Organic electronics is fastly growing field of science and technology and semiconducting conjugated polymers are playing a significant role for the advancement of optoelectronic device applications. Photoluminescent and electroluminescent conjugated polymers made scientific and technological profound growth a wide range of light-emitting applications. Polyfluorenes is one of the most important class of luminescent materials with bright blue emission, thermal and environment stability and high quantum efficiency and tunable properties though modifications and copolymerisation with a great potential for various applications in electronics and photonics. The Thesis deals with the design, synthesis and study of novel class of functionalised polyfluorenes, namely with 4-substituted polyfluorenes and polyfluorenones. While most of previous researches on polyfluorenes were focused on their functionalisation at the C-9 carbon atom to change solubility, morphology and other properties or on copolymerisation of fluorenes with other building block to tune electronic properties of materials, in this work we systematically study novel polyfluorenes (and polyfluorenones) functionalised at position 4 of the benzene ring of the fluorene moiety. Such functionalisation allowed directly effect on electronic energy levels of polymers tuning their characteristics. Two families of polyfluorenes and polyfluorenones have been synthesised and the effects of such substitution with electron-donating and electron-withdrawing groups have been studied by electrochemical and photophysical methods demonstrating a potential of novel materials in further development in this field. The thesis consists of six chapters. Chapters 1 and 2 give general introduction into the area of conjugated polymers and overview of the field of polyfluorenes, including their synthesis and the most important properties (related to this work), ending with the formulation of the aims and objectives of the Thesis. Chapter 3 describes our studies toward functionalisation of fluorene in position 4 with various groups to access the monomers for 4-functionalised polymers. Two main approaches for such functionalisation have been developed that allowed to synthesise 4-functionalised fluorene monomers, i.e 2,7-dibromo-4-X-9,9-R,R-fluorenes with a number number of electron-donating and electron-withdrawing groups: X = OCH3, SCH3, SC4H9, C≡C-C6H5, F, CN, CO2C4H9, SO2CH3, SO2C4H9 (with solubilising groups R = n-octyl or 4-octyloxyphenyl). Chapter 4 describes synthesis and characterisation of polyfluorenes X-PF8 and XPF6/8 obtained from these monomers by Ni(0)-mediated Yamamoto polymerisation. The obtained polymers showed high molecular weights (Mw = 24,700–185,000 Da, Mn = 9,700–78,700 Da) and excellent thermal stability (Td ~ 400 oC). They absorb at λabs = 379–411 nm and demonstrate bright emission in a blue region with finely resolved vibronic structure of their photoluminescence spectra (λPL = 415–435 nm for the first vibronic peaks). The polymers show very high photoluminescence quantum yields in both the solution (ΦPL = 63–93%) and the solid state ((ΦPL = 12–41%). Both series of polymers (X-PF8 and X-PF6/8)showed quasi-reversibe or irreversible p- and n-doping in cyclic voltammetry experiments, from which the energies of their HOMO and LUMO levels and the band gaps have been estimated. It has been shown that EDG and EWG substituents in position 4 of the fluorene moiety can efficiently tune the HOMO and LUMO energy levels of the X-PF8 and X-PF6/8 polymers by over 0.5 eV, whereas their band gaps remain almost the same as in the unsubstituted polyfluorene. The experimental data have been supported by DFT calculations. The formation of β-phase in the X-PF8 polymers have been studied and it has been demonstrated pronounces effect of groups X on its stability. Chapter 5 describes an alternative approach to 4-functionalised polyfluorenes by direct chlorosulfonation reaction of the fluorene moiety and further modifications at the sulfone site to access a wider range of possible substituted polyfluorenes. Two novel polymers XSO2-PF6 (X = PhO- or Et2N-) have been synthesised and characterised by electrochemical and optical methods, expanding the range of accessible 4-substituted polyfluorenes. Chapter 6 deals with 4-substituted polyfluorenones as electron-deficient polymers for electron transporting materials. Basing on the chemistry developed in Chapter 3, here we have elaborated advanced methods for the synthesis of several novel soluble 4-substituted polyfluorenones (X-PFon) and have demonstrated that they possess high electron affinity with reversible electrochemical n-doping and an emission in an orange-red region. Overall, this research has developed a new class of polyfluorenes and polyfluorenones functionalised at position 4 of the fluorene moiety for efficient tuning of the electronic properties of materials. Considering the huge number of fluorene-based homopolymers and copolymers synthesised and studied to date and their wide range of applications (organic light emitting diodes, solar cell field-effect transistors, lasing materials, sensors etc), described approach is expected to have large impact on further progress in the field, developing and application of novel materials based on 4-functionalised fluorenes in organic electronics