Polymer structure dyad units

In this context a particularly interesting and unique example of the alternating copolymerisation of enantiomers was demonstrate in the polymerisation of 1-methylnorbornene with the ReCl j catalyst [63,75]. The analysis relied on the fact that the hydrogenated forms of these polymers (but see more recent work, p. 52), unlike their unsaturated precursors, exhibited fine structure due to the presence of ring dyad units of different tacticities. 

Measurement of polymer structure using dyad units... 

It should be stressed that this treatment of polymer stereochemistry only deals with relative configurations whether a substituent is "up or down" with respect to that on a neighboring unit. Therefore, the smallest structural unit which contains stereochemical information is the dyad. There are two types of dyad meso (m), where the two chiral centers have like configuration, and racemic /-), where the centers have opposite configuration (Figure 4.1). 

A preferable system is poly(p-fluorostyrene) doped into poly(styrene). Since rotations about the 1,4 phenyl axis do not alter the position of the fluorine, the F spin may be regarded as being at the end of a long "bond" to the backbone carbon. In standard RIS theory, this polymer would be treated with dyad statistical weights to automatically take into account conformations of the vinyl monomer unit which are excluded on steric grounds. We have found it more convenient to retain the monad statistical weight structure employed for the poly(methylene) calculations. The calculations reproduce the experimental unperturbed dimensions quite well when a reasonable set of hard sphere exclusion distances is employed. 

Signals are split into two closely located singlets and comparison of the C-NMR spectrum of the racemic polymer, obtained from the racemic monomer, with that of the optically active polymer (prepared from (+)-( R, 5S)-6,8-dioxabicyclo[3,2,l]oc-tane indicate, that the lower field, smaller peak of each signal pair comes from the D-L (syndiotactic) dyad. The hi er field signal can thus be ascribed to the dyad structures of D-D and L-L consecutive units (isotactic dyads). 

Several different structural factors influence the properties of the mesophase in these polymers, including dipolar effects, the planarity and rigidity of the mesogenic unit, and its length-to-width ratio among others. These factors are difficult to quantify, either absolutely or relatively, but some idea of their influences can be obtained by comparing the properties of polymers with different mesogenic units when combined with the same flexible spacer. This comparison has already been made for the dyad and triad esters in Table 2, and in this section it will be extended to other types of liquid crystalline polymers which contain a common decamethylene spacer. 

If the vinyl monomer produces a stereogenic center upon incorporation into the polymer backbone, then additional structural permutations are possible. In the simplest case, we can consider the relative stereochemistry of adjoining monomer units in a polymer dyad. If these centers produce a mirror plane or a C2 axis of symmetry, then the dyad is said to have meso (m) or racemic (r) relative stereochemistry, respectively, as shown in Figure 1. The relative stereochemistries of higher n-ad sequences are defined by the stereochemistries of the component dyad sequences as shown for the triad components of isotactic (nun), syndiotactic (rr), and heterotactic (mr) polymers. 

The extensive structural characterization of iPP produced by ZN catalysts has established unequivocally that the polymer backbone arises from a predominant head-to-tail propagation of monomer units, with occasional head-to-head dyad regioerrors being incorporated. In terms of the Cossee-Arlman mechanism, these structural data can be accounted for by assuming a preferred primary 1,2-regioseIective mode of migratory insertion, with secondary 2,1-regioisomeric... 

In the case of polymers derived from unsymmetrically substituted norbornene derivatives, such as 1- and 5-alkylnorbornenes, the possibility of regioisomerism exists.The substituents of neighboring units may be oriented in the same direction or in opposite directions, giving rise to head-tail (tail-head), head-head, and tail-tail structures (Figure 20.4). A head-tail dyad is stmcturally identical with a tail-head dyad. A particular carbon atom, which is part of a head unit that is neighbored by a tail unit, is labeled HT a carbon atom belonging to a tail unit neighbored by a head unit is labeled TH. 

EIGURE 20.7 Dyad structures of hydrogenated poly((-t-)-enrfo,exo-5,6-dimethylnorbornene) (a) and stereoblock polymer composed of repeating monomer (M) units linked in a racemic (r) and meso (m) fashion (b). 

