Isotactic polystyrenes

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

In the crystalline region isotactic polystyrene molecules take a helical form with three monomer residues per turn and an identity period of 6.65 A. One hundred percent crystalline polymer has a density of 1.12 compared with 1.05 for amorphous polymer and is also translucent. The melting point of the polymer is as high as 230°C. Below the glass transition temperature of 97°C the polymer is rather brittle. 

The brittleness of isotactic polystyrenes has hindered their commercial development. Quoted Izod impact strengths are only 20% that of conventional amorphous polymer. Impact strength double that of the amorphous material has, however, been claimed when isotactic polymer is blended with a synthetic rubber or a polyolefin. 

Figure 8.1. (a) Spherulites growing in a thin film of isotactic polystyrene, seen by optical microscopy with crossed polars (from Bassett 1981, after Keith 196.3). (b) A common sequence of forms leading to sphertililic growth (after Bassett 1981). The fibres consist of zigzag polymer chains. 

A polymorphic behavior involving packing of chains having completely different conformations has been found also for isotactic polymers. For instance, isotactic polystyrene, under suitable experimental conditions, can produce crystalline gels in which the chains assume a nearly fully extended conformation [11,12], very close to a truns-planar, rather than the classical conformation of three-fold helix [13]. The two possible conformations proposed for the two crystalline forms of i-PS are shown in Fig. 2. 

Fig. 2 a, b. Side view and projection along the chain axis of models of isotactic polystyrene in the a) s(3/l) helical conformation b) nearly /raw-planar conformation, proposed for the crystalline gels [12]... 

A general method has been developed for the estimation of model parameters from experimental observations when the model relating the parameters and input variables to the output responses is a Monte Carlo simulation. The method provides point estimates as well as joint probability regions of the parameters. In comparison to methods based on analytical models, this approach can prove to be more flexible and gives the investigator a more quantitative insight into the effects of parameter values on the model. The parameter estimation technique has been applied to three examples in polymer science, all of which concern sequence distributions in polymer chains. The first is the estimation of binary reactivity ratios for the terminal or Mayo-Lewis copolymerization model from both composition and sequence distribution data. Next a procedure for discriminating between the penultimate and the terminal copolymerization models on the basis of sequence distribution data is described. Finally, the estimation of a parameter required to model the epimerization of isotactic polystyrene is discussed. 

Epimerization of Isotactic Polystyrene.This application considers modelling the epimerization of isotactic polystyrene by Monte... 

Figure 2.10 Maps of conformational energy of various isotactic polymers as function of backbone torsion angles 0i and 02 (a) Isotactic polystyrene, (b) polypropylene, (c) poly(l-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i020i02 [s(M/N) symmetry] has been assumed. Isoenergetic curves are reported every 10 (a,c,d) or 5 (b) kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero.

In the crystal structures of many other isotactic polymers, with chains in threefold or fourfold helical conformations, disorder in the up/down positioning of the chains is present. Typical examples are isotactic polystyrene,34,179 isotactic poly(l-butene),35 and isotactic poly(4-methyl-l-pentene).39,40,153,247... 

Isotactic polystyrenes (IPS), 10 180 23 365 Isotactic propylene polymers, 17 703, 704 Isotactic-syndiotactic stereoblock PP, 16 110... 

As polystyrene obtained by free radical polymerisation technique is atactic it is therefore non-crystalline. The isotactic polystyrene is obtained by the use of Ziegler-Natta catalysts and n-butyl lithium. Isotactic polystyrene is having a high crystalline Melting point of 250°C. It is transparent. It is more brittle than the atactic polymer. 

Table 9. LS results for fractions of isotactic polystyrene in chlorobenzene. M denotes apparent molecular weight obtained directly, 6 is the anisotropy parameter and M is the molecular weight obtained after correcting for anisotropy via the method of Utiyama124 ...

Among the uncommon stmctures of stereoregular polymers determined in recent times, is that of isotactic polystyrene first observed by Keller and coworkers in crystalline gels (185) and later studied by Corradini et al. (186). In this case too, a highly stretched helix [ (6/1)] is observed, with unit height h = 5.1 A and imit twist t = 60°. The repeating unit contains two independent monomer units with rotation angles close to 180°. 

G. Natta, P. Corradini, I. W. Bassi. Crystal structure of isotactic polystyrene ,... 

Beers KL, Douglas JE, Amis EJ, Kaiim A (2003) Combinatorial measurements of crystallization growth rate and morphology in thin films of isotactic polystyrene. Langmuir 19 3935-3940... 

Figure 1.4- Dependence of spheralitic growth rate (cm/s) on supercooling (AT) for isotactic polystyrene [27].

The crystallization of the acetone-insoluble polystyrene is completed by boiling for 2 h in freshly distilled butanone it is then allowed to stand overnight at room temperature and finally filtered and dried in vacuum at 60 °C. Yield of crystalline isotactic polystyrene 95-100% of the acetone insoluble portion.The crystalline melting range and the density see Sect. 2.3.4.1) are determined, as is also the limiting viscosity number in toluene at 20 °C. 

Meyerhoff and Cantow (118) compared the relationships between [ /] and MW for polystyrenes prepared in various ways they found that for given Mw isotactic polystyrenes produced with Ziegler catalysts had the highest [ij], followed by low-conversion free-radical polymers both high-conversion (80%) and anionic (Szwarc) polymers had lower [ij]. These differences were all attributed to differences in LCB, though in principle differences in tacticity such as those between Ziegler and free-radical or anionic polymers could produce differences in the coil size in solution and hence in [iy]. 

In particular, Table 2.1 shows for atactic polystyrenes (in ref. 68 also isotactic polystyrene has been investigated, yielding C = —10300 Br in bromoform) that the influence of molecular weight (measurements in bromo-benzene and on the bulk polymer) is negligible or rather small. 

Fig. 3. Variation of the dimensions of the a axis of isotactic polystyrene as a function of the content of copolymerized units of differently substituted styrenes. Abscissa molar fraction of CH,=CH—C6H4—R (from Natta, G., et al. J. Polymer Sci.

The formation of solid solutions appears from the X-ray diffraction spectrum for a system formed by a 1 1 (weight) mixture of isotactic polystyrene and of a styrene//>-methylstyrene copolymer (30 moles-% / -methylstyrene), obtained after melting followed by a proper annealing treatment (2). The solid solution is possible because the two different... 

Very efficient delocalization of singlet excitation between non-conjugated benzene rings has been observed in isotactic polystyrene,... 

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