This review summarizes our work on folding and aggregation of polymer chains in water.We started with the folding or coil-to-globule transition of linear poly(N-isopropylacrylamide) (PNIPAM) homopolymer chains in water

and then studied the folding of hydrophilically or hydrophobically modified PNIPAM copolymer chains.The roles of hydrophobic and hydrophilic interactions in the folding were investigated.The intermediates in the folding process

i.e.

the molten globular state and the "ordered-coil" state between the random-coil and compacted globular states were observed.On the other hand

the aggregation and stabilization of amphiphilic polymer chains in water were also studied.Our results revealed that if the polymer chains form aggregates with a core-shell structure

the surface area occupied per hydrophilic moiety is a constant.Such a relation does not hold when some hydrophilic moieties are buried inside the aggregates

and the aggregates would develop into hyperbranched clusters as the amount of hydrophilic moieties increases.