Given the undeniable relevance of polyolefin production, it is of great interest to understand the keyphenomena determining the quality of the polymerization process. lndustrial scale polymerization reactions could be metaphorically compared to the launching of a space rocket, in which the success of the entire process is highly dependent on the mastery of the initial moments. This so-called 'nascent phase' of the polymerization (10-1 to 102 s) is crucial in ensuring a satisfactory operation and properties of the final polymer. lt is then that the catalyst activation takes place, the particle morphology is defined and the system is in its highest potential for mass transfer and heat transfer limitations.Studying such short time-frames is challenging for a number of reasons (that we will see throughout this work) and requires specially adapted tools. Previous works from our research group have shown how the stopped-flow technique is a promising method to study aspects related to heat transfer and morphology evolution at early stages in gas-phase. lt is mainly for this reason that the focus of the current project was on the improvement of such tools. The novel reactor developed in this work was especially designed to tackle some of the limitations encountered with the previous versions, mainly aspects related to poor heat evacuation, lack of robustness and imprecision in experimental measurements.The first part of this PhD describes all steps taken until successful optimization of the hardware component of the novel stopped-flow reactor. The new set-up was a product of our innovative approach towards the reactor geometry (annular) and the tailored execution from a specialized engineering firm. Several improvements were achieved, such as the optimization of the reactor dimensions, the automated control of all functions and a wider range of operation conditions. Moreover, the capacity was highly improved by the possibility of injecting several gases simultaneously, as well as components that are liquid at room temperature. The new tool allows to perform polymerization reactions as short as 3s in gas phase, in reaction conditions that are representative of heat and mass transfer phenomena present in industrial scales.ln the second part of this work, we have developed a software component for the novel reactor, which consisted of a reactor model and state-observer. This tool was developed to estimate the dynamic polymerization rates from the temperature measurements and overcome the one-point character of such experiments (which provide no real information on the catalyst kinetics). For this purpose, a simplified one-dimensional model at the macromolecular level was developed and validated with experimental data.At last, with hardware and software components at hand, we have demonstrated the usefulness of the new tool in specific case studies with different silica supported metallocene catalysts. ln the first case study, we evaluated the impact of the pore structure of the support on the catalyst kinetics, polymer properties and particle morphology at short reaction times. On the second case study, we investigated the effect of various experimental conditions (reaction time, comonomer and hydrogen content) on the behavior of two different catalyst families. From both studies, the estimated reaction rates at early stages (<90s) reflected those observed in standard conditions (60 min). Moreover, the microscopy observations indicated that the catalyst surface seems to be inactive and polymer is being produced from the particle core. At last, we demonstrated how the new reactor set-up is a flexible androbust tool that can be used in the rapid evaluation of catalyst performance.