Composition and method for inhibiting polymerization and polymer growth

Abstract

A method for inhibiting the premature polymerization and the polymer growth of ethylenically unsaturated monomers is disclosed wherein the method comprises adding to said monomers an effective amount of at least one hydrogen donor or electron acceptor. In a preferred embodiment, the hydrogen donor or electron acceptor is used in combination with a stable nitroxyl free radical.

1 . A method for inhibiting the premature polymerization and the polymer growth of ethylenically unsaturated monomers comprising adding to said monomers an effective amount of at least one inhibitor that is a hydrogen donor selected from the group consisting of: 
 (A) a compound of the structure                          wherein    R 100  and R 101  are independently selected from the group consisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl, COR 102 , COOR 102 , CONR 102 R 103 , cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 100  and R 101  can be taken together to form a ring structure of five to seven members, and    R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members;    (B) a compound of the structure                          wherein    R 100  and R 101  are independently selected from the group consisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl, COR 102 , COOR 102 , CONR 102 R 103 , cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 100  and R 101  can be taken together to form a ring structure of five to seven members, and    R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members;    (C) a compound of the structure                          wherein    R 113 , R 114 , and R 115  are independently selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, and heterocyclic moieties; and    (D) a compound of the structure                          wherein    R 116 , R 117 , R 118 , R 119 , R 120 , R 121 , R 122 , and R 123  are independently selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, heterocyclic, substituted alkyl, substituted aryl, OR 110 , NR 110 R 111 , SR 110 , NO 2 , NO, CN, COR 112 , halogen, and/or any two adjacent groups can be taken together to form ring structure(s) of five to seven members,    R 110  and R 111  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, substituted alkyl or aryl where the substituents are C, O, N, S, or P, and COR 102  or R 110  and R 111  can be taken together to form a ring structure of five to seven members,    R 112  is R 102 , OR 102 , or NR 102 R 103 , and    R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members.    
     
     
         2 . The method of  claim 1  wherein the inhibitor is of the structure  
       
         
           
           
               
               
           
         
       
       wherein 
 R 100  and R 101  are independently selected from the group consisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl, COR 102 , COOR 102 , CONR 102 R 103 , cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 100  and R 101  can be taken together to form a ring structure of five to seven members; and  
 R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members.  
 
     
     
         3 . The method of  claim 1  wherein the inhibitor is of the structure  
       
         
           
           
               
               
           
         
       
       wherein 
 R 100  and R 101  are independently selected from the group consisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl, COR 102 , COOR 102 , CONR 102 R 103 , cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 100  and R 101  can be taken together to form a ring structure of five to seven members; and  
 R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members.  
 
     
     
         4 . The method of  claim 1  wherein the inhibitor is of the structure  
       
         
           
           
               
               
           
         
       
       wherein R 113 , R 114 , and R 115  are independently selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, and heterocyclic moieties.  
     
     
         5 . The method of  claim 1  wherein the inhibitor is of the structure  
       
         
           
           
               
               
           
         
       
       wherein 
 R 116 , R 117 , R 118 , R 119 , R 120 , R 121 , R 122 , and R 123  are independently selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, heterocyclic, substituted alkyl, substituted aryl, OR 110 , NR 110 R 111 , SR 110 , NO 2 , NO, CN, COR 112 , halogen, and/or any two adjacent groups can be taken together to form ring structure(s) of five to seven members;  
 R 110  and R 111  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, substituted alkyl or aryl where the substituents are C, O, N, S, or P, and COR 102  or R 110  and R 111  can be taken together to form a ring structure of five to seven members;  
 R 112  is R 102 , OR 102 , or NR 102 R 103 ; and  
 R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members.  
 
     
     
         6 . The method of  claim 1  wherein the inhibitor is selected from the group consisting of diethylhydroxylamine, cyclohexanoneoxime, dibenzylhydroxylamine, 2,4-dinitro-6-sec-butylphenol, N-phenyl-N′-(1,4-dimethylpentyl)-para-phenylenediamine, 2,5-di-t-butylhydroquinone, 2,5-di-t-amylhydroquinone, methylhydroquinone, 4-t-butylhydroquinone, 4-t-butylcatechol, octanethiol, 2,6-di-t-butyl-4-ethylphenol/Cu(I)naphthenate, dihydroanthracene, N-t-butyl-2-benzothiazole-sulfenamide, and N-methyl-4-nitroaniline.  
     
