Triads, configurational

Figure 2. Compositional tetrads (configurational triads) in chains of vinyl (Mj copolymers with a comonomer which places a single atom X in the main chain.

The measurement of polymer configuration was difficult and sometimes speculative until the early 1960 s when it was shown that proton NMR could be used, in several instances, to define clearly polymer stereochemical configuration. Bovey was able to identify the configurational structure of poly(methylmethacrylate) in terms of the configurational triads, mm, mr and rr, in a classic example (3). In the case of polypropylene, configurational information appeared available but was not unambiguously accessible because severe overlap complicated the identification of resonances from the mm, mr and rr triads (4). Several papers appeared on the subject of polypropylene tacticity but none totally resolved the problem (5). 

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. 

Table I. Configuration Triads of Three Successive Units...

G. BarbareUa, A. Bongini, and M. Zambianchi. Regiochemistry and Conformation of Poly(3-Hexylthiophene) Via the Synthesis and the Spectroscopic Characterization of the Model Configurational Triads. Macromolecules, 27(ll) 3039-3045, May 23 1994. 

Each configurational triad consists of two diads. The two diads may be isotactic-isotactic, isotactic-syndiotactic, or syndiotactic-syndiotactic. The mole fraction of what are known as heterotactic triads, + x i = x ,... 

Analogous reasoning gives for the three possible configurational triads... 

Each configurational triad consists of a pair of diads. The pair of diads can be isotactic-isotactic or syndiotactic-syndiotactic or the triad can consist of a syndiotactic and an isotactic diad. The sum of the mole fractions of these three kinds of triads must also be equal to unity. 

Appendix 2.A H, NMR chemical shifts (ppm CDCI ) of configurational triads in HT oligo- and poly(3-alkyl)thiophenes... 

G. Barbarella, A. Bongini and M. Zambianchi. Regiochemistry and conformation of poly(3-hexylthiophene) via the synthesis and the spectroscopic characterization of the model configurational triads. Macmmolecules 27(11), 3039-3045 (1994). 

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