The solid-state polymerization of bisphenol A polycarbonate (BPAPC) has been studied in amorphous and partially crystallized micro-layers (SSP_m) of low molecular weight prepolymers in presence of LiOH.H₂O catalyst at a temperature between the glass transition temperature and the melting point. When the prepolymers (14,000 g/mol) in micro-layers of a thickness range from 5 μm to 35 μm were solid-state polymerized at 230 °C, the polymer molecular weight increased rapidly to above 100,000 g/mol, exceeding the highest molecular weight obtainable by the conventional solid-state polymerization in micro-particles. It has also been observed that the final molecular weight reached as high as 600,000 g/mol even in presence of significant stoichiometric imbalances of end group mole ratios when the prepolymer having 21,000 g/mol is used at 230°C under low pressure (10 mmHg). Most notably, amorphous prepolymer micro-layers showed significantly higher increase in molecular weight than partially crystallized prepolymer micro-layers.

The chain branching and partially cross-linked structures in high molecular weight polycarbonates have been confirmed by 1H-NMR spectroscopic analysis as well as pyrolysis-gas chromatography mass spectrometry (Py-GC/MS). 13C-NMR analysis and SSP theoretical model simulation have shown that conventional linear step-growth polymerization is not responsible for the additional increase in molecular weight beyond 50,000 g/mol of polycarbonate MW. The ultra-high molecular weight is contributed to the formation of branched and partially cross-linked structures via Fries or Kolbe-Schmitt rearrangement reactions and radical recombination reaction, respectively. Micro-radical polycarbonate species can be produced via chain scission reaction and hydrogen abstraction at the solid-state polymerization temperature. The formation of cross-linked polymers by radical recombination reactions was attributed to the near complete removal of phenol (i.e. radical scavenger) from the micro-layers during the solid state polymerization. Branched structure polycarbonate was also confirmed by atomic force microscopy (AFM).

The presence of branched and cross-linked polymers contributed to the insolubility of the polymer in solvents such as chloroform, tetrahydrofuran (THF), and methlylene chloride. As SSP_m process extends for a long reaction time at 230°C, about 95% of the polymer was insoluble with excellent transparency (90-93% light transmission). Properties of ultra-high molecular weight nonlinear polycarbonates (SSP_m PCs) have been investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and rheometer.

The development of Multi-Layer Deposit and Reaction (MLDR) technique has shown that the SSP_m process is not limited to 5-35μm scale. The layer thickness can be expanded while keeping the merits (e.g. high transparency, good solvent resistance, and obtaining high molecular weight in short reaction time) of the SSP_m technique developed in this study.