Ethylene polymerization modes

The presence of methylenic bands shifted at higher frequency in the very early stages of the polymerization reaction has also been reported by Nishimura and Thomas [114]. A few years later, Spoto et al. [30,77] reported an ethylene polymerization study on a Cr/silicalite, the aluminum-free ZSM-5 molecular sieve. This system is characterized by localized nests of hydroxyls [26,27,115], which can act as grafting centers for chromium ions, thus showing a definite propensity for the formation of mononuclear chromium species. In these samples two types of chromium are present those located in the internal nests and those located on the external surface. Besides the doublet at 2920-2850 cm two additional broad bands at 2931 and 2860 cm are observed. Even in this favorable case no evidence of CH3 groups was obtained [30,77]. The first doublet is assigned to the CH2 stretching mode of the chains formed on the external surface of the zeolite. The bands at 2931 and... 

As stated by Gibson, the most striking feature in the structure of the Bis(imido) chromium(VI) precursor is the manner in which the two benzyl ligands are bonded to the chromium atom through and r coordination modes. Figure 3.30 shows the two cationic complexes prepared from the precursor complex to form ethylene polymerization catalysts. 

Scheme 19. Mode of lanthanide(III)-initiated polymerization of ethylene...

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate. By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially this includes the formation of polyesters and polyamides (nylons). Thus, there exists a large overlap between the terms stepwise kinetics and condensation polymers, and chainwise kinetics and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1. 

In addition to step and chain polymerizations, another mode of polymerization is of importance. This is the ring-opening polymerization (ROP) of cyclic monomers such as cyclic ethers, acetals, amides (lactams), esters (lactones), and siloxanes. Ring-opening polymerization is of commercial interest in a number of systems, including the polymerizations of ethylene oxide... 

Fig. 6.23. Effect of thermal (a) and UV initiation (b), type of comonomer, and percentage of 1-dodecanol in the polymerization mixture on the mode pore diameter of quinidine-functionalized chiral monoliths. (Reprinted with permission from [56]. Copyright 2000 American Chemical Society). Reaction conditions polymerization mixture, chiral monomer 25 8 wt%, glycidyl methacrylate ( ) or 2-hydroxyethyl methacrylate ( ) 16 wt%, ethylene dimethacrylate 16 wt%, porogenic solvent 60 wt% (consisting of 1-dodecanol and cyclohexanol), polymerization time 20 h at 60°C (a) and 16 h at room temperature (b).

Figure 29 Separation of the nonsteroidal anti-inflammatory drugs ibuprofen (peak 1), naproxen (2), ketoprofen (3), and suprofen (4) in anion-exchange CEC mode using a strong anion-exchange monolithic column. Conditions on-column alkylated monolith prepared from mixtures consisting of 8% 2-dimethylaminoethyl methacrylate, 24% 2-hydroxyethyl methacrylate, 8% ethylene dimethacrylate, 20% cyclohexanol, 40% 1-dodecanol UV-initiated polymerization at room temperature for 16 h cfpmode= 1423 nm. Column dimensions inner diameter 0.1 mm, total length 335 mm, effective length 250 mm. Mobile phase 0.4 mol/L acetic acid and 4 mmol/L triethylamine in acetonitrile/methanol (60/40), voltage -25 kV, injection -5 kV for 5 s, temperature 50°C, UV detection at 250 nm. (Reprinted from Ref. 127, with permission.)...

---