Comments on the articles: „Thermisch und UV‐photochemisch initiierte hochdruckpolymerisation des ethylens”︁

Abstract

In the articles's2) published recently, Buback et al. have presented data about the polymerization of ethylene initiated by thermal or photochemical method in a wide range of pressure (1600-2440 bar) and temperature (129-265°C). It was shown that the method of initiation has no effect on the molar mass, molar mass distribution and degree of long chain branching (A) of polyethylene (PE). Those structural parameters, however, are greatly influenced by pressure, temperature and ethylene conversion during the polymerization. The kinetics of polymerization was analysed on the basis of the classical theory of free-radical polymerization. Correlations of polymerization pressure and temperature with the structural characteristics of PE were presented.In spite of the fact that an approach of this kind is well established in the interpretation of the polymerization processes, some advances are necessary in the case of the high pressure polymerization of ethylene. It is known3) that pressure accelerates the polymerizations because it aligns the monomer molecules in a way which favours the chain propagation step. Compression causes the monomer molecules to lose some of their rotational and translational entropy, with the result that they have less to lose when they become built into the growing polymer chains. The acceleration of polymerization at high pressure is entirely due to the increase of activation entropy3).Since the density of ethylene in the polymerization conditions (0,4 -0,6 g/cm3) exceeds the density of loosely packed unoriented ethylene molecules (0,28 g/cm3), Hunter concluded that ethylene molecules under polymerization conditions are properly oriented and distorted4). It has recently been shown by us5) that during compression the ethylene molecules are organized in different supermolecular particles.This supermolecular organization of compressed ethylene, represented by the low ethylene entropy, has a decisive effect on the polymerization mechanism, the kinetics and the structure of PE6). As a rule, the same structure of PE should be expected if the entropy of ethylene is equal in different polymerization runs, even if the other polymerization conditions (pressure, temperature, type of initiation, etc.) are quite different ').In this cornmefits we want to explain how the data obtained by Buback et al. can be interpreted on the basis of the organization of monomer molecules, i.e. ethylene 0025-1 16X/85/$03.00

Origin of the molecular structure and properties of low density polyethylene

“…The structure and reactivity of the supermolecular particles are discussed elsewhere. 15 Low reactive molecular pairs are present in a-ET and in P-ET, but the highest concentration occurs near to critical isochore conditions. Very reactive bimolecules dominate in 0-ET, but the highest concentration occurs near to the 0.7 phase transition.…”

“…10 in Ref. 15.) Furthermore, it has been shown that marked changes in PE structure originate during polymerisation Fig.…”

“…The supermolecular structure has a decisive effect on the polymerisation rate [14][15][16] (Table 1). Polymerisation is a chain process incranated in an actual material chain, namely a macromolecular chain.…”

Supermolecular organization and polymerization of compressed ethylene

“…The period of rapid growth arises when a macroradical begins to connect ethylene molecules within a supermolecular particle. This rule was proved on the following structural characteristics of polyethylene: molecular mass [19], short chain branching [ I I , 251, long chain branching [32], and unsaturated bonds [25]. During the period of stagnation, a macroradical can isomerize or collide with other particles present in the system (impurities, chain transfer agent, comonomer, polymer, nonreactive supermolecular particles of ethylene, etc.).…”

“…v [31], A [32], A [33], [34] and o , 1351) The characteristics of polyethylene strongly depend on its origin.…”