Redox Systems in the Initiation of Radical Processes. Mechanism of Action

Abstract The following redox systems capable of initiating radical polymerizations are discussed: (1) Systems of the first type, where the reaction leads to the formation of a single radical: (a) Mechanism of action of reversible systems involving dienols and salts of metals of variable valency. (b) Mechanism of the decomposition of hydroperoxides by the action of salts of metals of variable valency in their higher oxidation states. (2) Systems of the second type, where the reaction leads to the formation of two radicals. Mechanism of action and effectiveness of systems involving hydroperoxides, quinol, and sulfite. (3) Systems of the third type, in which the formation of free radicals is not directly linked to the redox process. Mechanism of action of systems involving diazoamino compounds and “polyamine” systems. (4) Crosslinking of unsaturated polymers at low temperatures by the action of redox systems. Transition from crosslinking to destructive processes in presence of oxygen.</jats:p

Complex Formation and Chain Structure in the Polymerization of Divinyl with Butyllithium

Abstract It has been established by a number of studies that the chain structure in the catalytic polymerization of monoolefins and dienes is determined to a large degree by the nature of the catalytic complex, participating in the polymerization process. Also it was shown that the initial catalytic complex bears a direct relationship to each elementary act of the chain growth. The isolation of organolithium compounds in the pure state is associated with great experimental difficulties. Together with the formation of the organometal compounds the possibility of a complex mixture of their oxidation products being formed is not excluded. To elucidate the influence of the indicated oxidation products on the chain structure in the polymerization of butadiene it seemed expedient to investigate the influence of oxygen. As the result of the investigation made by us it was established that in the polymerization of 1,3-butadiene with organolithium compounds the introduction of comparatively small amounts of oxygen into the system leads to a substantial increase in the number of 1,2 units in the polybutadiene at the expense of a reduction in the number of 1,4 units. The addition of either alcohol or phenol exerts a similar influence on the structure of the polymeric chain (Table 1).</jats:p

Polymerization of Isoprene by Organomagnesium Compounds

Abstract 1. It was established that ether-free organomagnesium compounds promote the polymerization of isoprene at elevated temperatures. 2. The polyisoprene so-formed is mainly 3,4 linked. The nature of the halogen and radicals in the organomagnesium material has no influence on the microstructure of the polymer chains. 3. The presence of complex-forming agents (ethyl ether and triethylamine) affect an appreciable reduction in the velocity of polymerization and a marked lowering in the content of 3,4 links in the chains.</jats:p

cis-trans-Isomerization of 1,4-Polybutadiene under the Influence of Nitrogen Dioxide

Abstract It is well known that, under the influence of various catalytic agents of ionic nature, cis-trans isomerization of natural rubber takes place. Sulfur dioxide, organic aluminum compounds, titanium halides, anhydrous hydrogen chloride and other materials show similar action. Also established is the possible isomerization of cis forms of natural rubber at elevated temperatures, under the influence of selenium, thio acids and a number of compounds which are dissociated through a radical mechanism, an example being benzoyl disulfide. For cis-1,4-polybutadiene, the possibility of isomerization by means of ultraviolet rays and also by means of gamma radiation, in the presence of sensitizers has been demonstrated. This is also accomplished by means of thio acids and several organic sulfides. As may be surmised, isomerization takes place under conditions dependent on consecutive reactions involving addition of free radicals to the double bonds and the dissociation of the products thus formed:</jats:p

Cis-Trans Isomerization of Natural Rubber under the Influence of Organo-Aluminum Compounds and Titanium Tetrachloride

Abstract Ziegler catalysts have become of prime importance for synthesis of polymers with regular structure, particularly cis-polyisoprene. The disruption of the structure of the chain by the formation of trans units was, until now, connected only with the influence of the physical and chemical properties of the catalyst on the nature of the addition of the diene monomers during the polymerization process. In the present work it is shown that destruction of regularity of structure can be caused by isomerization of cis units in complete polymer chains into trans units under the influence of the compounds used for initiating the polymerization process. Attempts to isomerize natural rubber by means of ultraviolet light and iodine did not give the expected effect. Ferri established for the first time that under the influence of zinc chloride and titanium tetrachloride natural rubber undergoes changes accompanied by disappearence of crystallization on stretching as shown by x-ray structure analysis. On the basis of these results the authors presumed that under the influence of these agents isomerization of cis units to trans units occurs in the natural rubber chain. The isomerization of cis-1,4 units to trans units in polybutadiene by means of ultraviolet light in the presence of organic bromine or sulfur compounds was first accomplished and proved by Golub. The formation of trans units in natural rubber under similar conditions was not observed. The study of the isomerizing effect of TiCl4 and organo-aluminum compounds was conducted by us on solutions of milled natural rubber (NK) in benzene in sealed glass ampoules. The benzene used was dried and distilled over sodium. All work was conducted in an atmosphere of dry argon. Unsaturation and microstructure of the chain was determined on each sample. The study of microstructure of polyisoprenes was carried out by means of infrared spectra, the relative content of the different configurations of the polymer chain being determined by the absorption in the 800–1000 cm−1 region. The method previously developed by one of us was used for this purpose.</jats:p

Oxidation-Reduction Systems for Initiation of Radical Processes. VI. Systems Involving Oxygen for Initiation of Oxidative Degradation of Polymers

Abstract 1. The mechanism of action of various oxidation-reduction systems containing organic reducing agents, iron salts, and oxygen, is investigated, and it is shown that in hydrocarbon media these systems are an effective source of free radicals at low temperatures. 2. The possibility of utilization of these systems for initiation of a chain process of oxidative degradation of unsaturated and some saturated polymers, at low temperatures, is shown.</jats:p