Polymer isomerization rates

The temperature of esterification has a significant influence on isomerization rate, which does not proceed above 50% at reaction temperatures below 150°C. In resins produced rapidly by using propylene oxide and mixed phthaUc and maleic anhydrides at 150°C, the polyester polymers, which can be formed almost exclusively in the maleate conformation, show low cross-linking reaction rates with styrene. 

All solvents for these solution thermolysis reactions were freshly distilled and all reactions were done in sealed glass tubes heated in a thermostatted oven. Over a wide range of solvents (DMF, naphthalene, diphenylmethane, benzene, toluene, and decalin) there was no significant variation in either isomerization rate or product composition. Reactions were done at 125°C, 155°C and 195°C and the only limitation was that DMF could not be used as the solvent in reactions at 195°C it led to substantial substrate destruction (polymer forming reactions of substrate with DMF ). Isomer compositions were ascertained both by HPLC and by NMR. 

The process of reorientation during cis—>trans thermal isomerization can be seen at the value of in Equation 3.11, which shows that the cis anisotropy does not contribute to the trans anisotropy if the trans isomer loses total memory of the orientation in the cis isomer Q2 = 0). It is informative to note that in the realistic physical case—i.e., the case of the azobenzene molecule chemically attached to a polymer, where the cis and trans diffusion rates are negligible in comparison to the cis— trans isomerization rate—the relaxation of the cis and trans anisotropy, AA and can be written respectively in the form ... 

A comment must be made about the cis— trans thermal isomerization rate at pressure. At room temperature, the thermal back reaction of DRl-PMMA follows a complex, nonexponential recovery, most of which is completed after a few seconds with a rate of 0.25 s" and deviates from a single exponential decay after the first 10 seconds.Larger relaxation times at Tg -98°C include slow polymer motion coupled with the chromophores rotational diffusion. We confirmed that this behavior is true in the polymer... 

C. Barret, A. Natansohn, and P. Rochon, Cis-trans thermal isomerization rates of bound and doped azobenzenes in a series of polymers, Chem. Mater. 7, 899-903 (1995). 

The polymer microstructure is also sensitive to the reaction conditions. The mmmm pentad content increases with increasing propene pressure and decreasing polymerization temperature, ranging from 6.3% (7p = 45 °C, P = 1 bar) to 28.1% (7p = 20 °C, polymerization in liquid monomer). A qualitative prediction of this phenomenon is based on a polymerization rate proportional to monomer concentration and a catalyst isomerization rate independent of monomer concentration [mmmrn[ °c /rp[M]//Ci. The Colemam and Fox kinetic model predicts that the isotactic block length should increase with monomer concentration. 

Kawashima et al. [188] reported preparation of donor-acceptor type norbomadiene carboxylic acids compounds with carbamoyl groups, such as dipropylcarbamoyl, methylphenylcarbamoyl, propylcarbamoyl, and phenyl-carbamoyl. Benzyl esters were also prepared. Addition of these groups to polystyrenes formed polymers with pendant donor-acceptor type norbomadiene. Some were formed with 100% substitution. It was found that the polymers containing phenylcarbamoyl groups exhibit especially high photo-reactivity. In addition, the rate of the photochemical reaction in films of these norbomadiene polymers increases efficiently by an addition of 4,4 -bis(diethyl-amino)benzo-phenone photosensitizer. As a result, all the norbomadiene groups of the polymers isomerize to the... 

The photoisomerization of trans-azobenzene to the cis form and the thermally activated back reaction can be followed conveniently by UV spectroscopy. We used these processes to compare the behavior of azobenzene residues in the backbones of polymer chains and their low molecular weight analogs. This approach has the advantage over the NMR technique in that we are not limited to solutions of high fluidity, but can study also very viscous and even glassy systems. Data obtained on the dark reaction indicated again no difference between the rates of azobenzene residues in polyamide backbones and in small molecules (8). The isomerization rate of the polymer remained unchanged even when the solution concentration was raised to the point where the molecular chains were heavily intertwined. 

Isomerization is faciUtated by esterification at temperatures above 200°C or by using catalysts, such as piperidine and morpholine (6), that are effective in raising isomerization of fumarate to 95% completion. Resins made by using fumaric acid are exclusively fumarate polymers, demonstrate higher reactivity rates with styrene, and lead to a complete cross-linking reaction. 

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