Polymerization activation energy

This situation seems highly probable for step-growth polymerization because of the high activation energy of many condensation reactions. The constants for the diffusion-dependent steps, which might be functions of molecular size or the extent of the reaction, cancel out. 

Using typical activation energies out of Tables 6.2-6.4, estimate the percent change in the rate of polymerization with a 1°C change in temperature at 50°C for thermally initiated and photinitiated polymerization. 

Applying the Arrhenius equation to Eq. (6.116) shows that the apparent activation energy for the overall rate of polymerization is given by... 

Polymerization Solvent. Sulfolane can be used alone or in combination with a cosolvent as a polymerization solvent for polyureas, polysulfones, polysUoxanes, polyether polyols, polybenzimidazoles, polyphenylene ethers, poly(l,4-benzamide) (poly(imino-l,4-phenylenecarbonyl)), sUylated poly(amides), poly(arylene ether ketones), polythioamides, and poly(vinylnaphthalene/fumaronitrile) initiated by laser (134—144). Advantages of using sulfolane as a polymerization solvent include increased polymerization rate, ease of polymer purification, better solubilizing characteristics, and improved thermal stabUity. The increased polymerization rate has been attributed not only to an increase in the reaction temperature because of the higher boiling point of sulfolane, but also to a decrease in the activation energy of polymerization as a result of the contribution from the sulfonic group of the solvent. 

The primary cation CH20H is created in the cage reaction under photolysis of an impurity or y-radiolysis. The rate constant of a one link growth, found from the kinetic post-polymerization curves, is constant in the interval 4.2-12 K where = 1.6 x 10 s . Above 20K the apparent activation energy goes up to 2.3 kcal/mol at 140K, where k 10 s L... 

The typical systems are BPO-DHET(N,N-di(2-hy-droxyethyl)-p-toluidine) system, BPO-DHPT(N,N-di(2-hydroxypropyl)-p-toluidine) system, BPO-HMA(N-2-hydroxyethyl-N-methyl-aniline), and BPO-HMT(N-2-hydroxylethyl-N-methyl-p-toluidine) system [17-19]. Their polymerization rate and overall activation energies of polymerization Ea are determined and the data are compiled in Table 2. 

In general, the activation energies for both cationic and anionic polymerization are small. For this reason, low-temperature conditions are normally used to reduce side reactions. Low temperatures also minimize chain transfer reactions. These reactions produce low-molecular weight polymers by disproportionation of the propagating polymer ... 

The nature of this interaction is not yet clear, but there is no doubt that this is a manifestation of a polymer-monomer interaction, typical of PCSs. The process of polymerization of phenylacetylene on free ions is characterized by the 6th order in initiator and monomer and has an activation energy of=25 kJ/mol (6 kcal/mol). 

Stoicescu and Dimonie103 studied the polymerization of 2-vinylfuran with iodine in methylene chloride between 20 and 50 °C. The time-conversion curves were not analysed for internal orders but external orders with respect to catalyst and monomer were both unity. Together with an overall activation energy of 2.5 kcal/mole for the polymerization process, these were the only data obtained. Observations about the low DP s of the products, their dark colour, their lack of bound iodine and the presence of furan rings in the oligomers, inferred by infrared spectra (not reported), completed the experimental evidence. The authors proposed a linear, vinylic structure for the polymer, and a true cationic mechanism for its formation and discussed the occurrence of an initial charge-transfer complex on the... 

The steady-state polymerization in the presence of Cr (it-CsHb) was first order with respect to the monomer concentration (125) the effective activation energy was 4.7 0.5 kcal/mole. When the concentration of Crfir-CaHs was varied, first a linear rise of the polymerization rate occurred with an increase of tris-ir-allylchromium concentration to the upper limit then the rate does not depend on Crfx-CaHs concentration (126). The value of the upper limit of the polymerization rate increased with an increase in the water content of the solvent used. 

The number of active centers determined by the quenching technique was dependent on the polymerization temperature (98) that was the reason for the difference between the overall activation energy and the activation energy of the propagation step. 

In polymerization by one-component catalysts [chromium oxide catalyst (75), titanium dichloride 159) at ethylene concentrations higher than 1 mole/liter and temperatures below 90°C the transfer with the monomer is a prevailing process. The spontaneous transfer, having a higher activation energy, plays an essential role at higher temperatures and lower concentrations of the monomer. 

Radical additions are typically highly exothermic and activation energies are small for carbon30-31 and oxygen centered32,33 radicals of the types most often encountered in radical polymerization, Thus, according to the Hammond postulate, these reactions are expected to have early reactant-like transition states in which there is little localization of the free spin on C(J. However, for steric factors to be important at all, there must be significant bond deformation and movement towards. sp hybridization at Cn. 

The result indicates that the activation energy for combination is higher than that for disproportionation by ca 10 kJ mol"1. A similar inverse temperature dependence is seen for other small radicals (Section 2.5). However, markedly different behavior is reported for polymeric radicals (Section 5.2.2.2.1). 

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