     
         7 . The method of  claim 1  wherein a transition metal is added.  
     
     
         8 . The method of  claim 7  wherein the transition metal is copper.  
     
     
         9 . A method for inhibiting the premature polymerization and the polymer growth of ethylenically unsaturated monomers comprising adding to said monomers 
 (A) at least one first inhibitor that is a hydrogen donor selected from the group consisting of: 
 (1) a compound of the structure  
                     
  wherein  
 R 100  and R 101  are independently selected from the group consisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl, COR 102 , COOR 102 , CONR 102 R 103 , cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 100  and R 101  can be taken together to form a ring structure of five to seven members, and  
 R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members;  
 (2) a compound of the structure  
                     
  wherein  
 R 100  and R 101  are independently selected from the group consisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl, COR 102 , COOR 102 , CONR 102 R 103 , cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 100  and R 101  can be taken together to form a ring structure of five to seven members, and  
 R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members;  
 (3) a compound of the structure  
                     
  wherein  
 R 113 , R 114 , and R 115  are independently selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, and heterocyclic moieties; and  
 (4) a compound of the structure  
                     
  wherein  
 R 116 , R 117 , R 118 , R 119 , R 120 , R 121 , R 122 , and R 123  are independently selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, heterocyclic, substituted alkyl, substituted aryl, OR 110 , NR 110 R 111 , SR 110 , NO 2 , NO, CN, COR 112 , halogen, and/or any two adjacent groups can be taken together to form ring structure(s) of five to seven members,  
 R 110  and R 111  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, substituted alkyl or aryl where the substituents are C, O, N, S, or P, and COR 102  or R 110  and R 111  can be taken together to form a ring structure of five to seven members,  
 R 112  is R 102 , OR 102 , or NR 102 R 103 , and  
 R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members; and  
   (B) at least one second inhibitor having the following structural formula:                           wherein    R 1  and R 4  are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl,    R 2  and R 3  are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl, and    X 1  and X 2  
 (1) are independently selected from the group consisting of halogen, cyano, amido, —S—C 6 H 5 , carbonyl, alkenyl, alkyl of 1 to 15 carbon atoms, COOR 7 , —S—COR 7 , and —OCOR 7 , wherein R 7  is alkyl or aryl, or  
 (2) taken together, form a ring structure with the nitrogen.  
   
     
     
         10 . The method of  claim 9  herein the first inhibitor is of the structure  
       
         
           
           
               
               
           
         
       
       wherein 
 R 100  and R 101  are independently selected from the group consisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl, COR 102 , COOR 102 , CONR 102 R 103 , cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 100  and R 101  can be taken together to form a ring structure of five to seven members; and  
 R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members.  
 
     
     
         11 . The method of  claim 9  wherein the first inhibitor is of the structure  
       
         
           
           
               
               
           
         
       
       wherein 
 R 100  and R 101  are independently selected from the group consisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl, COR 102 , COOR 102 , CONR 102 R 103 , cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 100  and R 101  can be taken together to form a ring structure of five to seven members; and  
 R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members.  
 
     
     
         12 . The method of  claim 9  wherein the first inhibitor is of the structure  
       
         
           
           
               
               
           
         
       
       wherein R 113 , R 114 , and R 115  are independently selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, and heterocyclic moieties.  
     
     
         13 . The method of  claim 9  wherein the first inhibitor is of the structure  
       
         
           
           
               
               
           
         
       
       wherein 
 R 116 , R 117 , R 118 , R 119 , R 120 , R 121 , R 122 , and R 123  are independently selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, heterocyclic, substituted alkyl, substituted aryl, OR 110 , NR 110 R 111 , SR 110 , NO 2 , NO, CN, COR 112 , halogen, and/or any two adjacent groups can be taken together to form ring structure(s) of five to seven members;  
 R 110  and R 111  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, substituted alkyl or aryl where the substituents are C, O, N, S, or P, and COR 102  or R 110  and R 111  can be taken together to form a ring structure of five to seven members;  
 R 112  is R 102 ,OR 102 , or NR 102 R 103 ; and  
 R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members.  
 
