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FAIR-CT96-1829
Polyglycerols chemistry ecology and applications of polyglycerol esters |
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Proposal No: | FAIR-CT96-1829 |
| Date Prepared: | July 2001, September 1999 | |
| Source: | Final Report
Abstract and Synthesis Report Progress Report November 1998 |
Source: Final Report, January 2001
Consortium: The project was coordinated by Novance, Compiegne (France) and ONIDOL, Paris (France), in partnership with Henkel KGaA, Düsseldorf (Germany), Rhone Poulenc Industrialisation, Decines-Charpieu (France), Universite d'Orleans (France), ENSCR, Campus de Beaulieu, Rennes (France) and the Universidad Complutense Madrid (Spain).
Summary
Introduction
Conventional methods for polymerising glycerol require drastic process conditions, including high temperature and caustic content. As a result, the molecular composition of the product is very difficult to determine and correlations between structure and performances cannot be established. This limits the rational design of optimised structures. Several parameters, which have not yet been fully studied, need to be quantified, in terms of analytical data, performances, toxicology and enviroruncntal impact. These include:
These parameters are critical because some molecular descriptors such as linearity versus branching, or nature of the fatty chain can have a dramatic impact on biodegradation, and on meeting the market requirements, as far as performances is concerned.
Objectives
The overall aim of this project to provide a complete description of polyglycerol based surfactants, as a result of completing the following tasks:
Achievements
Task 1. Selection and synthesis of several representative target compounds
1.1. Synthesis of linear samples A successful strategy for the synthesis of linear di-, tri-, tetra- and pentaglycerols from commercially available derivatives and from a variety of synthetic key precursors has been developed. The target molecules were fully characterised and the syntheses scaled up. About 4 g of each sample was delivered to partners (Henkel, Rhone-Poulenc, Universitie of Madrid and Orleans).
1.2 Synthesis of cyclic samples An efficient four or five step strategy toward cyclic dimers and their monolauroyl or monolauryl derivatives has been developed. The 2,6-bis-hydroxymethyl p-dioxane and the corresponding surfactants have been produced via a new pathway, at a multigram scale for testing. Higher cyclic oligomers (triglycerol to pentaglycerol) have also been synthesized.
1.3. Synthesis of branched samples Previous reports have described various methods that lead to branched oligomers of glycerol, with a defined degree of oligomerisation from 2 to 6. These syntheses have been upscaled to 3-4 grams of each target compound and the pure products have been sent to various partners.
1.4. Enzymatic synthesis of polyglycerol esters - An enzymatic process for the production of (mainly) the diglycerol monoester of lauric acid has been carried out using Novozym 435 as catalyst.
Task 2. Analytical characterisation and structrual assignments
2.1. Selection of a chromatographic method - New analytical methods have been developed for the study of polyglycerols and polyglycerol esters:
An Evaporative Light Scattering Detector was used with the liquid chromatographs. This performs as a quasi-universal detection method in isocratic or gradient elution.
2.2. Structural characterisation - With the help of the analytical method selected in task 2.1. all the compounds prepared by synthesis teams were investigated in depth, using the classical tools of analytical organic chemistry, including mass spectroscopy. Purity was checked either by elemental analysis or HPLC, sometimes both methods were used. This enabled a full description of the molecular units to be obtained, in order to quantify the relationship between structure and activity.
Task 3. Evaluation of performance and toxicology (irritation)
This task dealt with those aspects of the project required to fulfill market requirements in terms of performances and toxicological impact. These criteria were used to select the most promising compounds. Such parameters are also essential for assessment of bioaccumulation (subtask 4.2.). In addition, irritation testing (applied to both polyglycerols and polyglycerol-based surfactants) was added to the evaluations.
