Isolated ethylene units

Fig. 2. Isolated ethylene unit in a vinyl chain can be found in a meso (M) or a racemic (R) environment.

Zambelli, A., Bajo, G. and Rigamonti, E. (1978). Model compounds and carbon-13 NMR investigation of isolated ethylene units in ethylene/propene copolymers. Makromolek. Chem., 179, 1249-1259. 

The same conclusion as in the case of propylene homopolymerisation has been drawn considering IR [396] and NMR [389,395] spectra of ethylene/propylene copolymers obtained with vanadium-based syndiospecific catalysts. The type of propylene insertion depends on the kind of last inserted monomer unit secondary insertion [scheme (40)] occurs more frequently when the last monomeric unit of the growing chain is propylene, while primary propylene insertion [scheme (39)] is more frequent when the last monomeric unit of the growing chain is ethylene [2]. The above explains the microstructure of ethylene/propylene copolymers obtained with vanadium-based Ziegler-Natta catalysts. These copolymers contain both m and r diads when the sequence of propylene units is interrupted by isolated ethylene units i.e. a propylene insertion after an ethylene insertion is substantially non-stereospecific [327,390,397], The existence of a steric interaction between the incoming monomer molecule and the last added monomer unit is also confirmed by the fact that the propagation rate for the secondary insertion of propylene in syndiospecific polymerisation is lower than for primary insertion in non-stereospecific polymerisation [398],... 

Capacchione, C. D Acunzi, M. Motta, O. Oliva, L. Proto, A. Okuda, J. Isolated ethylene units in isotactic polystyrene chain Stereocontrol of an isospecific post-metallocene titanium catalyst. Macromol. Chem. Phys. 2004, 205, 370-373. 

A relationship exists between the peak intensities of the characteristic products such as 1-0 (Cg), 2-MO (Cg), and 1-0 (Cjq) and the E/P content in EPDM. In general, as the ethylene content increases in the EPDM, the ethylene sequence length becomes longer, and the amormt of the isolated ethylene units decreases. As described previously, 1-0 (Cg) and 1-0 (Cjo) are formed from the longer ethylene sequences than the triad. Intensities of 1-0 (Cg) and 1-0 (Cm) monotonously increase with the rise of the ethylene content in the EPDM. Conversely, 2-MO (Cg), which is correlated to the isolated ethylene unit, monotonously decreases as the ethylene content increases. Consequently, these relations can be used to estimate the E/P composition in EPDM. 

IR spectroscopy also provides information on regioisomerism in ethylene propylene copolymers. The fact that the polypropylenes prepared with VOClj or VO(OR)3-containing catalysts show tail-to-tail arrangement means that tail-to-tail coupling of propylene units may also occur in ethylene-propylene copolymers. Because the content of (CH2)2 sequences in the copolymers is much higher than in the polypropylenes prepared with the same catalysts, a large part of these sequences probably stems from isolated ethylene units between two head-to-head oriented propylene units, their relative amount depending on the ratio of reaction rates of formation of the sequences ... 

The two remaining peaks at 39.22 and at 25.52 ppm from TMS have the same intensity and must be attributed to the isolated ethylenic units present in one of the previously shown structures, which reasonably should be structure a). The presence of structure a) only, should indicate that the steric control is essentially exerted by the catalyst. 

In the case of copolymers obtained with isospecific catalytic systems, both qualitative (125) and quantitative IR data (71,118) demonstrate that the distribution of ethyloie units roughly corresponds to that expected on the basis of the V2 values Some of the possible deviations are due to the presence of isolated ethylene units as (CH2)2 sequences. 

A signifkani disagrement between the statistics and IR data was found (122) for some copolymers obtained with TiClj-containing systems. This disagreement is probably partly due to the misassignment of the shaip ctystalline tend at 731 cm to the absorption of isolated ethylene units (see Section III.C.2e). The presence of long ethylene sequences in these copolymers was confirmed by NMR in the same paper. 

Veerkamp and Veermans [6] have reported on the differential measurement of (012)2 and (012)3 units at 13.42 and 15.87 pm, respectively, in ethylene-propylene copolymers. The band at 13.89 pm was assigned to units of 5 or more CH2 units. Thus, the absorbance ratio A13.42/A13.89 pm provides a measure of the number of isolated ethylene units, and when corrected for the effect of composition (by multiplying by the ratio mole% C3/mole% C2) is related to the inverse of the ratio A10.33 pm/A8.69 pm times the ratio mole/C2/mole%C3. The more random the copolymer, as indicated by the absorbance ratio A13.42 pm/A13.89 pm for the relative number of isolated ethylene units, the more diffuse the 10.33 pm band becomes. 

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