Structure of diene polymers

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... 

Whilst in many cases the structure of these polymers may be established by infra-red, NMR and other standard methods it has been seen how the information may be supported by the use. of model compounds and of deuterated polymers. In the next chapter we shall consider the further application of these and other techniques to elucidate cross-linking mechanisms and cross-linked structures of diene polymers which are less amenable to conventional analysis. 

TABLE 8.5 Structure of Diene Polymers Obtained by the Free Radical Polymerization Method... 

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. 

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 marked variation in stereostructure of diene polymers caused by changes in the counter-ion and solvent when butadiene or isoprene are polymerized anionically, are as yet not fully explained. Much progress has been made on elucidating the causes of variations in the cis/trans ratio of the l h structures in these systems (], , ), but the causes of the change in the ratio of 1 2 to 1 U structures in butadiene for example has been left largely unresolved. In dioxane, for instance, the amount of 1 2 structure decreased from with Li counter-ion at 15°, to hl% with Cs (I4). Less variation is found in THF because a substantial part of the reaction is carried by the free ion. Changes are also observed in polyisoprene ( ). 

Ionic polymerizations are remarkable in the variety of polymer steric structures that are produced by variation of the solvent or the counter ion. The long lived nature of the active chain ends in the anionic polymerization of diene and styrene type monomers lends itself to studies of their structure and properties which might have relevance to the structure of the polymer produced when these chain ends add further monomer. One of the tools that, may be used in the characterization of these ion pairs is the NMR spectrometer. However, it should always be appreciated that, the conditions in the NMR tube are frequently far removed from those in the actual polymerization. Furthermore NMR observes the equilibrium form on a long time scale, and this is not necessarily that form present at the moment of polymerization. 

C1 Cj to C4, and C4 to C4), but the channel directs the addition selectively only to C2 to C4. Polymerization of these dienes takes place in DC A and ACA channels, but with less selectivity. In all of these cases, it is the tight fit of the monomer within the cross section of the channels that enforces the geometry of inclusion and subsequently the structure of the polymer. The fact that the selectivities noted above correlate well with the channel cross-sectional area (lower selectivity obtained in the broader channels of DCA and ACA compared to urea and thiourea) is consistent with the hypothesis that alignment is controlled primarily by repulsive forces. 

A fairly considerable amount of research has gone into the determination of diene polymer structure. The technique which one now finds most often em- j ployed is quantitative infrared spectroscopy (54). Although this technique still appears to present a number of difficulties, its ease and convenience have led to wide usage. Ozonization and perbenzoic acid titration have also been used to distinguish 1,2, from 1,4 linkages (75). 

The structure of the polymer obtained in the polymerization of butadiene and isoprene with heterogeneous Ziegler-Natta catalysts depends on the nature of the monomer, catalyst system, and reaction conditions. Previously reported results are reviewed and a mechanism is proposed for the stereoregulated polymerization of conjugated dienes. The polymerization of cyclopentadiene with LiAlH -TiCl4 or LiAlR4-TiCl4 catalyst system yields a readily oxidized polymer for which a 1,2-structure is proposed. 

A long sought goal in mechanistic polymer chemistry has been the determination of those factors which lead to cis, trans or vinyl structures in diene polymers. Various proposals have been made jnd are summarized in the comprehensive revie edited by Saltman. The simplest proposal, advanced by Cossee and Arlman, assigns the dominant role to the nature of the diene coordination. In this mechanism bidentate coordination, e.g. of necessity involving the cisoid conformation of the diene, would lead to cis polymer. 

The role of ozone is also very important in the degradation of rubber. It was found that on exposing a stressed rubber sample to ozone, small cracks formed on the surface. These cracks are perpendicular to the direction of applied stress [450]. The velocity of cracking depends on the chemical structure of the polymer, time of exposure, magnitude of applied stress, content of plasticizers, and so on [13, 14, 111, 186, 239, 360, 361, 553, 554]. Several workers [13, 184] have indicated that the ozonization of diene elastomers is accompanied by an autocatalytic... 

The third major breakthrough was by Szwarc and coworkers who reported in 1956 (30. 31) that certain anionic polymerization systems gave "living polymers" to which a second monomer could be added without termination, so that block polymers could be formed. Interest of research groups in this system was dissipated somewhat by the finding that the sodium naphthalene diinitiator used by Szwarc resulted in undesirable 1,2- or 3,4-addition structures in diene polymers. 

Upon the transition to the polymerization on ion pairs and, further, on the carbon-metal bond, the presence of the counterion in the active center will distort and decrease the local symmetry of the intermediate, thus weakening the spin exclusion principle and gradually leading to the predominant formation of the 1,4(4,l)-structure of diene units in the polymer. This situation is qualitatively shown in Fig 5. 

