the backbone of the resin of the present invention is formed either ( 1 ) by reacting a dihydroxyl polydiene prepolymer with a diisocyanate as a chain extender to form a polyurethane polydiene resin or ( 2 ) by reacting a dicarboxyl polydiene with a diepoxide to form an epoxy polydiene resin . because of their greater yield strengths , the polyurethane polydiene resins are preferred over the epoxy polydiene resins . the dihydroxyl or dicarboxyl terminated polydiene prepolymers used in the present invention should have a high residual vinyl or alkenyl content . polydienes derived from isoprene , dimethylbutadiene , methylpentadiene , or butadiene are suitable , with dicarboxyl 1 , 2 - polybutadiene and dihydroxyl 1 , 2 - polybutadiene being preferred and dihydroxyl 1 , 2 - polybutadiene being more preferred . dihydroxyl and dicarboxyl polydiene prepolymers having molecular weights of from about 600 to about 3000 are preferred . if the molecular weight exceeds 3000 , the prepolymers become too viscous for proper casting and potting operations . the chain extenders for the dihydroxyl polybutadiene prepolymers may be aromatic as represented by toluene - 2 , 4 - diisocyanate or naphthalene - 1 , 5 - diisocyanate . however , the non - aromatic diisocyanates ( e . g . hexamethylene - 1 , 6 - diisocyanate , 2 , 2 , 4 - trimethylhexamethylene - 1 , 6 - diisocyanate , isophorone diisocyanate , dicyclohexylmethane diisocyanate ) are preferred because they undergo urethane and allophanate formation rather slowly thus displaying low viscosities during the casting and vacuum potting operation . isophorone diisocyanate is the more preferred chain extender . the key feature of this invention is the grafting of a cylic monomer selected from the group consisting of n - vinyl - 2 - pyrrolidone , n - vinyl - 3 - pyrrolidone , n - vinylcarbazole , n - vinylimidazole , n - vinylpyridine , coumarone , indene and mixtures thereof onto the polymer backbone . the preferred graft monomers are n - vinyl - 2 - pyrrolidone , n - vinyl - 3 - pyrrolidone , and mixtures thereof . although the free radical initiator used is not a critical limitation , 0 , 0 &# 39 ;- bis ( t - butylperoxy ) diisopropylbenzene works well as the catalyst . preferably about 2 parts by weight of the free radical initiator should be used for every 100 parts by weight of the dihydroxyl or dicarboxyl polydiene prepolymer . the preferred range of the molar ratio of aliphatic diisocyanate to dihydroxyl polydiene prepolymer is from about 0 . 75 : 1 to about 1 . 30 : 1 , with from 0 . 95 : 1 to 1 . 10 : 1 being the more preferred range . for instance , preferably from 12 to 20 and more preferably from 15 to 18 parts by weight of isophorene diisocyanate chain extender ( mw = 222 ) should be used with every 100 parts by weight of dihydroxyl 1 , 2 - polybutadiene prepolymer ( mw = 1420 ). if aromatic diisocyanates are reacted with the dihydroxyl polydiene prepolymer , an excess of the aromatic diisocyanate should be used for proper viscosity control . thus , the preferred range of the molar ratio of aromatic diisocyanate to dihydroxyl polydiene prepolymer is from about 0 . 95 : 1 to about 1 . 65 : 1 , with from 1 . 20 : 1 to 1 . 50 : 1 being the more preferred range . for example , preferably from about 12 to about 20 and more preferably from 15 to 18 parts by weight toluene diisocyanate chain extender ( mw = 174 ) should be used for every 100 by weight of the dihydroxyl 1 , 2 - polybutadiene prepolymer ( mw = 1420 ) used . the preferred range of the molar ratio of the cyclic graft comonomer used in this invention to the dihydroxyl or dicarboxyl polydiene prepolymer is from about 0 . 60 : 1 to about 6 . 40 : 1 with from 4 . 45 : 1 to 5 . 