Norbornene with metallocenes

The copolymers of ethylene with norbornene produced with metallocene catalysts [199] are interesting thermoplastic materials characterized by high glass transition temperatures, good chemical and thermal resistance and excellent transparency [200,201]. The properties of these copolymers depend on the comonomer composition, the distribution of the comonomers and the chain stereoregularity. 

Polymerization of norbornene with heterogeneous Ziegler-Natta catalysts is accompanied by ROMP, whereas homogeneous metallocene, Ni, and Pd catalysts promote addition polymerization. The polymers feature two chiral centers per monomer unit and therefore are ditactic. 

Thus, it was not until 1990 that the group of Kaminsky and Arndt-Rosenau took a more detailed look at the homopolymerization of norbornene and the structures of the resulting polymers. Driven by the growing interest in copolymers with high norbornene contents and high glass transition (Tg) temperatures, as well as the unusual properties of PNBs, Arndt-Rosenau et al. used the hydrooligomerization technique to produce saturated model norbornene dimers and trimers with metallocene catalysts known to produce atactic, isotactic, and syndiotactic poly(a-olefins) (1-3, Table 16.1). 

Cyclic alkenes such as cyclobutene, cyclopentene, and norbornene can be polymerized by double-bond opening with metallocene/methylalumoxane or late transition catalysts [548-551]. The activities for the polymerization of cyclobutene and cyclopentene are high, whereas the activity of norbornene is significantly lower. The melting points are... 

Kaminsky and coworkers foxmd that copolymerization of cyclopentene or norbornene with ethylene, using various metallocene compounds activated with MAO, produced new thermoplastic amorphous materials [55] that were transparent and had high stiffness and excellent stability. 

Copolymerizations of ethene with bicyclic olefins, such as 2,5-norbornadiene, 5-vinyl-2-norbornene, have been investigated with metallocene catalysts. The secondary groups do not interfere with metallocene copolymerizations and post-polymerization functionalization makes it possible to synthesize functionalized polyolefins. 

Cyclohexene does not polymerize by either route except when it is part of a bicyclic structure as in norbornene. Stereochemistry in the ROMP of norbomene is complicated since the polymer, LXVI in Sec. 7-8, has possibilities of isomerism at both the ring and the double bond. Most polymerizations by the typical ROMP initiators yield cis stereochemistry at the cyclopentane ring with varying amounts of cis and trans placements at the double bond [Ivin, 1987]. Metallocene initiators yield predominantly double-bond polymerization with 1,2-placement [Janiak and Lassahn, 2001]. 

Metallocene catalysts show low r values, which allows easy incorporation of bulky cycloolefins into the growing copolymer chain. Surprisingly, the ethylene reactivity ratio in copolymerisation with cyclopentene in the presence of a (ThindCH2)2ZrCl2-based catalyst (r = 2.2) and in copolymerisation with norbornene in the presence of catalysts characterised by Cs and Ci symmetry (ri 3.4 and 3.1 respectively) is considerably lower than that for the copolymerisation of ethylene with propylene (r = 6.6 at 37 °C). Various catalysts produce copolymers of structures that are between statistical and alternating [468]. 

The alternating copolymers are characterized both by a glass transition temperature (130°C for ethylene/norbornene copolymers) and a melting point (295°C for the totally alternating copolymer). The melting point and the crystallinity of these copolymers may be influenced by choice of the metallocene and the conditions of polymerization. Compared with the statistical copolymers, the alternating structures show better resistance to nonpolar... 

Single-site metallocene catalysts are also highly reactive vis-a-vis cycloolefins such as cyclobutene, cyclopentene or norbornene. While homopolymers of these cycloolefins have melting temperatures (>380°C), much too high for technical processability, ethylene-cycloolefin copolymers (COC s) - e.g. ethylene-norbornene copolymers - are amorphous materials with glass transition temperatures, above which they become soft and processable [W. Kaminsky, J. Polym. Sci. A, Polym. Chem., 2004, 42, 3911]. 

