-C16)
and their lithium salts were performed, which resulted in ω-esters of lactic acid.
The developed method for the synthesis of lactic acid esters can be used for the preparation
of alkyl lactates on a laboratory scale and in the long term to develop a technological process
of PLA recycling.