75 : 1 being the more preferred range . for instance , from about 5 to about 50 , and more preferably from 35 to 45 parts by weight of n - vinyl - 2 - pyrrolidone ( mw = 111 ) should be used for every 100 parts by weight of dihydroxyl 1 , 2 - polybutadiene prepolymer ( mw = 1420 ). the amount of n - vinyl - 2 - pyrrolidone which can be used for a given weight of prepolymer is limited by the hygroscopic properties of the vinyl pyrrolidone . these cyclic graft comonomers are grafted onto an already existing polydiene polymer backbone and therefore are to be distinguished from polymers where the cyclic comonomers are part of the backbone itself . the difunctional polydiene prepolymer , chain extender , cyclic graft comonomer , and free radical initiator are all mixed together and allowed to react at ambient temperature for about 1 hour . later the reaction mixture may be poured into casts or structural spaces and cured at elevated temperatures . moreover , by embedding glass or phenolic microspheres into the reaction mixture before the curing step at elevated temperatures , syntactic buoyancy foams having superior strength and buoyancy characteristics are obtained . note that before the curing step at elevated temperatures , the reaction mixture contains some unreacted isocyanate groups which have an inherent reactivity with silanol ( or other metal hydroxide ) groups located in the surface of the glass microspheres . the pyrrolidone moieties contribute further to the adhesiveness of the resin matrix . and finally , adhesion promotors derived from silyperoxides ( e . g . tris ( tert - butylperoxy ) vinyl silane ) drastically increase bond strength and they reduce water absorption of the glass resin interface . the following examples serve to illustrate the present invention without , however , limiting the same thereto : molecular weight 1420vinyl content ,% 91 . 5hydroxy content , meg / gr 1 . 19viscosity , poises at 45 ° c 55 there was added a mixture of 15 parts by weight isophorone diisocyanate , 40 parts by weight n - vinyl - 2 - pyrrolidone and 4 parts by weight 0 , 0 &# 39 ;- bis ( t - butylperoxy ) diisopropylbenzene ( commercially known as &# 34 ; vul - cup r &# 34 ;). after stirring until homogenous , the resulting reaction product maintained over a period of one hour the following temperature - viscosity relationship : the product was poured into a mold and cured consecutively 48 hours at 90 ° c plus 24 hours each at 100 ° c , 110 ° c and 120 °- 200 ° c . yield strength ( 0 . 2 % offset ), psi 11800ultimate strength , psi 27500compressive modulus , psi 450000density ( gr / cc ) 1 . 04 example 1 was repeated omitting the n - vinyl - 2 - pyrrolidone . the viscosity rose from 350 to 1250 centipoises at 90 ° c over a period of 1 hour and the following strength characteristics of the cured resin were determined : yield strength ( 0 . 2 % offset ), psi 7400ultimate strength , psi 36000compressive modulus , psi 367000density ( gr / cc ) 1 . 04 to successive portions of 200 parts by weight dihydroxyl 1 , 2 - polybutadiene having properties as described in example 1 , there were added 36 parts by weight toluene - 2 , 4 - diisocyanate and subsequently 10 , 20 , and 40 parts by weight n - vinyl - 2 - pyrrolidone . finally , 4 parts by weight 0 , 0 &# 39 ;- bis ( t - butylperoxy ) diisopropylbenzene was stirred into the mixture . castings made were cured consecutively 24 hours each at 90 °- 115 ° c , 105 °- 120 ° c , and 120 °- 140 ° c . the following table summarizes the compositions ( in parts by weight ) and the strength characteristics of the cured castings : a b c d__________________________________________________________________________dihydroxyl 1 , 2 - polybutadiene 200 . 0 200 . 0 200 . 0 200 . 0toluene - 2 , 4 , diisocyanate 36 . 0 36 . 0 36 . 0 36 . 