Process for the preparation of polyisoprene with a mainly alternating cis-1,4-alt-3,4 structure comprising polymerizing isoprene in the presence of a catalytic system comprising:
(a) at least one pyridyl iron complex having general formula (I):




wherein:
R1 is selected from linear or branched C1-C20, preferably C1-C15, alkyl groups, optionally substituted cycloalkyl groups, optionally substituted aryl groups;
R2 is selected from linear or branched C1-C10, preferably C1-C3, alkyl groups; X, mutually identical or different, represent a halogen atom such as, for example, chlorine, bromine, iodine; or they are selected from linear or branched C1-C20, preferably C1-C15, alkyl groups, —OCOR3 groups or —OR3 groups wherein R3 is selected from linear or branched C1-C20, preferably C1-C15, alkyl groups.
n is 2 or 3;
(b) at least one co-catalyst selected from organo-derivative compounds of aluminum, preferably from
(b1) aluminoxanes having general formula (II):

(R4)2—Al—O—[—Al(R5)—O—]m—Al—(R6)2  (II)

wherein R4, R5 and R6, mutually identical or different, represent a hydrogen atom, or a halogen atom such as, for example, chlorine, bromine, iodine, fluorine; or they are selected from linear or branched C1-C20 alkyl groups, cycloalkyl groups, aryl groups, said groups being optionally substituted with one or more silicon or germanium atoms; and m is an integer ranging from 0 to 1000;
(b2) aluminum compounds having general formula (III):

Al(R7)(R8)(R9)  (III)

wherein R7 is a hydrogen atom, or is selected from linear or branched C1-C20 alkyl groups, cycloalkyl groups, aryl groups, alkylaryl groups, arylalkyl groups, alkoxy groups; R8 and R9, mutually identical or different, are selected from linear or branched C1-C20 alkyl groups, cycloalkyl groups, aryl groups, alkylaryl groups, arylalkyl groups;
wherein the molar ratio between the aluminum present in the co-catalyst and the iron present in the iron pyridyl complex having general formula (I) is ranging from 5 to 60, preferably from 8 to 55.