## 1 Introduction  
The preparation and characterization of porous micro- and nanopolymeric particles is a very attractive and longstanding research topic in view of the actual and potential applications of such particles. Well-established techniques, mainly based on emulsion and suspension polymerization, allow one to prepare in a controlled manner meso- and microporous particles with porosities higher than 1 cm\({}^{3}\)/g, surface areas in the hundreds of square meters and average diameters ranging from some tenths of micrometers to millimeters [1; 2; 3; 4; 5; 6]. Recently, this kind of structures has become interesting in the development of drug delivery systems [7; 8], catalysts [9; 10], enzyme and cell immobilization [11], among others. A common feature of these particles is that their sizes are typically higher than 100 nm in average diameter. Research on mesoporous polymeric nanoparticles with average diameters smaller than 50 nm has received little attention [12; 13; 14]. The interesting thing about this kind of nanoparticles is their enhanced area/volume ratio and the increased capacity for avoiding the immunological system, a feature especially useful for drug delivery systems due to the fact that it leads to a longer circulation period in the bloodstream [15]. The reports in the specialized literature on the preparation of such small nanoparticles include the work of [PERSON] and [PERSON] [12], who prepared mesoporous polymeric nanoparticles with average diameters smaller than 50 nm and pores of 2-6 nm in diameter by a sol-emulsion-gel method. More recently, [PERSON] _et al._[14] documented the preparation of porous hyperbranched conjugated polymeric nanoparticles with diameters ranging 20-60 nm by miniemulsion polymerization. On the other hand, our group has reported the preparation of mesoporous polymeric nanoparticles by polymerizing styrene in the presence of cross-linking and porogen agents using the so called semicontinuous heterophase polymerization technique [13]. The obtained nanoparticles showed average diameters close to 30 nm, pore diameters in the range 6-8 nm and porosities around 0.40 cm\({}^{3}\)/g. Semicontinuous heterophase polymerization is a technique developed by our group [16; 17; 18] that allows one to obtain polymeric nanoparticles with diameters smaller than 50 nm, solids contents up to 25%-30% and all the surfactant in the latex stabilizing the particles. However, this technique requires one to operate under the so-called _monomer-starved conditions_ during the addition period [19]. [PERSON] and [PERSON] [19] coined this term to explain the smaller particle sizes obtained in the emulsion polymerization of styrene carried out in semicontinuous mode compared to the batch process. These authors resorted to the correlation developed for emulsion polymerization by [PERSON] and [PERSON] [20] for predicting the number of particles (\(N_{\mathrm{P}}\)) for the case-II kinetics, in which \(N_{\mathrm{P}}\) is inversely proportional to the volume growth rate of polymer particles during the nucleation period. When the particles are saturated with monomer, they grow at their maximum rate. As a consequence the particle nucleation is minimum. In semicontinuous emulsion polymerization particle monomer saturation is attained by operating at the so-called _monomer-flooded conditions_. In contrast the operation under monomer-starved conditions, that is to say, when monomer in the particles is below the saturation concentration, slows down the particle growth rate resulting in a larger number of smaller particles. These conditions are achieved by adding the monomer at very slow dosing rates. As described above, semicontinuous heterophase polymerization operating under monomer starved conditions allows the preparation of mesoporous polymeric nanoparticles having average diameters smaller than 50 nm [13]. As far as we know, ours is the only report on the use of this technique forpreparing such particles.