Thermodynamic Control in the Catalytic Insertion Polymerization of Norbornenes as Rationale for the Lack of Reactivity of Endo-Substituted Norbornenes

Abstract

The catalytic insertion polymerization of substituted norbornenes (NBEs) leads to the formation of a family of polymers which combine extreme thermomechanical properties as well as unique optical and electronic properties. However, this reaction is marred by the lack of reactivity of endo substituted monomers. It has long been assumed that these monomers chelate the metallic catalyst, leading to species which are inactive in polymerization. Here we examine the polymerization of cis-5-norbornene-2,3-dicarboxylic anhydride (so-called carbic anhydride, CA) with a naked cationic Pd catalyst. Although exo-CA can be polymerized, the polymerization of endo-CA stops after a single insertion. Surprisingly, no chelate is formed between the catalyst and endo-CA. Using DFT calculation, it is shown that while the insertion of exo-NBEs is exergonic, the insertion of two endo-CA in a row is endergonic. In this latter case, the enthalpy gain corresponding to the insertion of a double bond is not sufficient to overcome the entropic penalty associated with ligand binding. Thus, the different reactivity between endo and exo NBEs is thermodynamic in nature, and it is not controlled by kinetic factors. Interestingly, thermodynamics is also the main factor controlling the stereochemistry of the chain. For CA polymerization, and even for unsubstituted NBE polymerization, the formation of r and m dyads is, respectively, exergonic and endergonic, resulting in a polymer which is essentially disyndiotactic. Thus, this study demonstrates that thermodynamics can control the chemo- and stereoselectivity of a catalytic polymerization.

Addition Homo‐ and Copolymerizations of Dicyclopentadiene and 5‐n‐Hexylnorbornene in the Presence of Pd‐N‐Heterocyclic Carbene Complexes

“…The second reason is the used DCPD was enriched by a less reactive endo ‐isomer, while HexNB consisted of approximately equal amounts of exo ‐ and endo ‐isomers. Thus it was shown that in addition polymerization of norbornenes, exo ‐derivatives as a rule are more reactive than endo ‐isomers . The last reason is the capability of DCPD to form stable chelates with the catalyst active species ( Figure ).…”

“…Thus it was shown that in addition polymerization of norbornenes, exo-derivatives as a rule are more reactive than endo-isomers. [44,45] The last reason is the capability of DCPD to form stable chelates with the catalyst active species (Figure 7). Thus, the presence of the second double bond in DCPD leads to the possibility to coordinate with Pd-centers in different ways (Figure 7, structures I-III) and there is a competition between these double bonds for the vacant coordination place at Pd-complexes.…”

Highly efficient vinylic addition polymerization of 5-vinyl-2-norbornene using benzylic palladium complexes as precatalysts

“…As for many other substituted norbornenes, the endo isomer (the major one) is the least reactive. [10][11][12] Also, both endocyclic and exocyclic unsaturations in endo-VNB are perfectly arranged to coordinate to the metal in a chelating fashion, which could slow down the polymerization process. Even in a monodentate fashion the terminal double bond in VNB is a competing ligand.…”

Addition copolymerization of norbornene lactone catalyzed by Pd complexes

“…The Diels‐Alder reactions between cyclopentadiene with dienophiles readily produce a variety of functionalized NBs with predominant formation of the endo ‐isomers. Generally, the exo ‐isomers show the higher polymerizability than the endo ‐isomers, which is believed to be due to steric hindrance of an endo ‐substituent and/or chelation of a terminal transition metal with an endo ‐substituent 8–11 . Addition polymerizations of 5‐norbornene‐2‐carboxylic acid esters, ones of the most investigated functionalized NBs, have been performed by using less oxophilic but highly active Pd catalysts, including [( η 3 ‐allyl)PdL 2 ]SbF 6 (L = solvent), 10,12 ( t Bu 3 P)Pd(Me)Cl, 13,14 [( η 3 ‐allyl)Pd(NHC)Cl] (NHC = N‐heterocyclic carbene), 15 Pd(acac) 2 , 16 Pd(dba) 2 (dba = dibenzylideneacetone), 17 and Pd(acac)(MeCN) 2 BF 4 18 .…”

“…Generally, the exo-isomers show the higher polymerizability than the endo-isomers, which is believed to be due to steric hindrance of an endo-substituent and/or chelation of a terminal transition metal with an endo-substituent. [8][9][10][11] Addition polymerizations of 5-norbornene-2-carboxylic acid esters, ones of the most investigated functionalized NBs, have been performed by using less oxophilic but highly active Pd catalysts, including [(η 3 -allyl)PdL 2 ]SbF 6 (L = solvent), 10,12 ( t Bu 3 P)Pd(Me)Cl, 13,14 [(η 3 -allyl)Pd(NHC)Cl] (NHC = N-heterocyclic carbene), 15 Pd(acac) 2 , 16 Pd(dba) 2 (dba = dibenzylideneacetone), 17 and Pd(acac)(MeCN) 2 BF 4 . 18 Living polymerization was also performed by using a catalytic system of ( t Bu 3 P)Pd(Me)Cl/NaB[3,5-(CF 3 ) 2 C 6 H 3 ] 4 .…”