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## Biodegradable polyesters obtained via solid-state polycondensation

### Marcin Kaczorowski

#### Abstract

Introduction Polylactide (PLA) is a biodegradable, aliphatic polyester which can be produced from renewable resources, such as plants' waste. PLA's properties are similar to these of typical polymers produced from crude oil, e.g. polyolefins, polystyrene, or poly(ethylene terephthalate). PLA is produced via ring-opening polymerization of lactide. This method is an expensive one, because of the high cost of the monomer. High molecular weight polymer cannot be obtained via direct melt polycondensation of lactic acid. The method with azeotropic dehydratation requires the use of large amounts of solvent, which is ecologically disadvantageous. Solid state polycondensation (SSP) is an alternative route to obtain high molecular weight polymer in a relatively low temperature. Prepolymers subjected to SSP must be partially crystalline and their molecular weight is usually between 10 000 and 100 000. The process is carried out at a temperature between the glass transition temperature and melting temperature of crystallites. The reaction proceeds in the amorphous phase, where mobile end groups of the polymer chains are present. Reaction byproducts are removed under vacuum or by using inert gas flow. The aim of the study was to investigate the effectiveness of different catalysts, the effect of crystallization time and time of SSP on the properties of the resulting polymer. The influence of the type of reaction system on the melt polycondensation and the influence of the purity of lactic acid on the molecular weight of the product were also investigated. Results and discussion The synthesis of PLA consisted of three stages: the dehydration of aqueous lactic acid, polycondensation in the melt followed by crystallization, and solid-state polycondensation. Reactions were carried out using solutions of lactic acid from two different manufacturers. Firstly, effectiveness of various catalysts was investigated by conducting several melt polycondensation reactions. Selected prepolymers after grinding (grain diameter 200-500 µm) were crystallized at 70 or 105 °C in an oven at atmospheric pressure or in a flask heated in an oil bath under reduced pressure. After the crystallization step prepolymers were subjected to SSP at a temperature of 140-150 °C (depending on the melting temperature of the prepolymer) at a pressure of 0.5 Torr for 20, 40 or 70 hours. Gel permeation chromatography (GPC) measurements indicated that the most effective of the tested catalysts were stannous chloride, titanium(IV) butoxide and antimony(III) oxide. Differential scanning calorimetry (DSC) measurements showed that prepolymers with the highest melting temperatures were these obtained using heterogeneous catalysts. The addition of a nucleating agent (talc) caused the increase in the degree of crystallinity of the prepolymer. Polymers with the highest molecular weights of about 100 000-120 000 were made from prepolymers obtained using tin chloride as catalyst. MALDI-ToF measurements showed that the presence of monofunctional alcohols or carboxylic acids in the lactic acid solution may be the molecular weight limiting factor. Conclusions Studies have shown that the molecular weight of the final product depends on the purity of the lactic acid solution, the type of catalyst, the crystalline phase content in the prepolymer, the time and temperature of the SSP. The degree of crystallinity is influenced by the type of catalyst, the nucleating agent content, the time and conditions of crystallization. The highest molecular weight polymer was obtained using tin chloride - p-toluenesulfonic acid catalyst system. The prepolymer containing 0.05 wt% of talc after 1 hour of crystallization was subjected to SSP at 150 °C for 40 hours. Weight average molecular weight of the resulting polymer was about 120 000.
Record ID
WUT726d1f2f5ccf4c22b8f07ef13cf75df3
Diploma type
Master of Science
Author
Marcin Kaczorowski (FC/CPCT) Marcin Kaczorowski,, Chair Of Polymer Chemistry And Technology (FC/CPCT)Faculty of Chemistry (FC)
Title in Polish
Poliestry biodegradowalne otrzymywane metodą polikondensacji w stanie stałym
Supervisor
Gabriel Rokicki (FC/CPCT) Gabriel Rokicki,, Chair Of Polymer Chemistry And Technology (FC/CPCT)Faculty of Chemistry (FC)
Certifying unit
Faculty of Chemistry (FC)
Affiliation unit
Chair Of Polymer Chemistry And Technology (FC/CPCT)
Study subject / specialization
, Technologia Chemiczna
Language
(pl) Polish
Status
Finished
Defense Date
07-09-2012
Issue date (year)
2012
Keywords in Polish
-
Keywords in English
-
Abstract in Polish
urn:pw-repo:WUT726d1f2f5ccf4c22b8f07ef13cf75df3