Diene polymers, structure determination

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... 

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). 

Another review [36] discusses the determination of composition and structure, including tacticity, branching and end groups for diene polymers with the help of IR spectroscopy. 

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. 

Use of acetal/ketal monomers with reactive polymerizable functionality allows easier access to families of related polyacetals. This facilitates determination of structure-activity correlations that influence many properties including hydrolysis half-life. Hydrolytic stability is often a function of whether the degradable unit is a ketal or acetal, along with other features of the polymer structure (e.g., hydrophilicity or neighboring group effects). The use of a polymerizable bis-diene Michael acceptor as an A-A mono-... 

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. 

Depending on the used catalyst one or both diene double bonds are coordinated. Coordination of one of two bonds only leads to the polymerization such as 1.2. The coordination of both bonds provides polymer with 1.4 structure. According to another theory the coordination of Ti-allyl structure takes place and the polymer structure is determined by the movement direction of the approaching monomer molecule. If the monomer molecule is approaching the center of an active catalyst from the CH -titanium bond side the formed polymer has the 1.4 structure type. When the monomer molecule is approaching fi-om the CH2-titanium bond then the pol5mierization is of 1.2 type. 

In terms of polymer structure, the ROMP of norborn-2-enes and norborna-dienes using ruthenium-based systems generally results in the formation of polymers that predominantly contain trons-vinylene units (vide infra). For a detailed discussion on the stereochemistry of ROMP-derived polymers and the determination of tacticity, the reader should refer to the chapter by J.G. Hamilton in Ref. [1], and the references cited therein. Polymerizations initiated by Grubbs-lype initiators are best terminated by the use of ethyl vinyl ether, yielding methylidene-terminated polymers according to the equation [25] ... 

The tendency toward cyclization decreases considerably (lower kjkp value) for unsym-metric 1,6-dienes, such as allyl methacrylate, where the two double bonds have significantly different reactivities. The polymer contains linear repeat units, rings, and pendant double bonds in relative amounts determined by kc/kp. The pendant double bonds eventually react to yield a crosslinked structure. Reactants with more than two double bonds per molecule,... 

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

Empirically determined chemical shift additivity parameters have been determined- for diene-type polymers. The shift contribution of a quaternary carbon which is fllto the carbon in question was not determined by those authors. However, using their additivity parameters and the shift positions of the ( carbons in Figure 8, a value of +15.4 ppm can be estimated for the contribution of a neighboring (0() quaternary carbon. Using this value, the shift positions of the carbons in structures VII and VIII are calculated as shown. If the first 1,2 unit were on the chain in a 2,1 manner, the methylene carbon resonance would be at a considerably higher field, but it would be difficult to estimate its position with any certainty because the quaternary effect... 

Polymers of 1,4-dienes are obtained in the presence of titanium, and also with Co, Ni, and Rh, where allyl complexes can be isolated. 1,2-Polybutadiene can be produced in the presence of Pd, which is not generally regarded as a Ziegler catalyst. Chromium and molybdenum systems have also been used. Whereas structural isomerism is controlled by the metal in the catalyst center, the geometric isomerism is determined by the ligands and counterions. 

The properties of polymers are strongly influenced by details of the chain structure. The structural parameters that determine properties of a polymer include the overall chemical composition and the sequence of monomer units in the case of copolymers, the stereochemistry or tac-ticity of the chain, and geometric isomerization in the case of diene-type polymers. 

Subsequently, several studies over the years have been carried out to determine whether 1,6-dlene systems undergo cycliza-tlon in an Intramolecular mode by forming a 5-membered ring or 6-membered ring (9). Since polymer systems are structurally difficult to evaluate some very interesting studies on monomer substrates were carried out by Brace (10), Julia (11), Solomon (12), Lancaster (13), and Ottenbrite (.9). These studies have shown radical addition to a 1,6-diene system occurs readily in an intramolecular mode to preferentially form a 5-membered ring. There are several factors that influence this cyclization process (a) the relative conformational stability of a 6-member-... 

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. 

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