Carboxylate-based metal organic frameworks (MOFs) coordinated by multiple-functional carboxylic acid linkers and various metals represent a large family of MOFs. Carboxylic acid functionalized polymers, such as poly(acrylic acid), polystyrene-b-(acrylic acid) and poly(methacrylic acid)-co-(ethylene glycol di-methacrylate), have been used in the synthesis of carboxylate-based MOFs in order to modulate, or provide a template for, the growth of MOF crystals. Although this strategy has been used to prepare MOFs, the role of the carboxylic acid functionalized (co)polymers in the synthesis of MOFs is still not very clear. In this article, we report the synthesis of UiO-66 (a MOF designed at the University of Oslo) in the presence of core cross-linked poly(methacrylic acid)-b-poly(methyl methacrylate) (PMAA-b-PMMA) nanoparticles (NPs), with two different modulators (acetic acid (HAc) and hydrochloric acid (HCl)) and in the absence of any modulator. Additionally, a series of reactions were carried out where the classical terephthalic acid (TA) linker was replaced with PMAA-b-PMMA NPs. These experiments elucidated the role of NPs in the polymer-assisted synthesis as well as how the PMAA-b-PMMA NPs become incorporated into the final UiO-polymer hybrid investigated material. In this polymer-assisted UiO-MOF synthesis, the PMAA-b-PMMA NPs act as a stabilizer, a template and a (co)modulator to form the final UiO-polymer hybrid NPs with different colloidal aspects, sizes and crystallinity.