3.1. Assessment of detergent properties of polyglycerol monoesters and ethers The compounds obtained in this project behaved as nonionic surfactants with low cmc values, critical micelle concentrations and good surface activity as revealed by the low surface tensions. Even when very dilute, as may be the case during application, the formation of micelles (as well as related properties such as solubilisation) were expected. All polyglycerols have low interfacial tension againts n-decane and therefore good emulsififying capability and a very pronounced foaming behaviour.
However, even if their performances are quite good, any further development is dependent on the cost of manufacture (see below).
3.2 Assessment of the skin irritation potential of polyglycerols (PG's) and polyglycerol derivatives by in vitro HET-CAM and investigation of effects on the vitality of human reconstructed skin models - (additional, voluntary investigations)
In technical polyglycerol esters (i.e. mixtures of polyglycerol homologues, mono-/ poly esters of fatty acids with different chainlengths), irritation effects of certain molecular species might be compensated by "dilution" with non-irritating esters and/ or unreacted polyglcerols. Synergistic effects lowering the irritation potential of such roducts also be possible.
Task 4: Evaluarion of environmental impact
Nonionic surfactants concentrations in the environment are generally below 0.05mg/l. Higher concentrations can be exceptionally observed in the case of industrial wastes. In order to obtain environinentally friendly polyglycerol surfactants, they were assessed in order to fulfil the stringent requirements covering biodegradability, ecotoxicitv and bioaccumulation. The aim of task 4 was therefore to predict the behaviour and effects of polyglycerols in water. The work concerned the following aspects:
4.1. Exploration of biodegradability The ultimate aerobic biodegradability of polyglycerol based surfactants has been evaluated using screening tests in order to compare the relative biodegradability of the synthesized compounds and to select compounds of interest in terms of reduced environmental impact. One of the main objectives of the project has been reached. For the first time in the field of polyglycerols, a correlation was established between the structure of a given polyglycerol monoester or - ether (position of the fatty acid/ -allkyl chain) and the biodegradation score observed although, there was no surprise here: linear polyglycerols were biodegradable, ramified and cyclic structures showed increased persistence.
4.2. Ecotoxicity and Bioaccumulation A series of ecotox tests were cvarried out on selected polyglycerol samples. In terms of structure/toxicity relationships the following was concluded:
Task 5 Economic assessment: balance between performance and market requirement
As the project concerned basic research, from which improved compounds could emerge, industrial development would be occur only in the future, since this aspect was not scheduled within the framework of the present project. An economic assessment was therefore a necessity. It had been proposed to investigate the food additives market as a priority and then consider the markets for cosmetics and other uses. In the event, only the former was reviewed, due to the small size of the other markets.
If a higher price has to be charged for materials having the PG structures suggested by the present project, this has to be justified by some unusual properties compared with existing market products (active ingredients market, no bulk applications possible). For instance a diglycerol monolaurate can be compared with pentaerythritol monolaurate.
As a result of extensive testing, no such unusual properties could be found - making it very difficult to introduce such new products into the market.
The situation is worse for detergent applications of pure PG esters, since it is necessary to compare these with established and moderately priced products like sorbitan esters or APG™ (Cognis and others) as benchmarks.
Summary
Introduction Conventional methods for polymerising glycerol require drastic process conditions including high temperature and alkali content. The molecular composition of the resulting mixtures is very difficult to determine and correlations between structure and performances cannot be established, which limits the rational design of optimised structures. Several parameters, which have not yet been fully studied, need to be quantified, in terms of analytical data, performances, toxicology and environmental impact in order to generate better products. In particular information is required concerning:
Such information is critical because some molecular characteristics, such as linearity versus branching, or nature of the fatty chain can have a dramatic impact on both biodegradation and on the extent to which products meet market requirements in terms of performance performances. Hence, the initial task is the evaluation of performances and biodegradability, with mixtures of polyglycerols and polyglycerol esters resulting from industrial processes using caustic solutions and elevated temperatures.
