The dynamic mechanical properties of poly(acrylonitrile)s (PANs) with NMR triad isotacticities ranging from 0.25 to 0.68 and drawn to a draw ratio (DR) of 1-60 were measured at 0.5-110 Hz in the temperature range of -150 to 200 °C to study the effects of stereoregularity and DR on the relaxation behavior in PAN. Four kinds of relaxations were observed: R and R c relaxations at around 150 °C, a βc relaxation at around 100 °C, and a γ one at around 25 °C were observed as tan δ peaks measured at 3.5 Hz depending on the stereoregularity and DR. The magnitudes of the loss modulus E′′ and tan δ peaks were sensitive to the isotacticity, DR, and thermal history of the samples. These results confirm the previous assignments of the R and βc relaxations in at-PAN to the molecular motions in amorphous and paracrystalline phases, respectively, and they are also applicable to both iso-and at-PANs. Furthermore, the observed effects of the stereoregularity and DR on the relaxation behavior, crystallinity, and chain conformations studied by wide-angle X-ray diffraction suggested more details about the origins of these relaxations. The β c and γ relaxations are predominantly ascribed to the segmental motions of helical sequences and planar zigzag sequences, respectively, both in paracrystalline phases. The lower tail of the loss peak due to the γ relaxation is likely ascribed to the local mode motions of conformationally disordered sequences. It is noted, however, that an ultradrawn at-PAN fiber with a DR of 60 exhibited a sharp tan δ peak at around 150 °C (R c relaxation) that was associated with a sudden decrease in the storage modulus E′ and a DSC endothermic peak. Therefore, both the molecular motion in amorphous regions of PANs (R relaxation) and that associated with the first-order thermal transition in paracrystalline phases of an ultradrawn at-PAN fiber (Rc relaxation) contribute to the relaxation at around 150 °C.

Uniaxial drawing of isotactic poly(acrylonitrile) (iso-PAN, isotactic triad fraction of 68%) and the resultant structure and tensile properties of drawn products were studied. The results were compared to those of atactic-PAN (at-PAN). Dried gel films prepared from 2 to 10 wt % solutions in N,N ‘-dimethylformamide were initially drawn by solid-state coextrusion (first-stage draw) to an extrusion draw ratio of 16, followed by a further tensile draw at 100−200 °C (second-stage draw). The ductility of iso-PAN increased rapidly above 100 °C, due to the onset of molecular motion in crystalline regions, as found by WAXD at elevated temperatures. In contrast, the ductility of at-PAN increased above the first-order crystal/crystal transition at around 150 °C. Thus, the temperature for optimum second-stage draw of iso-PAN, 130−140 °C, was significantly lower than that (160−180 °C) of at-PAN, reflecting their crystal softening temperatures. The maximum achieved total draw ratios (DRt), after the two-stage draw, were comparable for these PANs. The shapes of stress/strain curves for highly drawn products recorded at room temperature were significantly different between iso- and at-PAN. The meridional WAXD patterns of these samples revealed that the difference is ascribed to their chain conformations which change with the applied tensile stress. The iso-PAN likely takes a predominantly 3/1 helical chain conformation, whereas at-PAN seems to consist of both planar zigzag and helical sequences, as previously suggested. However, upon increasing the tensile stress on oriented fibers, the helical sequences progressively transform into a planar zigzag conformation which shows a higher modulus. Such an effect of the stress was more prominent in at-PAN than in iso-PAN fibers. Thus, the maximum achieved tensile modulus, as well as the modulus at a given DRt, was slightly higher for iso-PAN than for at-PAN (28.5 ± 1.0 vs 23.0 ± 1.0 GPa). However, the maximum tensile strength at the break was comparable for each PAN, at 0.90 ± 0.05 GPa.

