Over the past three decades, polypropylene in-reactor alloys have been widely used in various fields such as packaging, automotive and construction due to their excellent mechanical properties and good impact resistance. These excellent properties are derived from their complicated compositions and phase structures, which have attracted significant research interest in recent years. Polypropylene in-reactor alloy is a multicomponent system containing PP homopolymer, ethylene-propylene random copolymer (EPR) and ethylene-propylene block copolymers with different sequence lengths (EbP). These complex components form multiphase in the polypropylene in-reactor products. The original products are in the forms of powers or particles and have complicated morphology. After the processing, polypropylene in-reactor alloys will form abundance microstructures which have crucial influence on the impact strength of the final product. However, the relationship between molecular structures, morphology and properties of the product are still not well understood. For example, it was found that the formation of core-shell structure lead to the best balance of rigidity and toughness. However, studies on the formation condition and the composition of the core-shell structure are still limited. In the present review, we aim to highlight the recent progress in the studies on the morphology of polypropylene in-reactor alloys, and the prospective tendency of this field is proposed.

Contents
1 Introduction
2 Synthesis and composition of polypropylene in-reactor alloys
3 Morphology of polypropylene in-reactor alloys
3.1 Morphology of the polypropylene in-reactor alloys original particles
3.2 Influence of thermal treatment and processing on the morphology of the polypropylene in-reactor alloys
3.3 Core-shell structure of the polypropylene in-reactor alloys
4 Summary and outlook