A solid, particulate catalyst comprising: (i) a complex of formula: wherein M is zirconium or hafnium; each X is a sigma ligand; L is a divalent bridge selected from -R' 2 C-, -R' 2 C-CR' 2 -, -R'2Si-, -R' 2 Si-SiR' 2 -, -R'2Ge-, wherein each R' is independently a hydrogen atom, C1-C20-alkyl, tri(C1-C20-alkyl)silyl, C6-C20-aryl, C7-C20-arylalkyl or C7-C20-alkylaryl; each R 1 independently is a linear or branched C1-C20 hydrocarbyl radical optionally containing one or more heteroatoms from groups 14-16; T 1 and T 4, which can be the same or different, are an OR 2, a SR 2, CH(R 18) 2, CH 2 R 18, aryl or heteroaryl group; each R 2, which can be the same or different, is a C1-C20 hydrocarbyl radical; each R 18, which can be the same or different, is a C1-C20 hydrocarbyl radical or two R 18 groups together with the carbon atom to which they are attached may form a cyclic C4-C20 hydrocarbyl group; T 2 and T 3, which may be the same or different, are a C(R 18) 3 group; W 1 and W 2, which may be the same or different, are a 5 or 6 membered aryl or heteroaryl ring wherein each atom of said ring is optionally substituted with an R 5 group; each R 5, which can be the same or different, is a C1-C20 hydrocarbyl radical optionally containing one or more heteroatoms belonging to groups 14-16; and optionally two adjacent R 5 groups taken together can form a further mono or multicyclic ring condensed to W 1 or W 2, optionally substituted by one or two groups R 5; and (ii) a cocatalyst. (i) a complex of formula: wherein M is zirconium or hafnium; each X is a sigma ligand; L is a divalent bridge selected from -R' 2 C-, -R' 2 C-CR' 2 -, -R'2Si-, -R' 2 Si-SiR' 2 -, -R'2Ge-, wherein each R' is independently a hydrogen atom, C1-C20-alkyl, tri(C1-C20-alkyl)silyl, C6-C20-aryl, C7-C20-arylalkyl or C7-C20-alkylaryl; each R 1 independently is a linear or branched C1-C20 hydrocarbyl radical optionally containing one or more heteroatoms from groups 14-16; T 1 and T 4, which can be the same or different, are an OR 2, a SR 2, CH(R 18) 2, CH 2 R 18, aryl or heteroaryl group; each R 2, which can be the same or different, is a C1-C20 hydrocarbyl radical; each R 18, which can be the same or different, is a C1-C20 hydrocarbyl radical or two R 18 groups together with the carbon atom to which they are attached may form a cyclic C4-C20 hydrocarbyl group; T 2 and T 3, which may be the same or different, are a C(R 18) 3 group; W 1 and W 2, which may be the same or different, are a 5 or 6 membered aryl or heteroaryl ring wherein each atom of said ring is optionally substituted with an R 5 group; each R 5, which can be the same or different, is a C1-C20 hydrocarbyl radical optionally containing one or more heteroatoms belonging to groups 14-16; and optionally two adjacent R 5 groups taken together can form a further mono or multicyclic ring condensed to W 1 or W 2, optionally substituted by one or two groups R 5; and (ii) a cocatalyst. M is zirconium or hafnium; each X is a sigma ligand; L is a divalent bridge selected from -R' 2 C-, -R' 2 C-CR' 2 -, -R'2Si-, -R' 2 Si-SiR' 2 -, -R'2Ge-, wherein each R' is independently a hydrogen atom, C1-C20-alkyl, tri(C1-C20-alkyl)silyl, C6-C20-aryl, C7-C20-arylalkyl or C7-C20-alkylaryl; each R 1 independently is a linear or branched C1-C20 hydrocarbyl radical optionally containing one or more heteroatoms from groups 14-16; T 1 and T 4, which can be the same or different, are an OR 2, a SR 2, CH(R 18) 2, CH 2 R 18, aryl or heteroaryl group; each R 2, which can be the same or different, is a C1-C20 hydrocarbyl radical; each R 18, which can be the same or different, is a C1-C20 hydrocarbyl radical or two R 18 groups together with the carbon atom to which they are attached may form a cyclic C4-C20 hydrocarbyl group; T 2 and T 3, which may be the same or different, are a C(R 18) 3 group; W 1 and W 2, which may be the same or different, are a 5 or 6 membered aryl or heteroaryl ring wherein each atom of said ring is optionally substituted with an R 5 group; each R 5, which can be the same or different, is a C1-C20 hydrocarbyl radical optionally containing one or more heteroatoms belonging to groups 14-16; and optionally two adjacent R 5 groups taken together can form a further mono or multicyclic ring condensed to W 1 or W 2, optionally substituted by one or two groups R 5;