liquid and then 2 reacting the thus obtained solution with a halogen of a transition metal to produce the solid catalyst suitable titanium compounds include the titanium halides and titanium alkoxides having 1 4 carbon atoms preferably ticl 4 suitable organo phosphate include the trialkyl phosphates having 1 10 carbon atoms preferably tri n butyl phosphate suitable solvents include any inert organic solvent preferably an aliphatic hydrocarbon having 4 8 carbon atoms a titanium tetrachloride catalyst supported on neodymium trichloride is surprisingly effective with the aid of an aluminum alkyl cocatalyst preferably a tri alkyl aluminum such as tri ethyl aluminum in producing an app of negligible crystallinity such a catalyst can be formed by 1 using tri n butyl phosphates to dissolve anhydrous ndcl 3 in a dry organic solvent and then 2 reacting this solution with ticl 4 a ticl 4 catalyst supported on praesydium trichloride which can be produced in the manner described above with the aid of cocatalysts described above is also effective in producing an app of negligible crystallinity similarly a ticl 4 catalyst supported on a mixture of ndcl 3 and prcl 3 can be used to obtain the desired amorphous propylene polymer the polymerization of propylene.