Polymers with Stiff, Bulky Substituents

Some of the physical characteristics of the synthesized PPOBr are shown in Table 1. The data in Table 1 indicate that there exists a direct correlation between the degree of bromination of PPO and their physical properties. The densities of PPO increase with an increase in the degree of bromination. The high density of the brominated PPO is attributed to the heavy bromine substituents. The Tg of the brominated PPO was higher than that of unmodified PPO and increase with an increase in the degree of bromination. This is probably due to the reduced torsional motion of the repeat unit caused by the bulky bromine atom. Similar trends have been reported for brominated low molecular weight (0.49 dL/g) PPO [6, 16]. Since polymer with stiff backbones often demonstrate high Tg, one may conclude that bromination of PPO increases polymer chain stiffness. 

The exponent a in the intrinsic viscosity-molecular weight relationship ([rj] = K.M ) of a polymer is associated with the expansion of the polymer in solution, and hence with the conformation and stiffness of the polymer (Table 24). The a values of tobacco mosaic virus, Kevlar and helical poly(a-amino acids) are close to 2, which means that they take rigid-rod structures. The a values of vinyl polymers are usually 0.5-0.8, indicating randomly coiled structures. In contrast, the a values of substituted polyacetylenes are all about unity. This result indicates that these polymers are taking more expanded conformations than do vinyl polymers. This is atrributed to their polymer-chain stiffness stemming from both the alternating double bonds and the presence of bulky substituents. 

Figure 2. Modification concepts for para-linked aromatic polymers including typical monomer structures (a) monomer units of different length, (b) kinked comonomers, (c) double kinked comonomers, (d) crankshaft comonomers, (e) flexible lateral substituents, (f) bulky and stiff lateral substituents, and (g) monomers with non-coplanar conformation.

It is rather amazing that 1-adamantylacetylene, an acetylene sterically even more crowded than ferf-butylacetylene, also polymerizes in high yield20. Poly(l-adamantylacetylene) is insoluble probably because the bulky rigid substituent makes the main chain very stiff. Its copolymerization with /ert-butylacetylene produces a soluble copolymer. 3,3-Dimethyl-1-pentyne and 3,3-dimethyl-1-nonyne produce polymers as well20). 

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