Diad, configurational

Actually, the fixation of diad configuration in the polymer chain proceeds only when the next monomer molecule is attached. Until this moment, the end unit can still undergo isomerization becoming either planar or its mirror antipode. In the absence of isomerization, the formation of a stereoregular syndiotactic polymer should be expected. In the case when an end unit has the time (before the attack of the following monomer molecule) to adopt a planar configuration, the distribution of diads in the polymer is described by Bernoulli s statistics (binomial distribution) [42]. If the state to two end units of... 

The heterotactic chain (Figure 7) where the diad configuration alternates along the chain. 

Methyl a-chloro acrylate-methylmethacrylate Measurement of diad configurations and mean sequence length [95]... 

Such configurational as well as conformational effects have been also reported by MILLAN et al. in the case of nucleophilic substitution of poly(vinyl chloride) with sodium thiophenate (14) and with sodium isooctylthioglycolate or isooctylthiosalicylate (15). The authors have shown that these reactions proceed selectively on the isotactic TT diads which can only exist either in the GTTG isotactic or in the TTTG heterotactic triads, the former ones being much more reactive than the latter ones. 

Note Configurational sequences comprising two configurational units are termed diads, those with three such configurational units triads, and so on. In order of increasing sequence lengths they are called tetrads, pentads, hexads, heptads, octads, nonads, decads, undecads, etc. 

When it is necessary to specify the internal stereochemistry of the group, a prefix is required. In vinyl polymers there are meso (m) and racemic (r) diads and mm, mr, rr triads. The latter may be called isotactic, heterotactic and syndiotactic triads, respectively. Stereoregular vinyl polymers can be defined in terms of the regular sequences of diads thus an isotactic vinyl polymer consists entirely of m diads, i.e., it corresponds to the following succession of relative configuration -mmmmmm-, whereas a syndiotactic vinyl polymer consists entirely of r diads, corresponding to the sequence -rrrrrrr-. Similarly, a vinyl polymer consisting entirely of mr (= rm) triads is called a heterotactic polymer. 

Copolymerization of a monomer having two styrene moieties attached to a chiral template molecule with a comonomer e.g., methyl methacrylate) gives copolymers with strong optical activity after removal of the template molecules. In this case styrene diads of an S,S configuration separated from other styrene diads by comonomeric units are responsible for the optical activity. 

Stereoregularity. Representative stereoregularities of the two adjacent constitutional units in the homopolymer and co-polymers of terminal epoxides and symmetrical m-epoxides are listed in Figure 1. A diad is described as meso m) if the absolute configurations of the two adjacent constitutional units are the same, and racemo (r) if they are the... 

The influence of the chain expansion produced by excluded volume on the mean-square optical anisotropy is studied in six types of polymers (PE, PVC, PVB, PS, polylp-chlorostyrene), polylp-bromostyrenel. RIS models are used for the configuration statistics of the unperturbed chains. The mean-square optical anisotropy of PE is found to be insensitive to excluded volume. The mean-square optical anisotropy of the five other polymers, on the other hand, is sensitive to the imposition of the excluded volume if the stereochemical composition is exclusively racemic. Much smaller effects are seen in meso chains and in chains with Bernoullian statistics and an equal probability for meso and racemic diads. 

The proton NMR shifts of diad, triad, and tetrad protons in an isolated chain of PP are calculated, taking into account its configurations and conformations using Pople s approximation. The agreement between the observed and calculated results, however, is insufficient with respect to the large difference between syn and anti protons and the order of tetrad protons. Here, a priori probabilities of specified conformations necessary to the estimation of the chemical shifts are calculated using Flory s matrix method. 

The configurational-conformational characteristics of PP are discussed by considering every polymer chain as constituted by the periodic repetition of a sequence of monomeric units in a given configuration. Calculations are presented for the special case in which mesa and racemic diads are distributed according to Bemoullian statistics. Numerical results show that the characteristic ratio of atactic PP reaches an asymptotic value of 5.34 when the size of the periodic sequence corresponds to six monomeric units. 

The proton NMR spectrum of a highly isotactic sample of PP is examined. The polymer is shown to contain 2% racemic diads occurring randomly at junctions of isotactic sequences of opposite configurations. The mean-square end-to-end distance of this polymer is measured under 9 conditions. Comparison of the value obtained with theoretical predicitons of Flory, Mark, and Abe [J. Am. Chem. Soc. 1966, 88, 639] permit an approximate measurement of the strength of the steric interactions within the PP chain. 

In order to describe the stereosequence of such chains, meso or m diads with identical and racemic or r diads with opposite configuration of succeeding asymmetric carbon atoms are defined ... 

Stereocontrol of free radical polymerization is influenced by monomer constitution, solventy and temperature. Most polymerizations seem to follow at least a Markov first-order one-way mechanism. Ratios of the four possible rate constants ki/iy ki/8, k8/i, and k8/8 can be calculated from the experimentally accessible concentrations of configurational triads and diads. With increasing temperature, more heterotactic triads are formed at a syndiotactic radical whereas the monomer addition at an isotactic radical favors isotactic and not heterotactic triads. Compensation effects exist for the differences of activation enthalpies and activation entropies for each of the six possible combinations of modes of addition. The compensation temperature is independent of the mode of addition whereas the compensation enthalpies are not. 

Isoblock polypropylene is characterised by the appearance of stereosequences of m diads bridged by pairs of r diads [20,21], Such polypropylene, containing a small number of long blocks, is usually referred to as isotactic polypropylene. Note that the polymer chains in this case contain helix sequences of the same handedness (right or left) in the chain, which is due to the same relative configuration of tertiary carbon atoms in these sequences. Stereoblock polypropylene, which is also called block-isotactic polypropylene,... 

The 13C NMR analysis of polypropylene obtained with the Me2C(Cp)(Flu) ZrCh —[Al(Me)0]x catalyst showed that the polymer chains consist of long blocks of r diads separated by pairs of m diads (Figure 3.47b), which is indicative of polymerisation stereocontrol by the enantiomorphic catalyst site [23]. In this case, it is obvious that an occasional change in the configuration of the last inserted monomeric unit has no influence on the chirality and tends to remain isolated. 

BD/St-copolymers were also prepared by the use of the catalyst system Nd(OCOCCl3)3/TIBA/DEAC [503]. The copolymer exhibited 79% cis-1,4-structure in the BD units at a content of incorporated styrene of 23 mol%. In this study diades were also determined. According to the authors BD moieties which are adjacent to St moieties predominantly exhibit a transit-configuration whereas BD moieties in BD-BD-diades exhibit a cis-1,4-structure [503]. It therefore can be concluded that the microstructure of an entering BD monomer is controlled by a penultimate effect. This effect can be best described by a model in which backbiting coordination of a penultimate BD unit to Nd is involved [177,367]. 

Diads (triads, pentads) View of two (three, five) combined monomer units at any position in a polymer chain and investigating (e.g., by 13C NMR spectroscopy) the configuration of the tertiary carbon atoms. The term meso is used if they have the same configuration the term racemo is used if they have the opposite configuration. 

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