Poly(7-MeNBD) prepared with 10b and 11a contains between 47% and 52% trans C=C units, which are atactic. It is reported that the cis sequences in these samples comprise more r than m dyads. A model has been proposed suggesting that the change in polymer tacticity from syndiotactic to atactic is due to switching from a mode of coordination of the previously formed C=C double bond (similar to structure E in Scheme 20.15) to a mode of noncoordination. 

This chain-end control mechanism was found to be ineffective in the polymerizations of ent(o,exo-5,6-Me2NB, tra i-2,3-(C02Me)2NB, and trans-2,3-(MeOCIl2)2NB catalyzed by 16a,b. The resulting polymers are highly trans but atactic. In the case of 7-MeNBD, Mo compounds 16a,b produce polymers that contain 68% cis-vinylene structures. It is reported that the cis-enchained units are highly syndiotactic and that dyads containing trans C=C structures are predominantly isotactic (82% m). This result is markedly different from the stereochemistry results obtained for... 

As Randall emphasizes in his book, there are close similarities between the determination of vinyl polymer tacticity and of copolymer sequence structure. It is convenient therefore to use the n-ad notation (dyad, triad, tetrad, etc.) originally applied to the former to describe the latter also. When there is any possibility of ambiguity, the term compositional -ad is used to refer to sequences of n units differing in chemical structure in a copolymer (AA, AB, BB, etc.), and the term configurational -ad is used to refer to stereochemically distinct sequences, following Johnsen et al. ... 

Siiailarly, poly(2,3-dimethy 1-1,3-butadiene) prepared with a butyl lithium/hexane system was found to contain predominantly 1,4-cis and 1,4-trans units, arranged rcUidomly [l00,10ll. The methylene carbon and proton resonances were used to obtain dyad distributions in these studies, and the methyl proton resonances provided a measure of triad distributions. Although polymers prepared from 2,3-dimethyl-1,3-butadiene had about 7% 1,2-structure, these polymers were also fovind to have Bernoullian structures. 

Suzuki and coworkers [102] investigated the structures of poly(1-phenylbutadienes) prepared by anionic or coordinated polymerization. The polymers contained predominantly 1,4-enchainments of the 1-phenylbutadiene inits and dyad distributions were measvirable from the olefinic carbon resonances. A predominantly trans-1,4-polymer was isomerized to yield a l,4-cis/l,4-trans copolymer in which the cis and trans units were distributed randomly along the polymer chain. The polymers obtained by anionic polymerization also had random structures. 

Several procedures are available for preparing polybutadienes that contain equal amounts of 1,4-cis and 1,2- units [104,105]. The structures of these equibinary" polymers have been investigated extensively by cmr spectroscopy [21,22,106,107]. The olefinic carbons of the 1,4-cis units are particularly useful for determining cis unit centered triad distributions, and some of the aliphatic carbon resonances have provided measures of dyad distributions. The... 

Polyisoprene prepared with a Ti(OR)4 -Et3Al catalyst system contains 3,4- and 1,4-cis-isoprene units almost exclusively and these are enchained mostly in a head-tail fashion [13]. The 3,4-3,4 dyads have a predominantly meso configuration. The cmr spectrum of this polymer has been interpreted in terms of a structure which obeys first order Markoffian statistics [108]. This is an interesting result because the structures of all other polymers prepared from dienes seem to have Bernoullian structiires, even when 1,4-cis, 1,4-trans, 1,2- or 3,4- units are present simultaneously [11,13,23]. 

The number of monomer units constituting a polymer chain is called the degree of polymerization (DP) it is directly proportional to the molar mass of the polymer. An assembly of a small number of monomer units within a macromolecular chain is called sequence and the first terms of the series of sequences are referred to as dyad, triad, tetrad, pentad, and so on. Chains made up of a small number of monomer units are called oligomers typically, the degrees of polymerization of oligomers vary from 2 to a few tens. Synthetic polymers are obtained by reactions known as polymerization reactions, which transform simple molecules called monomer molecules (or monomers) into a covalent assembly of monomer units or polymer. When a polymer is obtained from the polymerization of different monomer molecules (indicated in this case by comonomers) exhibiting different molecular structure, it is called a copolymer. 

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