Polymer of dienes

Polymers of dienes such as butadiene frequently contain a substantial portion of gel which will not dissolve in a good solvent, though it may swell to a volume 20 to 100 or more times that of the polymer itself. This gel, which may comprise up to 90 percent or more of the polymer, consists of a space-network structure formed as a result of a very few cross-linkages provided by occasional (perhaps 1 in 1000 or less) diene units both double bonds of which have entered into the polymerization (see Chap. VI). 

The diethylzinc-a-titanium trichloride system gives low molecular weight polymers of diene hydrocarbons 253, 523). Diethyl-zinc and titanium tetrachloride polymerizes alkynes 535). 

Polymers of dienes (hydrocarbons containing two C—C double bonds) have the potential for head-to-tail and head-to-head isomerism and variations in doublebond position as well. The conjugated diene butadiene can polymerize to produce 1,4 and 1.2 products ... 

This can be used with many polymers such as polyolefins, polymers of dienes, and others. The reactions with natural rubber can be illustrated as follows ... 

Block polymers of dienes and styrene are usually prepared in solution polymerization systems using alkylmetal initiators although exact methods of producing commercial polymers are not often disclosed. Block structures can be foimed by sequential addition of different monomsrs combined with coupling of polymer chains if desired. Possible synthesis methods have been described in a number of publications (6,... 

Copolymerization can be carried out with styrene, acetonitrile, vinyl chloride, methyl acrylate, vinylpyridines, 2-vinylfurans, and so forth. The addition of 2-substituted thiazoles to different dienes or mixtures of dienes with other vinyl compounds often increases the rate of polymeriza tion and improves the tensile strength and the rate of cure of the final polymers. This allows vulcanization at lower temperature, or with reduced amounts of accelerators and vulcanizing agents. 

The primary use of TiCl is as a catalyst for the polymerisa tion of hydrocarbons (125—129). In particular, the Ziegler-Natta catalysts used to produce stereoregular polymers of several olefins and dienes, eg, polypropylene, are based on a-TiCl and A1(C2H3)3. The mechanism of this reaction has been described (130). SuppHers of titanium trichloride iaclude Akso America and Phillips Petroleum ia the United States, and Mitsubishi ia Japan. 

It is an observed fact that with most synthetic polymers the head-to-tail structure is formed. In the case of diene polymers differences may arise in the point of addition. Reaction can take place at the 1 and 4 positions, the 1 and 2 positions or the 3 and 4 positions to give the structures indicated in Figure 4.9. 

The first material to be marked, Fluoroprene, was introduced by Du Pont in 1948. A polymer of 2-fluorobuta-1,3-diene it was the fluoro analogue of polyehloroprene. However, its properties were far from outstanding and produetion was soon discontinued. 

Unlike polyethylene and other simple aikene polymers, natural rubber is a polymer of a diene, isoprene (2-methyl-l,3-butadiene). The polymerization takes place by addition of isoprene monomer units to the growing chain, leading to formation of a polymer that still contains double bonds spaced regularly at four-carbon intervals. As the following structure shows, these double bonds have Z stereochemistry ... 

Early work on the microstructurc of the diene polymers has been reviewed.1 While polymerizations of a large number of 2-substituted and 2,3-disubstituted dienes have been reported,88 little is known about the microstructure of diene polymers other than PB,89 polyisoprene,90 and polychloroprene.91... 

Chemistry on solid support has gained tremendous importance during the last few years, mainly driven by the needs of the pharmaceutical sciences. Due to the robust and tolerable nature of the available catalysts, metathesis was soon recognized as a useful technique in this context. Three conceptually different, RCM-based strategies are outlined in Fig. 11. In the approach delineated in Fig. 1 la, a polymer-bound diene 353 is subjected to RCM. The desired product 354 is formed with concomitant traceless release from the resin. This strategy is very favorable, since only compounds with the correct functionality will be liberated, while unwanted by-products remain attached to the polymer. However, as the catalyst is captured in this process by the matrix (355), a higher catalyst loading will be required, or ancillary alkenes have to be added to liberate the catalyst. 

With polymer-bound diene 356, two different strategies are possible. Following path A, RCM results in the formation of a (volatile) alkene 357a and a... 

Three kinds of polymer segments are formed in the polymerization of dienes 1-4 cis-, 1-4 trans-, and 1-2 segments (or 3-4 in polymerization of isoprene or other monosubstituted dienes). The latter may form isotactic or syndiotactic diads when the proportion of the 1-2 form is sufficiently high, e.g. a syndiotactic, highly 1-2 polybutadiene was described recently by Ashitaka et al. 123), although the so far examined 1-2 polybutadienes produced by homogeneous anionic polymerization were found to be atactic (unpubl. results of Bywater, Worsfold). 

STRUCTURE AND ARRANGEMENT OF UNITS IN POLYMERS FROM DIENES... 

This discussion of the structures of diene polymers would be incomplete without reference to the important contributions which have accrued from applications of the ozone degradation method. An important feature of the structure which lies beyond the province of spectral measurements, namely, the orientation of successive units in the chain, is amenable to elucidation by identification of the products of ozone cleavage. The early experiments of Harries on the determination of the structures of natural rubber, gutta-percha, and synthetic diene polymers through the use of this method are classics in polymer structure determination. On hydrolysis of the ozonide of natural rubber, perferably in the presence of hydrogen peroxide, carbon atoms which were doubly bonded prior to formation of the ozonide... 

Cationic polymerization of dienes using boron trifluoride or aluminum chloride as catalysts seems also to favor the rans-1,4 structure, although 1,2 and 3,4 units also are present. These catalysts also cause cyclization of the structural units, with a consequent decrease in the unsaturation in the polymer. 

No mention has been made of diene polymers in this discussion. Among 1,4 units the substituents are well separated from one another, and the steric repulsions between them should therefore be negligible. A succession of 1,2 units of isoprene (VIII), on the other hand, would form a chain structure of the sterically hindered type XI. 

The equivalent function of the degree of conversion is encountered in the cross-linking of diene polymers discussed below. It is plotted in Fig. 73 in relation to the latter problem. For present purposes it is necessary merely to replace the ordinate in Fig. 73 with p/Cp. Regardless of the absolute magnitude of the branching transfer constant, the relative amount of branching must increase rapidly with conversion. 

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