Objectives The overall objective of this programme is to provide a complete description of polyglycerol based surfactants following on from selection and synthesis of several representative targets compounds, followed by analysis, characterisation and structural assignments. These will be evaluated for performances, determined through measurements of the physico-chemical properties, and toxicology (irritation). Their potential environmental impact and economic value, in terms of performance and market requirements will also be evaluated.
Activities - Task 1: Synthesis
1. 1 Synthesis of linear samples The scientific objectives for the first period of the programme were to synthesise and characterise linear oligoglycerols as well as the corresponding fatty esters with a selected degree of oligomerisation ranging from di- to penta-glycerol. A general strategy has been adopted , based on the use of commercially available monomer derivatives and a number of synthetic key dimeric and trimeric precursors possessing orthogonal protective groups or functionalities. These precursors allowed convergent preparations of higher oligomers i.e. tetra- and pentaglycerol. The synthesis of these defined linear oligomers of glycerol was achieved in an efficient procedure that involved the regio-specific opening of one or two epoxides by hydroxylated nucleophiles under basic conditions, and by using convenient phase transfer catalysis. Tri- and tetraglycerol monoesters with well defined structure (primary, internal or central positions) and a lauroyl acyl chain were produced under mild reaction conditions, using N-lauroylthiazolidine-2- thione or lauroyl chloride in order to avoid any intramolecular isomerisation. Structural characterisations were performed by elemental analysis, mass spectrometry, NMR spectrometry and chromatography. Most of compounds described in the work have been prepared for the first time, to the best of knowledge, with a high degree of purity allowing therefore their physico-chemical properties to be determined. Linear polyglycerols of degree 2, 3 and 4 and three regio-isomers of lauroyl triglycerol were also prepared at a 3 to 4 g scale for chromatographic studies and for testing sensitisation as well as physicochemical evaluation.
1.2 Synthesis of cyclic samples Scheduled in the second year of the project.
1.3 Synthesis of branched samples The main objective is to prepare at the laboratory scale authentic samples of branched oligomers of glycerol and of the corresponding monoesters; to characterise the structures of the synthesised compounds by NMR, elemental analysis and chromatographic methods, and to scale-up the preparations in order to supply various partners with samples. The strategy for the elaboration of polyglycerols consists of assembling activated "monomer" and "dimer" units, with precursor of dimer plus precursor of monomer giving trimers and precursor of dimer plus precursor of dimer giving tatramers.
In order to synthesise the prim-sec type dimer, a large number of nucleophilic displacements by diverse alcoholates or openings of epoxides in various conditions have been tested. These attempts yielded at most 60 % of the desired product, using 4 equivalents of tosylate at a temperature of 80°C. With increasing degree of oligomerisation these yields would probably decrease and could come close to zero for the coupling between two secondary positions. Therefore a different strategy was used, based on the oxidising cleavage of selected anhydro-alditols, with several grams produced for toxicology and biodegradability studies, as well as for use in further stereospecific esterification.
1.4 Enzymatic synthesis of polyglycerol esters A methodology was developed to scale-down an enzymatic batch stirred reactor for-the reaction between diglycerol and lauric acid. Factorial design of experiments technique has been used for the optimization of the operating variables on the enzymatic synthesis using an standard reactor (250cm3) . The highest monoester yield (64 %) has been obtained using 3 % of Novozym as catalyst and a molar ratio of reactants of one. It was concluded that enzymatic esterification could lead to pure esters of the central or terminal hydroxyls, where synthetic methods fail to produce these.
Activities - Task 2: Analytical characterisation and structural assignments
2.1 Selection of a chromatographic method Properties of polyglycerols are linked to their structures. As the total number of topo-isomeric structures corresponding to each value of dimer, trimer, tetramer... (linear, branched, cyclic) is very high, efficient HPLC systems have been investigated in order to analyse samples. Two complementary systems, based on amino-column or porous graphitized column, were investigated with acetonitrile-water mixtures as mobile phase. As these compounds are non-UV absorbing, Evaporative Light Scattering Detection was used. The molecular weight of each eluted sample was determined using LC-MS. In addition general analytical methods were developed for use by the synthesis teams. LC-MS allowed partial identification when standards were missing, while synthesised standards, with a well defined structure, enabled a better identification of the isomers that had the same molecular weight and the same MS fingerprint.
