Z-RAFT star polymerization of butyl acrylate using multifunctional trithiocarbonate-type RAFT agents carrying methyl propionate as the leaving group were used to form star polymers having 3, 4, and 6 arms. The polymerizations showed well controlled behavior up to high monomer conversions. By using a mixture of mono- and multifunctional RAFT agents, arm and star polymer were obtained simultaneously within a single RAFT polymerization, which enabled the evaluation of the apparent arm numbers. The desired topology of the star polymers was found being present already from the early phase of polymerization on. The apparent arm number slightly increases at very high monomer conversions in accordance to earlier reported star coupling reactions. Correction factors for conventionally calibrated SEC showed excellent agreement with data for polystyrene stars, indicating that the contraction of the hydrodynamic volume in comparison to linear polymer is only dependent on the polymer topology. The evaluation of absolute molar masses of star polymers was however limited by the accuracy of the conventional calibration.

The goal of the work presented was the synthesis of star polymers with a defined number of arms, narrow molecular weight distribution and high degree of polymerization from controlled radical polymerization of different monomers. In order to reach this goal 2, 3, 4, 6 and 8 arm addition fragmentation chain transfer (RAFT)-agents were synthesized. In all cases the core of the stars was the stabilizing group (Z-group approach). Poly styrene, poly methylacrylate, poly butylacrylate, poly dodecylacrylate, and poly vinylacetate star polymers were produced and analyzed. For styrene polymerizations investigations regarding the RAFT pre equilibrium in star polymerizations were carried out via NMR measurements, Monte-Carlo simulations and the analysis of the conversion dependence of the apparent number of arms. It was shown that the rate in which the RAFT main equilibrium is reached depends on the leaving group of the RAFT agent. Thus, the RAFT leaving group has a significant influence on the topology of star polymers produced via RAFT star polymerization. Restrictions for the synthesis of styrene star polymers given by the high rate of termination were limited using high pressure. Esters of acrylic acid are known to have a high reactivity regarding intra and inter molecular transfer to polymer. During the polymerization of the above mentioned acrylates star-star coupling was observed, which is exclusively due to inter molecular transfer to polymer. By analysis of the molecular weight distributions in combination with modelling it was possible to determine the rate coefficients of the transfer to polymer reaction step for butyl acrylate and dodecyl acrylate for the first time directly. During the RAFT star polymerizations of vinyl acetate star-star coupling was observed also. Transfer reaction to monomer and polymer were identified as the limiting factors in the production of well defined vinyl acetate star polymer with a controlled molecular weight. A method to estimate the absolute number average of the molecular weight distribution of star polymers was developed using a mixture of linear and star like RAFT agents. A conventional SEC setup calibrated against linear polymer standards is sufficient for the method developed.

Reversible addition–fragmentation chain transfer (RAFT) polymerizations of styrene in bulk at 80°C using tri-, tetra-, and hexafunctional trithiocarbonates, in which the active RAFT groups are linked to the core via the stabilizing Z-group, were studied in detail. These Z-RAFT star polymerizations of styrene showed excellent molecular weight control up to very high monomer conversions and star sizes of more than 200kDa. The application of high pressure up to 2600bar was found to significantly increase the relative amount of living star polymer. Not even at very high monomer conversions and for large star molecules, a shielding effect of growing arms hampering the RAFT process could be identified. Absolute molecular weights of star polymers using a conventionally calibrated SEC setup were determined with high precision by using a mixture of linear and star-shaped RAFT agents. When using phenylethyl as the leaving R-group, well-defined star polymers that perfectly match the theoretical predictions were formed. However, when using benzyl as the leaving group, a pronounced impact of monomer conversion on the star polymer topology was observed and pure star polymers with the expected number of arms could not be obtained.

Arm-growth initiation in six-armed Z-RAFT star polymerization of styrene-d(8) was followed via the disappearance of H-1 NMR signals of the leaving group at their original position. These measurements revealed a dramatic impact of the leaving group on star polymer topology. Phenylethyl as leaving group induced formation of completely initiated stars already after 8% of monomer conversion. Benzyl as leaving group, however, drastically slows down arm-growth initiation, resulting in fully initiated star polymers only after about 60% of monomer conversion. Below that value, star-polystyrene of the expected topology cannot be generated with benzyl as leaving group. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.] Published online 25 September 2008

Six-arm star polymers of methyl acrylate (MA), butyl acrylate (BA), and dodecyl acrylate (DA) were generated in bulk at 60 degrees C via reversible addition-fragmentation chain transfer (RAFT) polymerization. A hexafunctional trithiocarbonate was employed as mediating compound, in which the active RAFT-agent moieties are interlinked to the central core molecule via the stabilizing Z-group. Well-defined star polymers with predictable number-average molecular weights of more than 1 x 10(6) g.mol(-1) were obtained by this Z-RAFT star polymerization of acrylates. Narrow and monomodal molecular weight distributions were found up to intermediate monomer conversions. At higher monomer conversions, an unexpected high molecular weight component occurred with increasing extent when going from MA over BA to DA polymerization. This high molecular weight material was assigned to a star-star couple containing two living cores, which formation is not in accordance to the basic Z-RAFT star polymerization mechanism, but most likely arises from an intermolecular chain transfer to polymer reaction. The amount of star-star couples, which is an indication for the extent of long-chain branching, was quantified as a function of the monomer conversion and subsequently modeled via kinetic simulations. By this approach, the rate coefficient of intermolecular transfer to polymer at 60 degrees C was estimated to be k(tr)(P,inter) = 0.33 L.mol(-1).s(-1) in BA polymerization and k(tr)(P,inter) = 7.1 L.mol(-1).s(-1) in DA polymerization. The living process was found to be very effective up to high monomer conversions, indicating that steric congestion next to the star polymer core, where the actual RAFT process takes place, is not significantly hampering the Z-RAFT star polymerization of acrylates.

Poly(vinyl acetate) and poly(vinyl propionate) star polymers with four arms were produced via reversible addition fragmentation chain transfer (RAFT) polymerization, employing a tetra-functional xanthate as the RAFT agent, in which the stabilizing groups are linked to the core. These novel star-like RAFT agents induced living/controlled behavior in both the vinyl acetate polymerization at 60 °C and in the vinyl propionate polymerization at 90 °C, respectively, leading to star polymers with minimum polydispersities of 1.2 and maximum apparent number average molecular weights of about 50,000 g·mol-1. The microstructure of the star polymers was confirmed by electrospray ionization mass spectrometry.