     
     
         14 . The method of  claim 9  wherein the first inhibitor is selected from the group consisting of diethylhydroxylamine, cyclohexanoneoxime, dibenzylhydroxylamine, 2,4-dinitro-6-sec-butylphenol, N-phenyl-N′-(1,4-dimethylpentyl)-para-phenylenediamine, 2,5-di-t-butylhydroquinone, 2,5-di-t-amylhydroquinone, methylhydroquinone, 4-t-butylhydroquinone, 4-t-butylcatechol, octanethiol, 2,6-di-t-butyl-4-ethylphenol/Cu(I)naphthenate, dihydroanthracene, N-t-butyl-2-benzothiazole-sulfenamide, and N-methyl-4-nitroaniline.  
     
     
         15 . The method of  claim 9  wherein a transition metal is added.  
     
     
         16 . The method of  claim 15  wherein the transition metal is copper.  
     
     
         17 . The method of  claim 9  wherein the second inhibitor is of the structure  
       
         
           
           
               
               
           
         
       
       wherein R 1  and R 4  are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R 2  and R 3  are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl, and the  
       
         
           
           
               
               
           
         
       
       portion represents the atoms necessary to form a five-, six-, or seven-membered heterocyclic ring.  
     
     
         18 . The method of  claim 17  wherein the second inhibitor contains one or more nitroxyls selected from the group consisting of: 
 N,N-di-tert-butylnitroxide;    N,N-di-tert-amylnitroxide;    N-tert-butyl-2-methyl-1-phenyl-propylnitroxide;    N-tert-butyl-1-diethylphosphono-2,2-dimethylpropylnitroxide;    2,2,6,6-tetramethyl-piperidinyloxy;    4-amino-2,2,6,6-tetramethyl-piperidinyloxy;    4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;    4-oxo-2,2,6,6-tetramethyl-piperidinyloxy;    4-dimethylamino-2,2,6,6-tetramethyl-piperidinyloxy;    4-ethanoyloxy-2,2,6,6-tetramethyl-piperidinyloxy;    2,2,5,5-tetramethylpyrrolidinyloxy;    3-amino-2,2,5,5-tetramethylpyrrolidinyloxy;    2,2,4,4-tetramethyl-1-oxa-3-azacyclopentyl-3-oxy;    2,2,4,4-tetramethyl-1-oxa-3-pyrrolinyl-1-oxy-3-carboxylic acid;    2,2,3,3,5,5,6,6-octamethyl-1,4-diazacyclohexyl-1,4-dioxy;    4-bromo-2,2,6,6-tetramethyl-piperidinyloxy;    4-chloro-2,2,6,6-tetramethyl-piperidinyloxy;    4-iodo-2,2,6,6-tetramethyl-piperidinyloxy;    4-fluoro-2,2,6,6-tetramethyl-piperidinyloxy;    4-cyano-2,2,6,6-tetramethyl-piperidinyloxy;    4-carboxy-2,2,6,6-tetramethyl-piperidinyloxy;    4-carbomethoxy-2,2,6,6-tetramethyl-piperidinyloxy;    4-carbethoxy-2,2,6,6-tetramethyl-piperidinyloxy;    4-cyano-4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;    4-methyl-2,2,6,6-tetramethyl-piperidinyloxy;    4-carbethoxy-4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;    4-hydroxy-4-(1-hydroxypropyl)-2,2,6,6-tetramethyl-piperidinyloxy;    4-methyl-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;    4-carboxy-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;    4-carbomethoxy-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;    4-carbethoxy-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;    4-amino-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;    4-amido-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine-1-oxyl;    3,4-diketo-2,2,5,5-tetramethylpyrrolidinyloxy;    3-keto-4-oximino-2,2,5,5-tetramethylpyrrolidinyloxy;    3-keto-4-benzylidine-2,2,5,5-tetramethylpyrrolidinyloxy;    3-keto-4,4-dibromo-2,2,5,5-tetramethylpyrrolidinyloxy;    2,2,3,3,5,5-hexamethylpyrrolidinyloxy;    3-carboximido-2,2,5,5-tetramethylpyrrolidinyloxy;    3-oximino-2,2,5,5-tetramethylpyrrolidinyloxy;    3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;    3-cyano-3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;    3-carbomethoxy-3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;    3-carbethoxy-3-hydroxy-2,2,5,5-tetramethylpyrrolidinyloxy;    2,2,5,5-tetramethyl-3-carboxamido-2,5-dihydropyrrole-1-oxyl;    2,2,5,5-tetramethyl-3-amino-2,5-dihydropyrrole-1-oxyl;    2,2,5,5-tetramethyl-3-carbethoxy-2,5-dihydropyrrole-1-oxyl;    2,2,5,5-tetramethyl-3-cyano-2,5-dihydropyrrole-1-oxyl;    bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)succinate;    bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipate;    bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)sebacate;    bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)n-butylmalonate;    bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)phthalate;    bis(1-oxy-2,2,6,6-tetramethylpiperidin-4-yl)isophthalate;    bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)terephthalate;    bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)hexahydroterephthalate;    N,N′-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipamide;    N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-caprolactam;    N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-dodecylsuccinimide;    2,4,6-tris-[N-butyl-N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)]-s-triazine; and    4,4′-ethylenebis(1-oxyl-2,2,6,6-tetramethylpiperazin-3-one).    
     
