chain transfer agents that are chlorine - free and liquid under polymerization conditions , useful for fluoromonomer polymerization , have the following structure : r 1 — o — r 2 they are superior to common chain transfer agents such as chloroform in being safer and environmentally benign . they are also superior to gaseous chain transfer agents such as ethane because they do not enter the fluoromonomer gas recycle streams and so do not complicate the analysis and purification of those streams .