The polysiloxane of greatest commercial importance and scientific interest is poly(dimethylsiloxane) (PDMS), [Si(CH3)2 –O –]x, a member of the symmetrical dialkyl polysiloxanes, with repeat unit [SiR2 –O –]x. This polymer is discussed extensively in the following chapters, particularly in chapter 5. Other members of this series are poly(diethylsiloxane) [Si(C2H5) –O–]x, and poly(di-n-propylsiloxane) [SiC3H7)2–O–]x. An example of an aryl member of the symmetrically substituted series is poly(diphenylsiloxane), with repeat unit [Si(C6H5)2–O–]x. This polymer is unusual because of its very high melting point and the mesophase it exhibits. The closely related polymer, poly(phenyl/tolylsiloxane), has also been prepared and studied. The unsymmetrically substituted polysiloxanes have the repeat unit [SiRR’O–]x, and are exemplified by poly(methylphenylsiloxane) [Si(CH3) (C6H5) –O–]xand poly(methylhydrosiloxane) [Si(CH3)(H) –O–]x. In some cases, one of the side chains has been unusually long, for example C6H13, C16H33, and C18H37, including a branched side chain—CH(CH3– (CH2)m–CH3. Another example has methoxy-substituted aromatic fragments as one of the two side chains in the repeat unit. Such chains have stereochemical variability in analogy with the vinyl polymers such as polypropylene [CH(CH3) –CH2–]xand vinylidene polymers such as poly(methyl methacrylate) [C(CH3)(C = OOCH3) –CH2–]xOne can also introduce optically active groups as side chains, the simplest example being the secondary butyl group—CH(CH3)(C2H5). Another example involves redox-active dendritic wedges containing ferrocenyl and carbonylchromium moieties. Other substituents have included phenylethenyl groups, cyclic siloxane groups, and Cr-bound carbazole chromophores. In a reversal of roles, some polymers were prepared to have PDMS side chains on a poly(phenylacetylene) main chain. Siloxane-terminated solubilizing side chains are used to improve the properties of thin-film transistors. Silalkylene polymers have methylene groups replacing the oxygen atoms in the backbone. Poly(dimethylsilmethylene) is an example, [Si(CH3)2–CH2]x. A variation on this theme is to include aryl groups, for example, in poly(dimethyldiphenylsilylenemethylene) [Si(CH3)2CH2Si(C6H5)2]x. Other aryl substituents, specifically tolyl groups, have also been included as side chains. It is also possible to insert a silphenylene group [Si(CH3)2–C6H4–] into the backbone of the polysiloxane repeat unit to give [Si(CH3)2–C6H4– Si(CH3)2O–], in which the phenylene can be para or ortho or meta. A specific example is poly(tetramethyl-p-silphenylene-siloxane).
Variation of the Side Chain Branch Position Leads to Vastly Improved Molecular Weight and OPV Performance in 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene/2,1,3-benzothiadiazole Copolymers