547.458.81 Dilute solutions of water-soluble cellulose esters and also solutions of model compounds -glucose and cellobiose -have been investigated. It has been shown that a definite influence on optical rotatory dispersion is exerted by the conformational difference of the pyranose rings of cellulose arising when the hydroxy groups in the macromolecule are partially replaced by functional groups of different natures.Optical activity characterizing features of the spatial structure of simple molecules was known even during the last century from the work of J. Biot, L. Pasteur, J. H. Van't-Hoff, A. M. Butlerov, and others. It is also known that features of the spatial structure of molecules determine various properties of a substance and play a major role in processes of synthesis.This question has been little studied as applied to cellulose and its derivatives. In the dissertation [1], which gathers together practically all the material on the optical activity of cellulose and celhilose acetate, there is no information on the connection of optical activity with the conditions of synthesis, the properties of the polymer, and the quality of the products. There are no clear ideas on the role of the functional groups of celhilose esters in the optical activities of their solutions.In order to elucidate the causes of the change in the optical activity of cellulose derivatives On the replacement of certain functional groups by others in the pyranose rings of the cellulose macromolecule, we have studied by the method of optical rotatory dispersion (ORD) dilute solutions of cellulose acetates (CAs) with different degrees of substitution, cellulose aminoacetate, and cellulose acetophthalate, and also solutions of model compounds -t~-ghicose and cellobiose.In this connection, we investigated solutions in acetic acid of CAs with various degrees of substitution obtained by the successive saponification of cellulose triacetate at temperatures of 59 and 20°C.The results of the optical rotatory dispersion investigations are given in Table 1, from which it can be seen that CAs with the same qualitative and quantitative compositions of the macromolecules can have negative or positive values of the ORD, depending on the conditions of synthesis. For solutions of CAs obtained at a low saponification temperature -20°C -a transition is observed from the negative to the positive region of optical rotatory dispersion. It can also be seen from Table 1 that the samples containing 54-56% of bound acetic acid obtained at 20°C were dextrorotatory, while those obtained at 59°C were levorotatory.In order to elucidate the factors causing CAs to change from levorotatory to dextrorotatory in the process of saponification, we studied model compounds of one functional composition -glucose and cellobiose -in acetic acid (Table 2). A difference in the optical activities of substances with one functional composition can be explained by a change in the conformational state of the pyranose rings and by rotational isomerism. However, rotational isomeris...

No abstract

The cross-linking reactions of acetylcelluloses having different levels of acetyt groups with pyromellitic dianhydride in homogeneous media have been investigated A relationship has been established between the aatic exchange capacity (SEC) and the content of hydroxy groups in the acetylcellulose and the esterification conditions (homogeneous or heterogeneous-solid phase). It has been shown that on the interaction of acetylcellulose with pyromellitic dianhydride under homogenous conditions there is a uniform distribution of cross-linkages over the whole length of the acetylcellulose macromolecule, which enables products with high SEC values to be obtaineflIn the majority of cases, the esterification of native cellulose under heterogeneous conditions begins at the surface of the fibers.The introduction of transverse covalent bonds between the cellulose macromolecules on esteriflcation with polyfunctional acids restricts their mobility and thereby inhibits the diffusion of an esterifying reagent into the depth of the cellulose'fibers. Moreover, in cellulosic materials hydrogen bonds also act similarly to covalent bonds, leading to a powerful intermolecular interaction [1]. Consequently, in the heterogeneous esterification of cellulose with di-and polyfunctional acids an intrafibrillar, rather than an intermolecular, formation of bridge bonds takes place. What has been said above may substantially affect the degree of modification of cellulose and the properties of the product obtained. In this connection, it appears of interest to study the influence of the esterification of soluble cellulose derivatives by polyfunctional acids under homogeneous conditions where bridge bonds may be distributed uniformly over the whole length of a cellulose ester macromolecule.Partially substituted esters of cellulose capable of dissolving in organic solvents are the most suitable objects and model compounds for studying features of the cross-linking of cellulose with polyfunctional acids. Under homogeneous conditions, the interaction of polyfunctional acids with partially substituted cellulose esters takes place through their free hydroxy groups. Therefore, in order to esterify partially substituted cellulose acetate with pyromellitic dianhydride a secondary acetylcellulose with a degree of substitution (DS) of 2.4 2.5 produced industrially and soluble in acetone was subjected to deep homogenous hydrolysis to different degrees of substitution.Figt~'e 1 presents kinetic curves of the change in the content of free hydroxy groups in acetylcellulose as a function of the time of hydrolysis. Saponification to low degrees of substitution (DS = 0.5----0.55) led to the appearance of free primary hydroxy groups in each pyranose unit. At such ratios of hydroxy groups and acetyl radicals, acetylcellulose is soluble in water [2]. With a further increase in the time of saponification the degree of substitution by acetyl groups in the acetylcellulose decreases. However, the reaction products again become insoluble in water, but, regardless of the...