Polycondensation in Monolayers

Besides the polymerization of diacetylenes, butadienes and acrylates, the polycondensation behavior of long-chain ot-amino acids, diamines and diesters has been investigated in monolayers 52.  

All polycondensation reactions were followed by FT-IR-spectroscopy. Polyamide formation could be demonstrated by the disappearance of the ester C = 0 stretching mode and simultaneous formation of amide C = 0 stretching modes32 . Additional proof for polymer formation is given by gel permeation chromatography. From the elution volumes a molecular weight between 2,000 and 10,000 can be estimated. 

The methyl esters (39) and (41) form polycondensates of higher molecular weights than the corresponding docosanyl esters (40) and (42). 

The results illustrated so far show that polyreactions in oriented planar monolayers are possible and lead to highly ordered and very stable model membranes. This raises the question whether polymerization is also possible in bilayers (like vesicles and BLM) and whether these bilayers exhibit a higher stability than their unpolymerized counterparts. 

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Besides polymerization, another type of polyreaction can be used for stabilizing model membrane systems. Recently, Fukuda et al.28) described polyamide formation via polycondensation in monolayers at the gas/water interface (definition of mono-layers see Sect. 3.2). Long-chain esters of glycine and alanine were polycondensed to yield non-oriented polyamide films of polyglycine and polyalanine. 

Polycondensation reactions in oriented monolayers and bilayers proceed without catalysis, and simply occur due to the high packing density of the reactive groups and their orientation in these layers. Bulk condensation of the a-amino acid esters at higher temperatures does not lead to polypeptides but to 2,5-diketopiperazines. No diketopiperazines are found in polycondensed monolayers or liposomes. Polycondensation in monolayers and liposomes leading to oriented polyamides represents a new route for stabilizing model membranes under mild conditions. In addition, polypeptide vesicles may be cleavable by enzymes in the blood vessels. In this case, they would represent the first example of stable but biodegradable polymeric liposomes. 

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