Abstract: A catalyst for addition polymerization comprising(a) an (initiator) first component comprising a plurality of atoms M as hereinafter defined, each in a moiety of formula (I):MX(Y.sub.n)Z.sub.p (I)whereinM is Si, Ge or Snn is 0 or an integer, and p is an integer such that (n+p)=3;X is a diradical ##STR1## bound via Q to M whereQ is N, P, As or P(=T)G.D where T is O or S and G and D are each independently a bond, O or S,R.sup.1 is optionally substituted hydrocarbyl, or, when Q is P, M(Y.sub.n)Z.sub.p as herein defined,R.sup.2 is optionally substituted hydrocarbadiyl, orR.sup.1 and R.sup.2 together are hydrocarbatriyl,all such R.sup.1 and R.sup.2 being inert in the conditions of the process of the present invention; andR.sup.2 being linked directly or indirectly to any group as defined for X, or an organic radical comprising further moieties of formula (I), or one or more M(Y.sub.

Abstract: A catalyst for addition polymerization comprising:(a) an (initiator) first component of Formula (I):MX(Y.sub.n)Z.sub.p (I)whereinM is Si, Ge or Sn;n is O or an integer, and p is an integer such that (n+p)=3;X is a radical ##STR1## where Q and Q.sup.1 are each independently N, P or As;Q.sup.2 is O, S, NR or PR where R is C.sub.1-4 alkyl;R.sup.1 is H or optionally substituted hydrocarbyl; andR.sup.2 and R.sup.3 are each independently optionally substituted hydrocarbyl, or R.sup.2 and R.sup.3 together are optionally substituted aliphatic hydrocarbadiyl;all such R.sup.1, R.sup.2 and R.sup.3 being inert in the process conditions of the present invention;Y is A or OA where A is optionally substituted hydrocarbyl inert in the process conditions of the present invention, or trialkylsilylalkyl; andZ is independently any group as definedd for Y, or an organic polymer radical comprising further MXY.sub.n moieties; orp is 2 andZ.sub.2 is a group Z.sup.1 .sub.2 of the formula --OSiR.sup.5 R.sup.8 O-- where R.sup.5, R.sup.