[10] Due to the problems with the separation and recycling of homogeneous carbonylation catalysts, heterogeneous polymer analogs have recently also been investigated, which deliver similarly high yields (up to 96%) at 60 bar CO pressure.

However, these catalysts do not yet appear to be promising candidates for industrial application as they show drastically lower catalytic activity in 50 mm molar laboratory batches.

[11] The cheap starting material butane-1,3-diol can be converted with the oxidizing agent barium manganate (BaMnO4) in acetonitrile under microwave irradiation within 1h to β-butyrolactone (74% yield).

[13] The commercialization of the polyhydroxybutyric acid (PHB) or of homo- and copolymeric polyhydroxyalkanoates as aerobically biodegradable thermoplastics isolated from bacteria under the brand name Biopol of Imperial Chemical Industries (ICI) in 1983 set the starting point for the search for synthetic alternatives which were to avoid the disadvantages of PHB such as brittleness and stiffness, thermal decomposition at temperatures just above the melting temperature (175 - 180 °C) and in particular uncompetitive costs[14] due to expensive fermentation, isolation and purification.

[23] Synthetic PHB variants, which were developed as homopolymers of β-butyrolactone or copolymers with other lactones, have so far not been able to compensate for the weaknesses of the biogenic material - in particular unfavourable mechanical and thermal properties and high price.

Instead, new problems with toxic heavy metals in the catalysts (e.g. tin, cobalt or chromium) and atactic polymer components (liquid and difficult to separate) with undesirable material properties have been introduced.