Low molecular weight model olefins

Therefore most progress towards the understanding of sulfur-vulcanisation chemistry was originally made by vulcanising low-molecular-weight model olefins. Numerous... 

The results of the optical spectroscopy studies into sulfur vulcanisation of polydiene rubbers correspond well with the results obtained via low molecular weight model olefin studies and solid state 13C NMR studies. From all these studies the mechanism for accelerated sulfur vulcanisation as shown in Figure 6.2 has emerged [14-18], which is... 

Figure 6.3 Accelerated sulphur vulcanization of EPDM as demonstrated by low-molecular weight model olefin studies [75,76]...

The sulfonation of low molecular weight model olefins was undertaken to determine the feasibility of this approach. Competitive sulfonations using acetyl sulfate were carried out on the model compounds below, representing the repeat structures of cis-l,4-polyisoprene (PIP), cw-l,4-polybutadiene (c-PBD), and trans-l,4-polybutadiene (Z-PBD), respectively. It was necessary to model both the cis and trans isomeric forms of 1,4-polybutadiene, since ttey have a nearly equal probability of occurrence when the anionic polymerization (Ii counterion) is conducted in a nonpolar hydrocarbon medium... 

A remarkable enhancement of optical activity has been observed for the most stereoregular fractions of poly-a-olefins, with respect to the optical activity of the low-molecular-weight model compounds (Table 9). 

As in the case of poly-a-olefins, only in some poly-vinyl-ethers in which the asymmetric carbon atom of the lateral chains is in the ft position with respect to the principal chain and in which the optical activity is remarkably higher than in low molecular weight models, it was found that the optical activity and its temperature coefficient remarkably depend on the stereoregularity of the sample. 

In fact, in poly-acrylates the closest position of the asymmetric carbon atom of the lateral chains, with respect to the principal chain, is the y-position (XXV) while, as we have already noted in the case of poly-a-olefins and poly-vinyl-ethers, the greatest differences between the rotatory power of polymers and low-molecular-weight models occur when the asymmetric carbon atom is in a (XXVI) or in S (XXVII)... 

Both in poly-a-olefins and in poly-vinyl-ethers, which are the most systematically investigated optically active vinyl polymers, the chromo-phoric systems responsible for the optical activity appear to be in the same spectral region of those of the low-molecular-weight models 66. 105,113a). 

Experimental and calculated rotatory power for isotactic poly(a-olefin)s and their conformational low molecular weight models... 

Vapor-phase alkylation of benzene by ethene and propene over HY, LaY, and REHY has been studied in a tubular flow reactor. Transient data were obtained. The observed rate of reaction passes through a maximum with time, which results from build-up of product concentration in the zeolite pores coupled with catalyst deactivation. The rate decay is related to aromatic olefin ratio temperature, and olefin type. The observed rate fits a model involving desorption of product from the zeolite crystallites into the gas phase as a rate-limiting step. The activation energy for the desorption term is 16.5 heal/mole, approximately equivalent to the heat of adsorption of ethylbenzene. For low molecular weight alkylates intracrystalline diffusion limitations do not exist. 

After these early studies an extensive FT-Raman study [77] was performed to bridge the gap between the low-molecular-weight ENBH model vulcanisation studies and the vulcanisation studies using high-molecular-weight EPDM. These studies will be presented in detail. First, a series of low-molecular-weight dialkenylsulfides will be discussed in order to determine the effect of sulfur vulcanisation on Raman spectra of olefins. Subsequently, the attachment of the sulfur crosslinks at the allylic positions, the conversion of ENB, the length of sulfur crosslinks and the network structure will be addressed for unfilled sulfur vulcanisates of amorphous EPDM. Some preliminary network structure/ properties relationships will also be presented. 

Near-quantitative conversion of monomer to polymer is standard in these polymerizations, as few side reactions occur other than a small amount of cychc formation common in all polycondensation chemistry [41]. ADMET depolymerization also occurs when unsaturated olefins are exposed to pressures of ethylene gas [42,43]. In this case, the equilibriiun nature of metathesis is shifted towards low molecular weight products under saturation with ethylene. Due to the high catalytic activity of [Ru] and the abihty of [Mo] and [Ru] to create exact structures, ADMET has proven a valuable tool for production of novel polymer structures for material applications as well as model copolymer systems to help elucidate fundamental structure property relationships [5]. 

These differences between model and data apparently arise from the expected increase in the severity of transport restrictions as thicker liquid layers (and pockets of liquid) between catalyst pellets become favored by low linear gas velocities. High conversions also increase the liquid load within the catalyst bed because vapor-liquid equilibrium constraints maintain a larger fraction of FT products in the liquid phase during reaction. Also, low CO concentrations favor H-addition steps that prevent a-olefin readsorption and chain initiation by a-olefins. In spite of the limited hydrodynamic scope of the model, it describes well the trends in product molecular weight and paraffin content with changes in bed residence time. These trends are clearly consistent with the observed increase in a-olefin readsorption as olefins remain longer within the catalyst bed. 

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