Polyethylene sequencing

The list of compounds identified in the pyrolysate of the copolymer shows the presence of polyethylene sequences, as well as of poly(ethylene glycol) sequences. However, a series of compounds such as 1-dodecanol, 1-tetradecanol, 1-hexadecanol, 2-(dodecyloxy)ethanol, 2-(tetradecyloxy)ethanol, 2-(hexadecyloxy)ethanol, hexadec-15-enyloxy-1-ethanol, and 2-(octadecyloxy)ethanol are also present in the pyrolysate. 

It seems that these reactions have a higher incidence than those leading to the cleavage of the C-C bonds in the polyethylene sequences or even than in the cleavage of C-O bonds in poly(ethylene oxide) sequences. 

The structure and composition of copolymers of ethylene and styrene are dependent on the catalyst system used and on the molar ratio of transition metal to cocatalyst (typically MAO). Polymers containing phenylethylene units bridging polyethylene sequences, that is isolated styrene units surrounded by PE segments, are generally formed with titanium-based catalysts. ... 

In copolymerization of ethylene with different class monomers, all the foregoing aspects are superimposed various chemical comonomer sequences can be generated, with various mutual stereospecific placements. Chemical comonomer sequences can be analyzed by statistical methods completely analogous to those described in Section IV (Tables 3 and 4), but practical analysis may be complicated by superposition of stereosequence statistics. Short-chain branching in longer polyethylene sequences is an additional factor to be considered. All these aspects can be demonstrated on the example of the ethylene-vinyl alcohol E/V copolymer prepared by a high pressure, free radical process, which has been characterized in great detail by modern NMR methods. 

The y relaxation takes place at the lowest temperature, overlaps with the )3 relaxation (Fig. 15), and coincides in location and activation energy with the typical y relaxation of polyethylene [35,36], and also of polyethers [37], and polyesters [38] with three or more consecutive methylene units. It appears, for 3 Hz and tan6 basis, at - 120°C (P7MB) and - 126°C (P8MB), and its location and activation energy (35-45 kJ mol ) agree with the values of a similar relaxation associated with kink motions of polymethylenic sequences. 

The propagating polymer then terminates, producing an isotactic polypropylene. Linear polyethylene occurs whether the reaction takes place by insertion through this sequence or, as explained earlier, by ligand occupation of any available vacant site. This course, however, results in a syndiotactic polypropylene when propylene is the ligand. 

Structurally, plastomers straddle the property range between elastomers and plastics. Plastomers inherently contain some level of crystallinity due to the predominant monomer in a crystalline sequence within the polymer chains. The most common type of this residual crystallinity is ethylene (for ethylene-predominant plastomers or E-plastomers) or isotactic propylene in meso (or m) sequences (for propylene-predominant plastomers or P-plastomers). Uninterrupted sequences of these monomers crystallize into periodic strucmres, which form crystalline lamellae. Plastomers contain in addition at least one monomer, which interrupts this sequencing of crystalline mers. This may be a monomer too large to fit into the crystal lattice. An example is the incorporation of 1-octene into a polyethylene chain. The residual hexyl side chain provides a site for the dislocation of the periodic structure required for crystals to be formed. Another example would be the incorporation of a stereo error in the insertion of propylene. Thus, a propylene insertion with an r dyad leads similarly to a dislocation in the periodic structure required for the formation of an iPP crystal. In uniformly back-mixed polymerization processes, with a single discrete polymerization catalyst, the incorporation of these intermptions is statistical and controlled by the kinetics of the polymerization process. These statistics are known as reactivity ratios. 

The general molecular structure of polyether-based polyurethanes is illustrated in Fig. 25.3 a). Typical polyether sequences include polyethylene glycol and polypropylene glycol. The length of the polyether sequences between urethane links can vary from one or two ether groups up to several hundred. As the length of the polyether sequences between urethane links increases, the polymer exhibits more of the properties normally associated with polyethers. 

The previous sections in this chapter have tried to stress upon the significance of distribution of sequence lengths in polyethylene-based copolymers. The sequence length of interest in a system of ethylene-octene copolymers would be the number of methylene units before a hexyl branch point. As was discussed, this parameter has a greater impact on the crystallization behavior of these polymers than any other structural feature like branch content, or the comonomer fraction. The importance of sequence length distributions is not just limited to crystallization behavior, but also determines the conformational,... 

In some very recent work by Karssenberg et al. [130], attempts have been made to improve the analytical ability of a technique like NMR spectroscopy to effectively predict the distribution of sequence lengths in polyethylene-alkene copolymers. They analyzed the entire [ C-NMR spectrum for homogeneous ethylene-propene copolymers. They used quantitative methods based on Markov statistics to obtain sequence length distributions as shown in Figure 22 [130]. The... 

Here m is the mode order (m — 1,3,5. .., usually 1 for polyethylenes), c the velocity of light, p the density of the vibrating sequence (density of pure crystal) and E the Young s modulus in the chain direction. The LAM band has been observed in many polymers and has been widely used in structural studies of polyethylenes [94—99,266], as well as other semi-crystalline polymers, such as poly (ethylene oxide) [267], poly(methylene oxide) [268,269] and isotactic poly(propylene) [270,271], The distribution of crystalline thickness can be obtained from the width of the LAM mode, corrected by temperature and frequency factors [272,273] as ... 

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