Polyisobutylene, structure

Polyisobutylene has a similar chemical backbone to butyl rubber, but does not contain double carbon-carbon bonds (only terminal unsaturation). Many of its characteristics are similar to butyl rubber (ageing and chemical resistance, low water absorption, low permeability). The polymers of the isobutylene family have very little tendency to crystallize. Their strength is reached by cross-linking instead of crystallization. The amorphous structure of these polymers is responsible for their flexibility, permanent tack and resistance to shock. Because the glass transition temperature is low (about —60°C), flexibility is maintained even at temperatures well below ambient temperature. 

Thermogravimetric data indicate that the structure of a polymer affects stability in a neutral environment (HI). A polymer such as Teflon, with carbon-carbon bonds which are (by comparison) easily broken, and with strong carbon-fluorine bonds, is quite stable thermally. However, polyethylene, also with carbon-carbon bonds but containing carbon-hydrogen bonds which are broken relatively easily in comparison with the carbon-fluorine bond, is less stable than Teflon. In turn, polyethylene is more stable than polypropylene. This difference in stability is probably caused by tertiary carbon-hydrogen bonds in polypropylene. Polypropylene is more stable than polyisobutylene or polystyrene, which decompose principally by unzipping mechanism. 

In view of the great structural similarity between the propagating sites in the cationic polymerization of P-PIN and isobutylene and their respective polymers (4), and our considerable experience accumulated with the LC Pzn of isobutylene [1-3], efforts have been made to adapt LC Pzn conditions found to yield living polyisobutylenes for the polymerization of p-PIN. 

These experimental results show conclusively that the deformation factor occurring in the theoretical equation of state offers only a crude approximation to the form of the actual equilibrium stress-strain curve. The reasons behind the observed deviation are not known. It does appear, however, from observations on other rubberlike systems that the type of deviation observed is general. Similar deviations are indicated in TutyP rubber (essentially a cross-linked polyisobutylene) and even in polyamides having network structures and exhibiting rubberlike behavior at high temperatures (see Sec. 4b). 

When this procedure is applied to the data shown for polystyrene in Fig. 116 and to those for polyisobutylene shown previously in Fig. 38 of Chapter VII, the values obtained for t/ i(1 — /T) decrease as the molecular weight increases. The data for the latter system, for example, yield values for this quantity changing from 0.087 at AT-38,000 to 0.064 at ilf = 720,000. This is contrary to the initial definition of the thermodynamic parameters, according to which they should characterize the inherent segment-solvent interaction independent of the molecular structure as a whole. 

Experiments have been made in which uncross-linkable polymer rubbers have been added to a similar rubber that is subsequently cross-linked (199). As an example, polyisobutylene was added to butyl rubber before it was cross-linked. The polyisobutylene molecules were not attached to the network structure, so they could be extracted by a solvent. As expected, the polyisobutylene greatly increased the creep compliance over that of the pure butyl rubber. 

Polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate) show very poor crystallization tendencies. Loss of structural simplicity (compared to polyethylene) results in a marked decrease in the tendency toward crystallization. Fluorocarbon polymers such as poly(vinyl fluoride), poly(vinylidene fluoride), and polytetrafluoroethylene are exceptions. These polymers show considerable crystallinity since the small size of fluorine does not preclude packing into a crystal lattice. Crystallization is also aided by the high secondary attractive forces. High secondary attractive forces coupled with symmetry account for the presence of significant crystallinity in poly(vinylidene chloride). Symmetry alone without significant polarity, as in polyisobutylene, is insufficient for the development of crystallinity. (The effect of stereoregularity of polymer structure on crystallinity is postponed to Sec. 8-2a.)... 

This review concerns the synthesis and characterization of octa-arm polyisobutylene (PIB) stars, allyl-terminated octa-arm PIB stars, and octa-arm star blocks by using a novel octafunctional caHx[8]arene-based initiator 1. Scheme 1 shows the structure of 1 and the target architectures. The syntheses were carried out under living carbocationic polymerization conditions. 

