Bioplastics, which will see rising demand in the next years, have to be compounded by using a twin screw extruder like fossil based plastics as well as non-biodegradable plastics. Polyhydroxybutyrate (PHB) belongs to the family of polyhydroxyalkanoates (PHA) and is both, biobased and biodegradable. PHB is insoluble in water and, in contrast to many other bioplastics, has a good barrier effect against water vapour. But PHB is like other bioplastics extremely brittle and cannot use due to that for film applications. This paper shows the synthesis of a PHB-block copolymer by using the twin screw extruder without any catalyst, solvents or purification processes. By using a twin-screw extruder with a special screw design the molecular weight of PHB can be controlled degraded. In a second step the PHB and soft phase can be compounded to a block copolymer by means of reactive extrusion. This improves the ductile properties of the material without changing the thermal or chemical characteristics. The analysis of the block copolymer is executed by using a plate-plate rheometer, a differential scanning calorimeter (DSC), an infrared spectroscopy as well as different microscopes. So by formation of a PHB block copolymer the elongation at break of 1 % for the pure PHB could be increased to 35 % in the block copolymer.