     
         19 . A composition comprising: 
 (A) at least one first inhibitor that is a hydrogen donor selected from the group consisting of: 
 (1) a compound of the structure  
                     
  wherein  
 R 100  and R 101  are independently selected from the group consisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl, COR 102 , COOR 102 , CONR 102 R 103 , cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 100  and R 101  can be taken together to form a ring structure of five to seven members, and  
 R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members;  
 (2) a compound of the structure  
                     
  wherein  
 R 100  and R 101  are independently selected from the group consisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl, COR 102 , COOR 102 , CONR 102 R 103 , cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 100  and R 101  can be taken together to form a ring structure of five to seven members, and  
 R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members;  
 (3) a compound of the structure  
                     
  wherein  
 R 113 , R 114 , and R 115  are independently selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, and heterocyclic moieties; and  
 (4) a compound of the structure  
                     
  wherein  
 R 116 , R 117 , R 18 , R 19 , R 120 , R 121 , R 122 , and R 123  are independently selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, heterocyclic, substituted alkyl, substituted aryl, OR 110 , NR 110 R 111 , SR 110 , NO 2 , NO, CN, COR 112 , halogen, and/or any two adjacent groups can be taken together to form ring structure(s) of five to seven members,  
 R 110  and R 111 , are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, substituted alkyl or aryl where the substituents are C, O, N, S, or P, and COR 102  or R 110  and R 111  can be taken together to form a ring structure of five to seven members,  
   R 112  is R 102 , OR 102 , or NR 102 R 103 , and    R 102  and R 103  are independently selected from the group consisting of hydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substituted alkyl or aryl where the substituents are C, O, N, S, or P, or R 102  and R 103  can be taken together to form a ring structure of five to seven members; and    (B) at least one second inhibitor having the following structural formula:                           wherein    R 1  and R 4  are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl,    R 2  and R 3  are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl, and    X, and X 2  
 (1) are independently selected from the group consisting of halogen, cyano, amido, —S—C 6 H 5 , carbonyl, alkenyl, alkyl of 1 to 15 carbon atoms, COOR 7 , —S—COR 7 , and —OCOR 7 , wherein R 7  is alkyl or aryl, or  
 (2) taken together, form a ring structure with the nitrogen.  
   
     
     
         20 . The composition of  claim 19  wherein the second inhibitor is of the structure  
       
         
           
           
               
               
           
         
       
       wherein R 1  and R 4  are independently selected from the group consisting of hydrogen, alkyl, and heteroatom-substituted alkyl and R 2  and R 3  are independently selected from the group consisting of alkyl and heteroatom-substituted alkyl, and the  
       
         
           
           
               
               
           
         
       
       portion represents the atoms necessary to form a five-, six-, or seven-membered heterocyclic ring.