The photo-cross-linkability of a polymer depends not only on its chemical structure, but also on its molecular weight and the ordering of the polymer segments. Vinyl polymers, such as PE, PP, polystyrene, polyacrylates, and PVC, predominantly cross-link, whereas vinylidene polymers (polyisobutylene, poly-2-methylstyrene, polymethacrylates, and poly vinylidene chloride) tend to degrade. Likewise, polymers formed from diene monomers and linear condensation products, such as polyesters and polyamides, cross-link easily, whereas cellulose and cellulose derivatives degrade easily. ... 

Polypropylene. A similar study on polypropylene is interesting because polypropylene has a molecular structure intermediate between polyethylene and polyisobutylene. An atactic polypropylene specimen was prepared by ether extraction and irradiated in a nitrous oxide atmosphere. The changes in gel fraction (insoluble in hot xylene) as a function of N-jO pressure are shown in Figure 6. Gel formation (cross-linking) of polypropylene is also promoted in the presence of nitrous oxide. 

The amplitude of shear velocity is a determinant of the degree of thixotropic destruction of a material s structure, independently of the frequency of vibration effect. The effect of reversible viscosity drop is also observed at ultrasonic frequencies. Vibrothixotropy of concentrated polyisobutylene (PIB) solution was sMudied earlier at a frequency of acoustic treatment of 18 kHz and an amplitude of up to 15 mcm74). The first difference of normal stresses determined by the degree of extrudate s swelling... 

These energy calculations can provide suitable and stable molecular models, and have been successfully utilized for the structure analyses of many other polymers, such as poly(tert-butylethylene oxide) (3 ) and polyisobutylene (35). 

Following quantitative methylation of the co-fert-chloro site by trimethyl aluminum, the methylated polyisobutylene methacrylate macromonomer was co-polymerized with MMA by Group Transfer Polymerization [87]. PMMA-g-PIB graft copolymers with controlled MW and composition were obtained. The structure and physical properties were determined by the [MMA]/[MA-PIB] and [MMA]/[Initiator] ratios. 

The mechanism of y-ray irradiation-induced scission of polyisobutylene was studied, based on the structural characterisation of end-groups by 13C-NMR as well as GC, GC/ MS, and SEC [77], The assignments of signals were made by comparison with those from model compounds and predictions based on empirical rules. Quantitative 13C-NMR measurements of chain-ends allowed the determination of radiation yield of products and of chain scission. 

Barrier Properties. Vinylidene chloride polymers are more impermeable to a wider variety of gases and liquids than other polymers. This is a consequence of the combination of high density and high crystallinity in the polymer. An increase in either tends to reduce permeability. A more subde factor maybe the symmetry of the polymer structure. It has been shown that both polyisobutylene and PVDC have unusually low permeabilities to water compared to their monosubstituted counterparts, polypropylene and PVC (88). The values listed in Table 8 include estimates for the completely amorphous polymers. The estimated value for highly crystalline PVDC was obtained by extrapolating data for copolymers. 

Stevens, J.R., Rows, R.M. (1973) Evidence for structure in polyisobutylene from positron lifetimes . J. Appl. Phys. 44,4328. 

A recent example of XPS analysis of polyurethane surfaces has been provided by Yoon et al. [30]. In these studies polyurethanes were prepared with 4.4 -methylenebisfphenyl isocyanate) (MDI)and hexafluoro- l.5-pentancdiol (FP) extenders as the hard segment, and poly(tetramethylene glycol) (PTMO) as the soft segment. The authors introduced hydrophobic soft segments, either poly-(dimethylsiloxan) (PDMS) or polyisobutylene (PIB) into this polyurethane structure. The concept underlying this polymer design was the desire to modify sur-... 

Figure 5.1. Molecular structures of the chemical repeat units for common polymers. Shown are (a) polyethylene (PE), (b) poly(vinyl chloride) (PVC), (c) polytetrafluoroethylene (PTFE), (d) polypropylene (PP), (e) polyisobutylene (PIB), (f) polybutadiene (PBD), (g) c/5-polyisoprene (natural rubber), (h) traw5-polychloroprene (Neoprene rubber), (i) polystyrene (PS), (j) poly(vinyl acetate) (PVAc), (k) poly(methyl methacrylate) (PMMA), ( ) polycaprolactam (polyamide - nylon 6), (m) nylon 6,6, (n) poly(ethylene teraphthalate), (o) poly(dimethyl siloxane) (PDMS).

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