Homopolymers, aliphatic-aromatic

The polyanhydrides synthesized by melt condensation have fiber-forming properties in the molten state. They hydrolyze when exposed to air and this degradation is mainly controlled by the composition of the polymer. Homopolymers of aromatic monomers, such as CPH, degrade at a rate that is several orders of magnitude lower than that of homopolymers of aliphatic monomers. 

The aliphatic-aromatic diacid monomers were prepared from the reaction of bromoalkanoic acid methyl ester and p-hydroxy benzoic acid methyl ester. The polymers of carboxyphenoxy alkanoic add of n = 3, 5, and 7 methylenes were soluble in chlorinated hydrocarbons and melted at temperatures below 100 °C. Copolymers of these monomers melted at lower temperatures than the respective homopolymers. These polymers displayed zero-order hydrolytic degradation profile ranging from 2 to 10 weeks. Increasing the length of the alkanoic chain, decreased the degradation rate of the polymer (Fig. 3). 

The polymer composition and the sequence of the comonomers can be determined by HNMR as demonstrated in the analysis of the aromatic-aliphatic homopolymers [52], The aliphatic-aromatic diacids can be connected in a polymer in three ways, aliphatic moiety to aliphatic moiety, aliphatic to aromatic, and aromatic to aromatic moieties. The HNMR spectra of the carboxyphenoxy valerate (CPV) diacetate prepolymer and polymer are shown in Fig. 7. The methylenic protons of the aliphatic residue conjugated to the anhydride bond in the polymer appeared as two triplets (e) at 2.54 ppm and 2.72 ppm chemical shifts. The aromatic protons ortho to the anhydride bond appeared in two chemical shifts (a), a doublet at 7.90 ppm and at 8.10 ppm. The peaks at 2.72 and 8.10 were not observed in the spectrum of the prepolymer (Fig. 7). These peaks were explained by a chemical shift effect across the anhydride bond, affecting the absorbancies of the a-protons to the anhydride bond. These peaks were attributed to the three types of anhydrides present in the polymen the 2.54 ppm signal corresponds to the aliphatic-aliphatic anhydride bond, 2.72 ppm to aliphatic-aromatic, and 8.10 ppm to aromatic-aromatic anhydride bonds. Examination of the integration of these peaks revealed a ratio of 1 2 1, aliphatic-aliphatic, aliphatic-aromatic, and aromatic-aromatic moieties. This ratio implies an equal statistical distribution of alternating aromatic-aromatic and aliphatic-aliphatic units throughout the polymer backbone [52]. 

The requirement, that all components of a plastic-composition must be degraded is of essential relevance for copolymers and especially for aliphatic-aromatic copolyesters. Here in one polymer chain structures are combined, which differ significantly in their degradation behaviour when the monomers are located in the corresponding homopolymers (aliphatic polyesters often are easily biodegraded while aromatic polyesters such as PET are quite biologically resistant). In this particular case it has to be ensured that no domains within the copolyester chains are poorly biodegradable and can accumulate in the nature. 

PVDF is a high-performance, high-molecular-weight homopolymer. It is resistant to most acids and bases, aliphatics, aromatics, alcohols, and chlorinated solvents. Strongly polar solvents, such as ketones and esters, cause partial solvation, especially at high temperatures. The methods for preparing the untreated film for adhesive bonding are similar to those of polyolefin and lluoropolymers (see Chapter 6). 

C and is easily processable, whereas the homopolymers do not melt before the onset of thermal degradation, at temperatures as high as 500°C.73,74 Varying copolymer composition permits the adjustment of melting temperature and of other properties (e.g., solubility) to desired values. This method is frequently used for aliphatic and aromatic-aliphatic polyesters as well. 

The general correlations of structure and properties of homopolymers are summarized in Table 2.13. Some experiments which demonstrate the influence of the molecular weight or the structure on selected properties of polymers are described in Examples 3-6 (degree of polymerization of polystyrene and solution viscosity), 3-15, 3-21, 3-31 (stereoregularity of polyisoprene resp. polystyrene), 4-7 and 5-11 (influence of crosslinking) or Sects. 4.1.1 and 4.1.2 (stiffness of the main chain of aliphatic and aromatic polyesters and polyamides). 

The standard method for synthesis of polysilanes follows the original preparation of (Me2Si) by Burkhard. Diorganodichlorosilanes are treated with finely dispersed sodium metal in an inert diluent, usually above the melting point of sodium. Homopolymers are obtained from single dichlorosilanes, while cocondensation of mixtures of dichlorosilanes yields copolymers (equation 33). Toluene is the most commonly used solvent, but other aliphatic or aromatic solvents are also effective. After completion of the reaction, the mixture is quenched with alcohol and/or water to destroy any excess sodium and silyl anion, then filtered and... 

Instead of block copolymers, the use of pseudo-random linear copolymers of an aliphatic a-olefin and a vinyl aromatic monomer has been reported [20], where the styrene content of the polymer must be higher than 40 wt%. Preferred are styrene and ethylene copolymers. These blends may contain, amongst other things, an elastomeric olefinic impact modifier such as homopolymers and copolymers of a-olefins. Presumably the styrene-ethylene copolymer acts as a polymer emulsifier for the olefinic impact modifier. Using 5 wt% of an ethylene-styrene (30 70) copolymer and 20% of an ethylene-octene impact modifier in sPS, a tensile elongation (ASTM D638) of 25 % was obtained. 

Typically, homopolymers are not studied because they possess unfavorable characteristics rendering their handling and manufacture difficult. Poly(SA), poly(CPP), and poly (FA) are semicrystalline and thus suffer from either being brittle or having high Tn,. Conversely, poly(FAD) is a liquid. Therefore, polyanhydrides are often prepared as copolymers of aliphatic and/or aromatic monomers. The most common copolymers under investigation in drug delivery applications include poly(FAD-SA) and poly(CPP-SA). 

The molecular and chemical composition of the polymer will influence its solubility characteristics. Park (A) has discussed the solvent-resin relationships in detail in "Advances in Chemistry Series 12A." They can be summarized as follows Aromatics such as toluene and xylene are primary solvents for only the most soluble of the vinyl resins. The homopolymers have very slight aromatic tolerances. Aliphatic-type solvents are not considered good solvents for vinyls. As with the aromatics, the extremely soluble resins will tolerate aliphatic solvents if a strong ketone is present. Only fair aliphatic tolerance is obtained with the low molecular weight high vinyl chloride content solution polymers. Aliphatic tolerance of the homopolymers is practically nil. The alcohol tolerance of vinyl resins is very limited. Recent studies with the high solubility type metal adhesion copolymers indicate that appreciable quantities of 2-propanol may be used, if a strong ketone solvent is used. 

Soluble Polystyrene. Styrene homopolymers (generally in admixture with other binders) are used in special applications such as zinc dust primers and bronze lacquers, in paints for mineral substrates and paper, as well as in adhesives. Polystyrene and poly(a-methylstyrene) have good water resistance, low permeability, resistance to chemicals and light, and also dry quickly. Suitable solvents include aliphatic and aromatic hydrocarbons, in some cases containing ketones or esters. 

---