In this case there are no symmetry restrictions, and regardless of the conditions of the process mainly the formation of the 1,4(4,l)-structure of diene units should be expected both in homo- and copolymers. The content of cis- or transunits in the polymer is entirely determined by the thermodynamical equilibrium conformation of the initial monomer. 

Naturally, the more complex the composition of the substances to be pyrolysed, the more characteristics are needed for identification. For example, in identifying isoprene rubbers (NK, SKN-3, SKIL, Natsyn, Coral, Cariflex IR), the characteristic pyrolysis products are isoprene and dipentene, whereas with butadiene rubbers (SKB, SKD, Budene, Diene NF, Buna CB, Asadene NF, Cariflex BR, Ameripol CB) they are butadiene and vinylcyclohexane. With copolymer rubbers, the number of characteristic products necessary for identification increases to three, viz., butadiene, vinylcyclohexene and styrene are used for butadiene -styrene rubbers (SKS-10, SKS-30, Buna S. Europrene-1500, Solprene) and butadiene, vinylcyclohexene and methylstyrene are used for butadiene-methylstyrene rubbers (SKMS-10, SKMS-30) [139, 140]. Fig. 3.12 [139, 140] shows as an example pyrograms of individual general-purpose rubbers and a four-component mixture of rubbers. The shaded peaks correspond to those components in the pyrolysis products which are used for identification. The ratio of the pyrolysis products changes depending on the composition of the copolymer and the structure of the polymer. 

Chain-growth polymerization exhibits a preference for head-to-tail addition. Branching affects the physical properties of the polymer because linear unbranched chains can pack together more closely than branched chains can. The substituents are on the same side of the carbon chain in an isotactic polymer, alternate on both sides of the chain in a syndiotactic polymer, and are randomly oriented in an atactic polymer. The structure of a polymer can be controlled with Ziegler-Natta catalysts. Natural rubber is a polymer of 2-methyl-l,3-butadiene. Synthetic rubbers have been made by polymerizing dienes other than isoprene. Heating mbber with sulfur to cross-link the chains is called vulcanization. 

The model compound, frans-4-chloro-4-octene, was chosen because it possessed the —C1C=CH— group of polychloroprene, but without the repeating 1,5-diene structure of the polymer. The autoxidation was similar to that observed for polychloroprene, although no induction period was observed (cf. hexachlorobutadiene oxidation above). The evolution of HC1 was proportional to the square of the time, but the oxidation kinetics were approximated more closely by the equation... 

Characteristics of Diene IV)lymer8. The physical and chemical properties of polymers are the resultant contributions of the fine structure and the gross properties of the polymer. Fine structure features of diene polymers are the ratio of the cts, tram, and vinyl configurations of the butadiene... 

Dienes. The structural analysis of diene polymers is complicated by the possible occurrence of (a) cis- or trans-, A addition, (b) 1,2- or 3,4-addition, and (c) head-head and tail-tail addition, but again n.m.r., C in particular, provides a powerful approach. Studies reported are of polymers of butadiene (type a structures ), penta-1,3-diene (a and a-i-b ), hexa-2,4-diene, isoprene (a-l-b and c ), 2,3-dlmethylbuta-l,3-diene (a ), chloropene (a+b, ... 

In light of the surprising cis substitution proposed for some of the dienyl iron tricarbonyl residues in the polymer, we wanted to corroborate our spectral evidence for the structure of the polymer by some additional analytical technique. In 1963> Mahler and Pettit -effectively demonstrated that the pentadienyliron tricarbonyl cation, which displays characteristic carbonyl bands at 2120 cm" and 2050 cm" can only be generated from cis diene iron tricarbonyl precursors. 

J.R. Ebdon, The characterization of diene polymers by high resolution proton magnetic resonance, in K. J. Ivin (Ed.), Structural Studies of Macromolecules by Spectroscopic Methods, John Wiley ns, London, 1976, p. 241. 

The first stereoregular 1,4-polybutadiene was obtained by Morton and his associates by using an Alfin catalyst in 1947 (34) and had a predominantly trans structure. The structure of this polymer was determined by careful X-ray and IR-excimination (61). A few years later the first synthesis of 1,4-cis-polyisoprene was accomplished (62) by using a Li metal dispersion. In the same period, Ziegler-type catalysts were reported to polymerize stereospecifically to 1,4-trans- or 1,4-cis-polymers a number of conjugated dienes. More recently, new types of catalysts for the stereospecific polymerization of dienes were prepared starting with TT-allyl derivatives of transition metals. Some catalysts for the stereospecific 1,4-polymerization of butadiene are indicated in Scheme 12. 

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