2.2 Structural characterisation Using the analytical method selected all the compounds prepared by synthesis teams were investigated in detail, to quantify the relationship between structure and activity, that will continue through the duration of the project.
Activities - Task 3: Evaluation of performances and toxicology This task concerns market requirements and performance and aims to select the most promising compounds, including aspects of bioaccumulation assessment. The activity of this task depends on the samples, both polyglycerols and polyglycerol-based surfactants, starting with the mixture of polyglycerols and polyglycerol esters resulting from the industrial processes using caustic solutions and elevated temperatures. The following samples were made available: Linear Polyglycerol 3, Linear Polyglycerol 4 , Lauroyl terminal ester, Lauroyl central ester Lauroyl internal ester, PG coprah fatty acids ester (mixture). Physicochemical characterisation started in late 1998 to assess their detergent properties. Experiments are still in progress, so no results were available by the end of the first year of the project. Some samples have been subject to a skin sensitisation test HET-CAM (hens egg test on the chorion allantoic membrane), again experiments are in progress.
Activities - Task 4: Evaluation of the environmental impact
The ultimate aerobic biodegradability of polyglycerol based surfactants was evaluated using screening tests in order to compare relative biodegradability of the synthesised compounds and to select compounds of interest in terms of reduced environmental impact. These screening tests were conducted in closed serum bottles containing a mixture of micro-organisms, a mineral medium and the surfactant. The initial microbial concentration and the surfactant concentration were optimised in order to reduce test duration, with biodegradability evaluated by measurement of either oxygen consumption or inorganic carbon production. The confirmation of overall biodegradation confirmed by measuring the dissolved organic carbon at the start and the end of the tests.
The first of compounds used were diglycerol, purified polyglycerol, and coprah based polyglycerol ester. Better results were obtained with the inorganic carbon test with 30%, 40% and 10% biodegradation after only 7 days in comparison with 68%, 64% and 31% after 28 days for diglycerol, coprah based polyglycerol ester and purified polyglycerol, respectively with the oxygen consumption test but with lower biodegradation percentages, A good prediction for comparison between tested compounds was obtained for the Inorganic Carbon Production test after 7 days. An inhibitory effect for high concentrations of tested compounds may be responsible for lower results obtained with the oxygen consumption test and it may be useful in future to conduct a preliminary respirometry test in order to verify that the initial concentration used is not toxic to the micro-organisms.
ConclusionsA successful strategy was developed for the synthesis of linear di-, tri-, tetra-, and pentaglycerols from commercially available monomer derivatives and from -a range of synthetic key precursors possessing orthogonal protective groups or functionalities. These precursors allowed efficient convergent preparations of higher oligomers, The successful coupling of mono- or diepoxides with hydroxylated nucleophiles under phase transfer catalysis conditions highlights the potential of the methods for the synthesis of well-defined linear oligomers of glycerol. Hence, linear tri-, tetra-, and pentaglycerols were prepared for chromatographic studies. It is a general rule that success in synthesis is not predictable. However, global strategies have been established that should ensure that significant quantities of new products will be prepared in the second year. Synthesis of branched samples has also been successful, with the synthetic moieties needed for continuation prepared in "bulk" quantities, ensuring success in year 2. It was also shown possible to esterify polyglycerols using lipases.
The required analytical methods have been established, for both structural characterisation and to establish the relationship between molecular weight and chromatographic parameters. The evaluation of performances and toxicology has been initiated following the availability of the first samples. Other aspects will start in the second year with the most promising polyglycerols.
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