A polymeric polyphosphite containing from 1 to 1000 repeating units of the formula: in which: R 2 is selected from the group consisting of (i) a C 1-20 alkyl group or C 2-22 alkenyl group which is optionally interrupted or terminated by a C 5-10 cycloalkyl or cycloalkenyl group, (ii) a C 2-20 polyalkylene glycol chain optionally terminated by a C 1-4 alkyl group, and (iii) a 3 to 7 membered ring containing a -CO-O- group and optionally substituted by a C 1-20 alkyl group; each of R 7 and R 9 independently represents a C 1-6 alkylene group; R 8 is a cyclohexylene group; and a and b are independently selected from the group consisting of 0 and 1; and from 0 to 1000 repeating units of the formula: in which Y represents a C 2-22 alkylene group and m is from 1 to 20; said polyphosphite being terminated adjacent the -P(OR 2)- group of the formula above by a group R 1 O-, and terminated at the other end of the chain by a group -P(OR 3)(OR 4), in which each of R 1, R 3, and R 4, which may be the same or different, has one of the meanings given for R 2; provided that when said polyphosphite contains more than 1 but less than 12 units of Formula B, it must contain 2 or more units of Formula A; and further provided that when said polyphosphite contains no units of Formula B, it must contain 8 or more units of Formula A. R 2 is selected from the group consisting of (i) a C 1-20 alkyl group or C 2-22 alkenyl group which is optionally interrupted or terminated by a C 5-10 cycloalkyl or cycloalkenyl group, (ii) a C 2-20 polyalkylene glycol chain optionally terminated by a C 1-4 alkyl group, and (iii) a 3 to 7 membered ring containing a -CO-O- group and optionally substituted by a C 1-20 alkyl group; (i) a C 1-20 alkyl group or C 2-22 alkenyl group which is optionally interrupted or terminated by a C 5-10 cycloalkyl or cycloalkenyl group, (ii) a C 2-20 polyalkylene glycol chain optionally terminated by a C 1-4 alkyl group, and (iii) a 3 to 7 membered ring containing a -CO-O- group and optionally substituted by a C 1-20 alkyl group; each of R 7 and R 9 independently represents a C 1-6 alkylene group; R 8 is a cyclohexylene group; and a and b are independently selected from the group consisting of 0 and 1; and from 0 to 1000 repeating units of the formula: in which Y represents a C 2-22 alkylene group and m is from 1 to 20; said polyphosphite being terminated adjacent the -P(OR 2)- group of the formula above by a group R 1 O-, and terminated at the other end of the chain by a group -P(OR 3)(OR 4), in which each of R 1, R 3, and R 4, which may be the same or different, has one of the meanings given for R 2; provided that when said polyphosphite contains more than 1 but less than 12 units of Formula B, it must contain 2 or more units of Formula A; and further provided that when said polyphosphite contains no units of Formula B, it must contain 8 or more units of Formula A. in which Y represents a C 2-22 alkylene group and m is from 1 to 20; said polyphosphite being terminated adjacent the -P(OR 2)- group of the formula above by a group R 1 O-, and terminated at the other end of the chain by a group -P(OR 3)(OR 4), in which each of R 1, R 3, and R 4, which may be the same or different, has one of the meanings given for R 2; provided that when said polyphosphite contains more than 1 but less than 12 units of Formula B, it must contain 2 or more units of Formula A; and further provided that when said polyphosphite contains no units of Formula B, it must contain 8 or more units of Formula A.