Chain transfer pressure effect

In solution polymerization, monomers mix and react while dissolved in a suitable solvent or a liquid monomer under high pressure (as in the case of the manufacture of polypropylene). The solvent dilutes the monomers which helps control the polymerization rate through concentration effects. The solvent also acts as a heat sink and heat transfer agent which helps cool the locale in which polymerization occurs. A drawback to solution processes is that the solvent can sometimes be incorporated into the growing chain if it participates in a chain transfer reaction. Polymer engineers optimize the solvent to avoid this effect. An example of a polymer made via solution polymerization is poly(tetrafluoroethylene), which is better knoivn by its trade name Teflon . This commonly used commercial polymer utilizes water as the solvent during the polymerization process,... 

Chain-transfer reactions would be expected to increase in rate with increasing pressure since transfer is a bimolecular reaction with a negative volume of activation. The variation of chain-transfer constants with pressure, however, differ depending on the relative effects of pressure on the propagation and transfer rate constants. For the case where only transfer to chain-transfer agent S is important, Cs varies with pressure according to... 

Radical chain polymerization of ethylene to polyethylene is carried out at high pressures of 120-300 MPa (17,000-43,000 psi) and at temperatures above the Tm of polyethylene (Fig. 3-18) [Doak, 1986]. Batch processes are not useful since the long residence time gives relatively poor control of product properties. Long-chain branching due to intermolecular chain transfer becomes excessive with deleterious effects on the physical properties. Continuous processes allow better control of the polymerization. 

We also observe a slowing down of the relaxation time T2 related to internal motions. The relaxation time T2 increases from 2.3 ps at ambient pressure to 3.6 ps at 6000 bar. This increase of T2 is accompanied by a slight decrease of the proportion of mobile protons (from 43% at ambient pressure to 39% at 6000 bar) and of the radius of the sphere in which motions occurs (from 3. 6 A at ambient pressure to 3.1 A at 6000 bar). At atmospheric pressure, QENS experiments on other proteins have shown that the motions observed, at similar wave vector and energy transfer ranges, concern protons belonging to the side chains at the surface of the protein . Thus the observed modifications of internal dynamics can only concern lateral chains. Moreover, molecular dynamics simulations have shown that backbone of BPTI is not affected by pressures up to 5000 bar ". A possible explanation about the pressure effects on internal protein motions is that the effect of pressure is to increase the density of first hydration layer at the protein surface. This water density increase induces... 

Impurities can also act as chain transfer agents. In some instances, as in the production of low density polyethylene via high pressure radical polymerization, impurities and/or the so-called inerts (methane, ethane, and propane), which come as impurities in the ethylene, are used as effective chain transfer agents to lower the MW of the polymer. 

Pressure exerts a marked effect on the polymerization reaction rate constant and can be used to control the reaction rate and molecular weight in addition to the more usual variables of initiator concentration and temperature. Since the number of short branches and the molecular weight are determined by chain transfer reactions which are more influenced by temperature and less by pressure than the polymerization reaction, it follows that the molecular weight decreases and the degree of short branching increases with increasing temperature (and vice versa with pressure). 

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