Alternating ethylene/norbornene copolymers

SCHEME 20.14 Strictly alternating ethylene-norbornene copolymers enriched in r dyads prepared through ROMP and subsequent hydrogenation (TsH = p-toluenesulfonhydrazide). 

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... 

Figure 25 Alternative concatenation microstructures in ethylene-norbornene copolymers.

This case has been recently described in the literature and is represented by the highly disordered crystalline form of alternated ethylene-norbornene (EN) copolymers [121]. The crystallinity of EN copolymers will be discussed in a following section, pointing out at the structural analogies with the conventional plastic crystals. 

Fig. 18 X-ray powder diffraction profiles of ethylene-norbornene copolymer samples with norbornene content close to 50% (A)-(C), and solid norbornane in f.c.c. polymorph stable a T>30.6°C, (D) redrawn from [121]. Sample (A) is a random copolymer and amorphous. Samples (B) and (C) have a prevailing alternating constitution and are both crystalline. Sample (C) is essentially atactic, whereas sample (B) has a prevailing diisotactic configuration. (Reprinted with permission from [121]. Copyright 2003 by the American Chemical Society)...

Recently, it has been shown that alternating ethylene-norbornene (EN) copolymers are crystalline [202-206] and that this crystallinity is not necessarily related to a regular succession of configurations of stereoisomeric centers in the norbornene units [121,207]. 

Kiji and co-workers [87] investigated the composition and microstructure of alternating olefin-carbon monoxide copolymers and their derivatives including ethylene-styrene-carbon monoxide alternating polymer, norbornene-amine copolymer and polymers modified with phosphorus pentasulfide or phosphorus pentoxide or primary amines. 

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]. 

Preferred olefins in the polymerisation are one or more of ethylene, propylene, 1-butene, 2-butene, 1-hexene, 1-octene, 1-pentene, 1-tetradecene, norbornene and cyclopentene, with ethylene, propylene and cyclopentene. Other monomers that may be used with these catalysts (when it is a Pd(II) complex) to form copolymers with olefins and selected cycloolefins are carbon monoxide (CO) and vinyl ketones of the general formula H2C=CHC(0)R. Carbon monoxide forms alternating copolymers with the various olefins and cycloolefins. 

Analogously to ethylene-carbon monoxide copolymers, alternating copolymers between cycloolefins such as norbornene and carbon monoxide have been synthesised using cationic Pd(II) complexes modified by phosphorus ligands such as [Pd(MeCN)n(PPh)4 J[BF4]2( = 1,2,3) [27]. General requirements for the... 

Discussion Point DP7i While ethylene and norbornene give essentially alternating, amorphous copolymers, attempts to copolymerize ethylene and cyclohexene give only crystalline polyethylene. Which factors might contribute to these observations Unsaturated norbornenyl chain ends cannot arise by ji-H transfer... 

Early attempts to produce E/NB copolymers utilized heterogeneous TiCl4/AlEt2Cl or vanadium catalysts, but real progress was achieved utilizing metallocene catalysts for this purpose. Metallocenes are about ten times more active than vanadium systems, and by choosing the metallocene, the norbornene/ethylene comonomer sequence distribution in the copolymer may be varied from statistical (random) to alternating. 

FIGURE 16.14 Structural units in E/NB copolymers with (a) isolated norbornene units and (b) alternating norbornene/ethylene units. 

Assignment of the Norbornene C1-C7 and Ethylene Q-C Carbon Atoms in E/NB Copolymers with Isolated or Alternating Norbornene Units to Different Groups of C NMR Peaks as Depicted in Figure 16.14... 

It was noted that norbomene-ethylene copolymers, obtained in the presence of the nickel ylide catalysts, can contain comonomer units in nearly equimolar proportions. The mechanism of the formation of these copolymers can be viewed as follows. The norbornene molecule coordinated with the Ni atom cannot insert itself into the Ni-phenyl or Ni-norbomyl bond for steric reasons, while the insertion of ethylene into these bonds takes place readily. This leads to copolymers in which comonomer units alternate with a high degree of regularity. 

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