Norbornene insertion polymerization

In the ethylene/5-vinyl-2-norbornene co-polymerization, only the endocyclic double bond undergoes insertion (up to 14 mol% with the catalyst and under the conditions investigated), leaving the exocyclic vinyl bond accessible for further reactions, leading to functionalized PEs (Scheme 26). 

A final illustration of the effect of coordinating functionalities on the polymerization of olefins by cationic metal complexes involves norbornene derivatives. The species, [Pd(PR3)(Me)]+, generated in situ by halide abstraction from [Pd(PR3)(Me)(Cl)]2, is an extremely active catalyst for the insertion polymerization of norbornene with a rate exceeding of 1000 tons norbornene/mol Pd hour at... 

The low activity was demonstrated to result from the difficulty of inserting a propene into the Mt—tertiary carbon bond formed after the norbornene insertion (Mt-N), which is even more sterically crowded than the sites formed after a propene (2,1) regioirregular insertion, less reactive than sites with a primary growing polypropene chain. However, at low norbornene/olefin ratio it is possible to obtain P—N copolymers that are relatively richer in norbomene than the E N copolymers prepared in similar conditions. At higher norbornene/olefin feed ratios, the great amount of 1,3 propene misinsertions clearly revealed that the steric hindrance of the Mt—tertiary carbon bond, when norbomene is the last inserted unit, makes the next propene insertion difficult, causing low polymerization activities, molecular masses, and Tg. 

Confirmation that the polymerizations proceed via metallacyclic intermediates was obtained by studying the ROMP of functionalized 7-oxanorbornadienes. These polymerize slower than their norbornene analogs, allowing NMR identification of the metallacyclobutane resonances and subsequent monitoring of ring opening to the first insertion product. In addition, the X-ray crystallographic structure of complex (212) has been reported.533... 

The microstructures of these polymers have been investigated using oligomers as models. Norbornene was shown to polymerize via cis exo insertion [188,189], while in the case of cyclopentene quite unusual cis and trans 1,3 enchainment of the monomer units is observed [190-192],... 

Within the family of cycloolefin co-polymers, the most important from a material properties standpoint, are the ethylene/norbornene co-polymers. These co-polymers, dubbed COC for cycloolefin co-polymers, are produced by Ticona and Mitsui under the tradenames Topas and Apel , respectively. An overview of properties and applications (for example, blisters for pills) can be found on Ticona s Topas homepage.607 Detailed ethylene/norbornene copolymerization studies with different 4/-symmetric and ansa-Cp-amido catalysts, with listing of co-polymerization parameters, have been published.608 611 NB is inserted exclusively in the cis-2,3-exo-modc (Scheme 25), and most of the metallocene catalysts tend to produce alternating co-polymers,609 612 due to the low reactivity of the M-NB intermediate toward further NB insertion. This mode of NB insertion prevents f3-H transfer, and thus ethylene/ norbornene co-polymers have increasing molecular masses at increasing NB content.611... 

Aqueous insertion-type polymerization of norbornene and substituted derivatives [Eq. (7)] has also been investigated. The tolerance of norbornene polymerization using [Pd(NCCH3)4](BF4)2 as an initiator towards added amounts of water was noted by Risse and Mehler in 1992 [91]. 

This has two detrimental effects on polymerization. First, chelation strengthens the metal-olefin interaction, thereby raising the barrier for the insertion step. Second, it forces insertion through the endo face, in sharp contrast to the known propensity for norbornene to insert into metal-carbon bonds through the less hindered exo face [3 a, 5]. Consistent with this hypothesis has been our observation of the preferential uptake of the exo isomer in the polymerization of functional norbornene derivatives by Pd(PRj)(Me). For example. Fig. 9.3 shows the uptake profile versus time for the polymerization of 5-norbornene-2-carboxyhc acid ethyl ester starting with a monomer isomer ratio of 22% exo to 78% endo. Indeed, under certain conditions a polymer can be obtained from the exo isomer but not the endo isomer [10]. 

SCHEME 16.2 Stereochemistry during the polymerization of a prochiral cycloolefin such as norbornene nsing Cs- (A) and C2-symmetiic (B) catalysts. Independent of the relative topicities of the single insertion events (contrasted by pathways a and b), apphcation of the mechanisms known from a-olefin polymerization predict an erythrodisyndiotactic polymer to be formed by C -symmetric catalysts and an erythrodiisotactic polymer to be formed by C2-symmetric catalysts. 

The polymerization of /o-functionalized norbornenes with groups containing hetcroatoms is slower than for exo-functionalized ones or for nb with the conventional cationic catalysts. This is thought to be due to the stabilization of intermediates by coordination of the heteroatom, E, which is only feasible for the < t>-functionalized molecule (Scheme 17). The insertion into the Pd-Me bond of PdMeCl(COD) of several nb derivatives functionalized either in the exo- or in the endo-t2ice has been studied in order to check whether this neutral less-electrophilic palladium center... 

Complex 2, which is effective for propene polymerization was found to conduct coordination-insertion-type polymerization of norbornene. In Table 3 are shown the effects of cocatalyst in norbornene polymerization with 2 in toluene at 20°C [33,34]. 

By reacting EtOH with I and with the trisubstituted titanacycle 3, obtained after the insertion of one molecule of norbornene on I, it is possible to obtain the corresponding ethoxides, which can polymerize ethylene after the addition of a chlorine containing cocatalyst, such as AlEtCla, TiCla. 

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