0n - vinyl - 2 - pyrrolidone 0 . 0 10 . 0 20 . 0 40 . 0vul - cup r 4 . 0 4 . 0 4 . 0 4 . 0compressive yield strength , 8100 9781 9970 11000psi ( 0 . 2 % offset ) ultimate strength , psi 42000 28000 21000 25800compressive modulus , psi 328000 331000 345000 385000density ( gr / cc ) 1 . 00 1 . 03 1 . 03 1 . 04__________________________________________________________________________ mixtures of dihydroxyl 1 , 2 - polybutadiene , isophoronediisocyanate , n - vinyl - 2 - pyrrolidone , trimethylolpropane trimethacrylate and 0 , 0 &# 39 ;- bis ( t - butylperoxy ) diisopropylbenzene were prepared from the following aliquot parts : a b c d e__________________________________________________________________________dihydroxyl 1 , 2 - polybutadiene 200 . 0 200 . 0 200 . 0 200 . 0 200 . 0isophoronediisocyanate 30 . 0 30 . 0 30 . 0 30 . 0 30 . 0n - vinyl - 2 - pyrrolidone 20 . 0 40 . 0 40 . 0 20 . 0 20 . 0trimethlolpropanetrimethacrylate 0 . 0 0 . 0 20 . 0 10 . 0 10 . 0vul - cup r 4 . 0 4 . 0 4 . 0 4 . 0 4 . 0__________________________________________________________________________ after mixing , casting and curing as described in example 1 , the following strength data were obtained : a b c d e__________________________________________________________________________yield strength 7800 9700 10350 8400 9150 ( 0 . 2 % offset ), psiultimate strength , psi 22000 17600 28400 31500 31000compressive modulus , psi 339000 378000 437000 387000 400000density ( g / cc ) 1 . 04 1 . 04 1 . 04 1 . 04 1 . 04__________________________________________________________________________ a syntactic foam buoyancy material was prepared by the maximum random packing of b - 35d hollow glass microspheres ( 3m company ) in a mixture of a low density prepolymer containing the modifiers . the basic resin matrix consisted of 325 parts ( by weight ) dihydroxy 1 , 2 - polybutadiene , 49 parts isophorone diisocyanate and 13 parts 0 , 0 &# 39 ;- bis ( tert - butylperoxy ) diisopropylbenzene ( commercially known as &# 34 ; vul - cup r &# 34 ;) used as vulcanizing agent . modification for increased stiffness and interfacial bonding comprised 130 parts vinylpyrrolidone , 65 parts trimethylolpropane trimethacrylate and 13 parts ( tert - butylperoxy ) vinyl silane . after deaeration in a vacuum potter , the obtained glass - resin composite was submitted to the following cure cycle : 48 hours at 90 ° c , 48 hours at 100 °- 110 ° c and 24 hours at 120 °- 200 ° c . the syntactic foam thus obtained had physical characteristics as follows : specific gravity 0 . 567 ( 35 . 3 pcf ) compressive hydrostatic strength , psi 16 , 200ultimate compressive strength , psi 11 , 5000 . 2 % offset yield strength , psi 11 , 100compressive modulus , psi 427 , 000water absorption after 168 hours ) at 9000 psi , percent 3 . 1 a syntactic foam buoyancy material was prepared by the maximum random packing of b - 35d hollow glass microspheres ( 3m company ) in a mixture of a low density resin prepolymer containing the modifiers . the basic resin matrix consisted of 325 parts ( by weight ) dihydroxyl 1 , 2 - polybutadiene , 49 parts isophorone diisocyanate and 13 parts 0 , 0 &# 39 ;- bis ( tert - butylperoxy ) diisopropylbenzene . after deareration in a vacuum potter , the obtained glass - resin composite was submitted to the following cure cycle : 48 hours at 90 ° c , then 48 hours at from 100 ° to 110 ° c , and finally 24 hours at from 120 ° to 200 ° c . the syntactic foam thus obtained had physical characteristics as follows : specific gravity 0 . 552 ( 34 . 4 pcf ) compressive hydrostatic strength , psi 14 , 900ultimate compressive strength , psi 10 , 9000 . 2 % offset yield strength , psi 9 , 800compressive modulus , psi 334 , 000water absorption ( after 168 hours at 900 psi ), percent 6 . 1