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... 

Cyclic olefin copolymers (COC) are copolymers of ethylene and norbornene (2,2,1 bicycloheptane), made using metallocene catalysts. They are amorphous polymers with excellent clarity, low density, high strength, and stiffness. Currently the major manufacturer is Ticona, which sells them under the trade name Topas. 

The copolymerizations were carried out under argon using a 1 L Biichi A6 Type I autoclave equipped with an additional external cooling system. For the standard experiments, the reactor was evacuated at 95 C for 1 h and subsequently charged with a solution of norbornene in toluene, 190 mL toluene solvent, 500 mg MAO in 10 mL toluene (from Witco/Crompton), and ethylene at different pressures. Norbornene was dried over triisobutylaluminum and subsequently distilled before use. Polymerizations were initiated by injection of a toluenic metallocene solution into the reaction vessel... 

In the case of the unsubstituted C -symmetric metallocenes 8 and 11, copolymerization proceeds under control of the last inserted monomer unit (chain-end control), that is, it can be described by a second order Markov model. Ethylene is inserted with these zirconocenes three times faster than norbornene. No norbornene block sequences longer than two (NN units) are formed, in agreement with parameters calculated for 8 (rsE = 2.40, r E = 4.34, rEN = 0.03, and rm = 0.00). This result easily explains the maximum observed Xn = 0.66. 

Catalysts 19 and 20 show notably different polymerization behaviors. Catalyst 19 incorporates norbornene much better than ethylene (Figure 16.13). Even at very low molar fractions of norbornene in the feed (e.g., xn = 0.05), the incorporation of norbornene is at about Yn = 0.44 (Table 16.6, Run 2). When metallocene/MAO catalyst systems are used similarly, it is necessary to carry out copolymerization at feed compositions of up to xn = 0.90 to incorporate norbornene in the same amount. At higher xns with 19, incorporation is nearly independent of feed composition and reaches a plateau of about Yn = 0.60. Polymerizations carried out at xn > 0.60 yielded partially insoluble polymers that were not evaluable by standard C NMR characterization protocols. 

To begin a discussion of the micro- and stereostructures of E/NB copolymers produced with different types of catalysts, trends for Ci-symmetric metallocenes will first be considered. While C2-symmetric catalysts (e.g., 2,4,6,7) produce random copolymers with small norbornene blocks if the norbornene concentration in the feed is high, C -symmetric catalysts produce more alternating (10) or purely alternating (9) polymers. The catalyst system lO/MAO was found to incorporate norbornene slightly better than 9/MAO (Table 16.4) whereas exclusively isolated and alternating norbornene... 

Single site catalysts, such as metallocene compounds, CGCs, and nickel or palladium diimine complexes, used in combination with MAO or borate cocatalysts, are highly active for the homopolymerization of norbornene and its copolymerization with ethylene. The structure of the norbornene homo- and copolymers can be widely influenced by the symmetry and structure of the ligands on the transition metal complexes. 

Nickel-0- and palladium-O-complexes are very active catalysts for the polymerization of norbornene and also for cyclopentene [552-554], Nickel catalysts produce soluble polymers with a molecular weight of over one million while polymers obtained with palladium or metallocene complexes are insoluble. The soluble polymers have an atactic structure. The microstructure of the polynorbornene depends on the catalyst used and is isotactic by synthesis with chiral metallocenes. 

Ansa-metallocenes with C2 and Cs symmetries generate random copolymers containing norbornene microblocks [43, 44, 61, 62, 64-68, 92]. Copolymers with norbomene content well above 50 mol% and Tg values as high as 220°C can be synthesized. Metallocene symmetry and ligand substituents dictate polymerizatiOTi activity, tacticity, and sequential distribution. The type of bridge has an influence oti polymerization activity, norbornene content, and molar masses (Fig. 5). 

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