Latterly, the formation cnthalpies were determined for many carbocations by means of thermochemical methods. Therefore a thermodynamical view is possible for the propagation reactions of the cationic polymerization to a larger extent.
Zzir Therniodynanrik der kationischen Polymerisation. 1. Kalionisches WachstumIn letzter Zeit wurden fur viele Carbokationen die Bildungsenthalpien thermochemisch bestimmt. Damit wird in erweitertem Umfang die thermodynamische Betrachtung der Wachstumsreaktionen der kationischen Polymerisation moglich. 0 mepfiodunamae tcamuounoiL noawepuiayuu.
Our study of the cationic polymerization of cis-1 ,Zdimethoxyethylene (DME)') indicated some characteristics in comparison with the cationic polymerization of vinyl ethers and P-substituted vinyl ethers (e. g. 1-propenyl alkyl ethers)'). Under comparable conditions (low-polarity solvent, initiation by means of a Lewis acid, -78 "C) DME-polymers have been received with relatively high molecular weight (up to SOOOOO) and narrow molecular weight distribution (ratio of weight to numberaverage molecular weight &,,/a,, = 1,3).As an explanation of this, intramolecular stabilization of cationic chain ends by oxygen atoms of the methoxy groups as donors has been cited. Such a stabilization was discussed for the case of vinyl ether polymerization by postulating a six-membered cyclic oxonium ion3) and for the cationic polymerization of oxygen heterocycles4). In contrast to normal vinyl ethers, DME can form 5-and 7-membered cyclic intermediates beside 6-membered rings4).
As a continuation of the thermochemical analysis of the cationic polymerization, the initiation as well as the transfer and termination reactions were thermodynamically treated as competing reactions to the cationic propagation using recent data for the formation enthalpies of the carbocations.
The influence of solvents on cationic polymerizations is well-known I). Starting from model calculations a rule could be established describing the influence of the solvent on the ratio of copolymerization reactivity ratios rl and rZzt3):"If in a copolymerization via ionized cationic chain ends the comonomer smaller in size, forming a cation with low charge delocalization, is indicated as monomer 1 and monomer 2 exhibits opposite behaviour, i. e. the latter is larger in size and the cation is stabilized by charge delocalization, then r2 /rl should decrease during the transition into a more polar solvent".The usefulness of this rule was shown by an extensive comparison with literature data (see part 1 of this series2)). Part 3 deals with theoretical and experimental investigations on the cationic copolymerization of isobutene (IB) with cyclopentadiene (CPD) according to the following equations:
IB-IB+(1-1) k l l , IB-CPD' (1-2) 4 2 , CPD-IB+ (2-1) k21 ., CPD-CPD ' (2-2) k22 , IB+ + IB IB+ + CPD CPD' + IB CPD' + CPD r, = k l l / k 1 2 ; r2 = kZ2/kalThe analysis of these processes by means of thermochemical kinetics4) leads to expectation values for the reactivity ratios of copolymerization.A method for the estimation of the reaction enthalpies of the first propagation steps in cationic polymerizations was given in ref.'). Using this method the following reaction enthalpies for the gas phase are calculated for steps (1-1) to (2-2):AHG/(kJ * mo1-l) = -104 (1-l), -122 (1-2), -90 (2-l), -108 (2-2).The Hammond principle is valid because of the distinctly negative values of these reaction enthalpies. This means that one can assume the direct proportionality AHT(i-j) a AH*(i-j), where AH* represents the activation enthalpyQ. This was a) Part 2 cf.3).
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