NOVELTY - Polymeric polyphosphite (IIIa) containing 8-1000 repeating units (A1) and 0-1000 repeating units (B1) is new, where the phosphite comprises one phosphorus atom and (IIIa) is a reaction product of: at least one monohydroxy-terminated reactants; and at least one dihydroxy-terminated reactant.    USE - (IIIa) is useful: as stabilizers for the rubber and plastics industry and an antioxidants; for stabilization of organic materials against oxidative, thermal or actinic degradation and for preparing various shaped articles e.g. film, pipe, profile, bottle, tank or container; in polyurethanes as an anti-scorch agent and flame lamination additives, and in e.g. floating devices, electronic moldings, panes for dashboards, headlamp glass, instrument panel, exterior linings, upholstery, automotive lights, pillar covers, chassis parts, exhaust systems, fuel filter/filler, fuel pumps, fuel tank, body side moldings, lock systems, side impact protection, sound deadener/insulator and sunroof, road traffic devices, car accessories, warning triangles, medical cases, helmets, tires, devices for electronic industry, plane, railway, motor car, space applications, architecture and design, mining applications, appliances, printer, television-sets, audio and video devices, electric appliances, preferably washing machines, tumblers, ovens, dish-washers, mixers, vacuum cleaners, in technical articles such as cogwheel, slide fittings, spacers, screws, bolts, handles, and knobs, solar devices, sanitary articles, hygienic articles, feminine hygiene articles, shower curtains, brushes, mats, tubs, mobile toilets, tooth brushes, and bed pans, pipes, aircraft, schools, extruded sheets, plates, extrusion-coating, wood composites, walls, surfaces, furniture, decorative foil, floor coverings, cement-, concrete-, composite-applications and covers, tiles tarpaulins, woven fabrics continuous and staple, fibers, filters, films, batteries and connectors, screw tops, tops and stoppers for bottles, and cans, foamed articles, materials for optical and magnetic data storage, kitchen ware, boxes and DVD electronic articles.    ADVANTAGE - (IIIa) exhibits improved thermal stability and hydrolytic stability, and excellent compatibility and color stability in PVC.    DETAILED DESCRIPTION - Polymeric polyphosphite of formula (R1O-(P(OR2)-O-(R7)a-R8-R9-O)x-P(O-R3)(O-R4)) (IIIa) containing 8-1000 repeating units of formula (-(P(OR10)-O-(R7)a-R8-(R9)b-O)-) (A1) and 0-1000 repeating units of formula (-(P(OR10)-(O-Y2)m)-) (B1) is new, where the phosphite comprises one phosphorus atom and (IIIa) is a reaction product of: at least one monohydroxy-terminated reactants comprising an alcohol compound of formula (R1-OH), (R2-OH), (R3-OH) or (R4-OH); and at least one dihydroxy-terminated reactant having an alcohol compound of formula (HO-(R7)a-R8-(R9)a-OH), or a trifunctional reactant comprising at least one phosphorus moiety.    R1-R4 = 1-20C-alkyl, 2-22C-alkenyl, 6-40C-cycloalkyl, 7-40C-cycloalkylene or Y1-OH serving as end capping moiety for R1-R4;    Y1 = 2-40C-alkylene, 2-40C-cycloaliphatic carboxylic esters or 3-40C-cycloalkyl;    R7 = straight or branched 1-6C-alkylene;    R9 = straight or branched 1-6C-alkylene or 5-10C-saturated carbocyclic rings;    R8 = 5-10C-saturated carbocyclic rings;    x = 8-1000;    a, b = 0-1;    R10 = 1-20C-alkyl or 2-22C-alkenyl (which is optionally interrupted or terminated by 5-10C-cycloalkyl or cycloalkenyl), 2-20C-polyalkylene glycol chain optionally terminated by 1-4C-alkyl, 3-7 membered ring containing -CO-O- group or optionally substituted by 1-20C-alkyl;    Y2 = 2-22C-alkylene; and    m = 1-20, where the polyphosphite is terminated adjacent -P(OR2)- group of (B1) by group R10-, or terminated at the other end of the chain by a group -P(OR3)(OR4), in which each of R1, R3, or R4, which may be the same or different, has one of the meanings given for R2, provided that when the polyphosphite contains more than 1 but less than 12 units of (B1), it must contain 2 or more units of (A1), and further provided that when the polyphosphite contains no units of (B1), it must contain 8 or more units of (A1).    INDEPENDENT CLAIMS are included for:    (1) the preparation of (IIIa); and    (2) a copolymeric polyphosphite, where (IIIa) further comprises two repeating units to form a copolymer of formula (R1O-((P(OR2)-O-(R7)a-R8-(R9)b-O)x1-(P(O-R3)-(O-Y1)m-O-))zw-P(OR5)(OR6)) (IVa).    R5, R6 = R1-R4;    x1 = 1-1000;    z = 0-1000, proviso that when z is greater than 1 but less than 12, then x1 ranges from 1-1000 and with the further proviso that when z is 0, then x1 is 8 or greater;    a, b = 0-1; and    w = 1-1000.