ABSTRACT:An NMR study was conducted on the mechanisms of side reactions in anionic polymerization of acrylonitrile (AN) with di-n-hexylmagnesium as an initiator in mixed-xylene at 25°C. In the initiation step the metalation of AN monomer leads to deactivation of the initiator. In the propagation stage both chain transfer to an a-proton in propagating chain and metalation of monomer proceed in parallel with addition reaction of the AN monomer to the propagating chain. The former two reactions result in I) formation of branching structures and 2) deactivation of active site, respectively. Some new NMR peaks resulting from above side reactions, which did not appear in 13 C NMR spectra of radically prepared PAN, were assigned. According to the assignments in this work, the number-average molecular weight, M. (330---5600) and branching density, which means the number of branching per 100 repeating units, were estimated for acetone-soluble or acetone-insoluble products.KEY WORDS Anionic Polymerization / Acrylonitrile / Side Reaction / NMR / Two-Dimensional NMR / Metalation / Intramolecular Chain Transfer / Branching Structure / Acrylonitrile (AN) can be polymerized to give polyacrylonitrile (PAN) not only by radical initiators but also by anionic initiators. 1 In general, the advantages of anionic polymerization are easiness of controling the molecular weight distribution by means of the "living" nature of its propagating chain and route that they provide for the syntheses of new functional polymers such as block copolymer and star-like polymer. Anionic polymerization also affords a possible route for the production of stereoregular polymers by nature of counter ion structure forming in its propagating end group.In the case of polymerization of monomer with polar group such as AN, side reactions based on the attack of propagating carbanion on the polar group result in some loss of the "living" nature of the system. 2 • 3 In fact, the "living" nature has never been found in the * To whom all correspondences should be addressed.anionic polymerization of AN because of the direct attack of the propagating chain on the cyano group 4 or an attack against electrophilic a-proton activated by the cyano group. 5 In order to obtain high molecular weight PAN (ca. 5 x 10 5 ) with high yield, it is necessary to polymerize under strictly limited conditions, for example, at very low temperature. 5 This may be a major reason why the anionic polymerization of AN has not been utilized commercially, though only short time is needed for completion of the polymerization owing to low activation energy.As mentioned above, side reactions cannot be ignored in anionic polymerization of AN. Many authors have proposed mechanisms for the polymerization 4 -8 : Kawabata and Tsuruta6 indicated that the metalation reaction of monomer occurs at an early stage with butyllithium as an initiator. Ottolenghi and 245

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An attempt was made to synthesize a catalyst having high activity for synthesis of highly isotactic polyacrylonitrile (PAN) with high molecular weight by anionic polymerization. The reaction products of di-n-hexylmagnesium [(n-C,H, 3)2Mg] with alicyclic alcohols such as 1,4-cyclohexanediol or 1,5-decalindiol were found to be effective as polymerization catalysts for preparation of PAN samples whose triad isotacticity [the content of mm (m, mEso configuration)] is about 0.64 and viscosity-average molecular weight (M,) exceeds 1 x lo5. The triad tacticities [mm, mr, and rr contents (r, racemo configuration)] of the polymers obtained were analysed by 13C NMR and their A?, s were determined by viscometry in dimethylsulphoxide at 25°C using Kamide et al.'s Mark-Houwink-Sakurada equation.A?, of PAN increased significantly by after-treatment of the catalysts, prepared from (n-C,H,,),Mg and alicyclic alcohols, with triethylaluminium (Et,Al). Ultrahigh molecular weight PAN samples ranging in A?, from 2 x 10, to 6 x 10, [(mm) = 0.57-0.663 were obtained in anisole at 135°C by using the above catalyst system. A main role of Et,A1 in these catalyst systems is considered to reduce, in advance, OH groups on the catalyst, resulting in a significant suppression of termination reactions of propagating PAN chains during the polymerization. K e y words: stereospecific polymerization, polyacrylonitrile, tacticity, anionic polymerization, dialkylmagnesium, polyhydric alcohol, ultra-high molecular weight. Recently, Kamide and coworkers' succeeded in synthe-merization temperature (T,) range above 100°C. In this sizing isotactic polyacrylonitrile (PAN) with triad iso-stereospecific polymerization of acrylonitrile (AN), however, O n o et ~2 1 .~7~ suggested that the termination * To whom correspondence should be addressed. reaction, chain transfer reaction, and deactivation of ini-Polymer International 0959-8 103/95/$09.OO 0 1995 SCI. Printed in Great Britain 87