Game theory is a mathematical formalism dealing with problem of competition between players. In cooperative games, players can negotiate and form coalitions to optimize their own gains. The theory gives some mathematical solutions, but not explains how coalitions will be formed. This paper describes a negotiation platform used to test various dynamics in a cooperative game theory setting. We focus our attention on dynamic of coalition formation between players. Players are described by three features: attraction for gain, risk aversion and strength of character. These features are used to define rationalities of players for negotiations. Simulations are performed and described with different combinations of players' behaviors. Some first results of coalitions gains distribution are displayed versus mathematical game theory solutions.