Ethylene polymers, molecular weight

Reactivity Is typical of an acrylamide. For example, compound 1 shows essentially 1 1 copolymerizablllty with butyl acrylate. Copolymerizablllty has also been demonstrated with styrene, other acrylates and methacrylates, vinyl acetate (VAc), VAc/ethylene and vinyl chlorlde/ethylene. High molecular weight polymers and copolymers remain soluble. Indicating any chain transfer to polymer, e.g. through abstraction of the acetal hydrogen. Is minor. 

The exchange reaction places an upper limit on the polymer molecular weight that is possible for polymerizations performed in the presence of alcohols or other protonic substances. There are very few reports of polymers with molecular weights above 10,000 for ethylene... 

Chain transfer to molecular hydrogen not only affects polymer molecular weight, but unlike other transfer agents, also affects polymerization rate. Hydrogen often decreases the rate of ethylene polymerization, but increases the rate of propene polymerization [Chadwick,... 

In the absence of H2 and other transfer agents, polymer molecular weight is limited by various P-hydride transfers—from normal (1,2-) and reverse (2,1-) propagating centers, before and after rearrangement [Lehmus et al., 2000 Resconi et al., 2000 Rossi et al., 1995, 1996 Zhou et al., 2001] (Sec. 8-4i-2). Vinylidene, vinylene, and trisubstituted double-bond end groups are formed in 1-alkene polymerizations, vinyl and vinylene in ethylene polymerization. [Vinyl groups are also produced in some 1-alkene polymerizations, not by P-hydride transfer, but by P-alkyl transfer (Sec. 8-4i-2).]... 

Explain how ethylene performs a control of polymer molecular weight in butadiene polymerisation with Ziegler-Natta catalysts. 

Di Colo, G., Burgalassi, S., Chetoni, R, Fiaschi, M. P, Zambito, Y., and Saettone, M. F (2001), Relevance of polymer molecular weight to the in vitro/in vivo performances of ocular inserts based on poly(ethylene oxide), Int. J. Pharm., 220(1-2), 169-177. 

Description Liquid or gaseous ethylene is fed, together with a solvent and required comonomer(s) into a stirred, liquid-filled, vessel-type reactor (1). The reactor is operated adiabatically thus, the feed is precooled. All heat of reaction is used to raise polymerization temperature up to approximately 200°C. Hydrogen is used to control polymer molecular weight. A high-activity, proprietaiy catalyst is prepared onsite from commercially available components. Ethylene... 

Poly (ethylene terephihalate) and nylon-6,6 manufacture are homogeneous bulk step-growth reactions. The molecular weight of the polymer produced is limited by the high viscosity of the reaction mixture at very high conversions. Post polymerization techniques such as that described in connection with reaction (5-39) can be used to increase the polymer molecular weight for some applications. 

In the polymerization of ethylene by (Tr-CjHsljTiClj/AlMejCl [111] and of butadiene by Co(acac)3/AlEt2Cl/H2 0 [87] there is evidence for bimolecular termination. The conclusions on ethylene polymerization have been questioned, however, and it has been proposed that intramolecular decomposition of the catalyst complex occurs via ionic intermediates [91], Smith and Zelmer [275] have examined several catalyst systems for ethylene polymerization and with the assumption that the rate at any time is proportional to the active site concentration ([C ]), second order catalyst decay was deduced, since 1 — [Cf] /[Cf] was linear with time. This evidence, of course, does not distinguish between chemical deactivation and physical occlusion of sites. In conjugated diene polymerization by Group VIII metal catalysts -the unsaturated polymer chain stabilizes the active centre and the copolymerization of a monoolefin which converts the growing chain from a tt to a a bonded structure is followed by a catalyst decomposition, with a reduction in rate and polymer molecular weight [88]. 

Olive and Olive [291] have studied transfer reactions in ethylene polymerization by (EtO)3TiCl/EtAlCl2. Polymerization reaches a maximum value at Al/Ti = 7 and is first order in monomer. As polymer molecular weight is independent of catalyst and monomer concentrations it is concluded that transfer involves monomer and that spontaneous termination is absent. 

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