Monodisperse Liquid Crystal Network Particles Synthesized via Precipitation Polymerization

The production of liquid crystalline (LC) polymer particles with a narrow size distribution on a large scale remains a challenge. Here, we report the preparation of monodisperse, cross-linked liquid crystalline particles via precipitation polymerization. This versatile and scalable method yields polymer particles with a smectic liquid crystal order. Although the LC monomers are randomly dissolved in solution, the oligomers self-align and LC order is induced. For the polymerization, a smectic LC monomer mixture consisting of cross-linkers and benzoic acid hydrogen-bonded dimers is used. The average diameter of the particles increases at higher polymerization temperatures and in better solvents, whereas the monomer and initiator concentration have only minor impact on the particle size. After deprotonating of the benzoic acid groups, the particles show rapid absorption of a common cationic dye, methylene blue. The methylene blue in the particles can be subsequently released with the addition of Ca2+, while monovalent ions fail to trigger the release. These results reveal that precipitation polymerization is an attractive method to prepare functional LC polymer particles of a narrow size distribution and on a large scale

The role of monomer in the chain transfer reaction in cobaloxime-mediated free-radical polymerization

The role of monomer in catalytic chain transfer polymerization was studied by determination of the chain transfer constants of the tetraphenyl derivative of cobaloxime boron fluoride (COPhBF) in methyl methacrylate at 60 degrees C varying the monomer concentration instead of the COPhBF concentration as is common practice. Toluene and tert-butyl acetate were used as diluents in these studies and it was found that the chain transfer constants obtained in the present studies were not significantly different from those observed in conventional experiments. These results suggest the absence of a direct participation of monomer molecules in the hydrogen abstraction step in catalytic chain transfer

Copolymerization of styrene and alpha-methylstyrene in the presence of a catalytic chain transfer agent

Copolymerizations of styrene (STY) and alpha-methylstyrene (AMS) have been performed at different monomer feed compositions and temperatures (40-70 degrees C) in the presence of the catalytic chain transfer agent bis(boron difluorodimethylglyoximate)cobaltate(II) (COBF). The average chain transfer constant, [C-s], was found to increase approximately 3 orders of magnitude upon going from pure Sm to pure AMS. The addition of only 10% AMS increased the (Cs) by 1 order of magnitude. This behavior can be predicted from the relative fractions of growing radical end groups, which results in the majority of polymer chains being formed with an unsaturated AMS end group. No significant penultimate unit effects in the chain transfer reaction were observed. The addition of 10-20% AMS results in the majority of the growing radicals having AMS end groups, and hence AMS dominates the catalytic chain transfer reaction. The [C-s] values continue to increase as the AMS content is increased beyond 20%; however, this can be mostly attributed to a decrease in [k(p)] rather than an increase in [k(tr)]. The expected end groups in the homopolymerization of AMS in the presence of COBF were confirmed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS)

Preparation and characterization of oligomeric terpolymers of styrene, methyl methacrylate and 2-hydroxyethyl methacrylate: A comparison of conventional and catalytic chain transfer

Batch solution terpolymerizations of styrene, methyl methacrylate and 2-hydroxyethyl methacrylate were carried out at 70 degreesC in the presence of either dodecanethiol or bis[(difluoroboryl)diphenylglyoximato]cob (II) (COPhBF), to yield polymer products with a number average molecular weight of about 2500. Conversion, molecular weight distribution and overall composition were monitored during the reaction, and the final products were investigated using differential scanning calorimetry and thermogravimetric analysis. It was found that the overall rates of polymerization do not differ significantly in the two systems, but that the molecular weight distribution of the polymer formed in the presence of the thiol becomes increasingly broader during the polymerization, whereas the COPhBF-mediated polymerization produces a relatively uniform product during the course of the reaction. The polymer product formed with COPhBF was found to be slightly less thermally stable than the product formed by the thiol, which can be explained by the formation of unsaturated endgroups in the case of the: former product.[GRAPHICS]DSC thermograms showing the glass transition behaviour of the terpolymer oligomers generated in the thiol and COPhBF mediated reactions

The effects of ester chain length and temperature on the catalytic chain transfer polymerization of methacrylates

The catalytic chain transfer polymerizations of methyl, ethyl, and butyl methacrylates were studied over the temperature range 40-70 degrees C with cobaloxime boron fluoride (COBF) and its tetraphenyl derivative (COPhBF). It was found that for both catalytic chain transfer agents the chain transfer constant decreases in going from methyl to butyl methacrylate, and that there is no significant temperature effect on the observed chain transfer constants. The results are consistent with a diffusion-controlled rate coefficient for the chain transfer reaction

Synthesis of 3-[tris(trimethylsilyloxy)silyl]propyl methacrylate macromers using catalytic chain transfer polymerization: a kinetic and mechanistic study

Macromers of 3-[tris(trimethylsilyloxy)silyl]-propyl methacrylate (TRIS) were synthesised using catalytic chain transfer polymerization, and the kinetic parameters governing the reaction were evaluated. A study on the radical solution polymerization of TRIS in the presence of the catalytic chain transfer agent bis[(difluoroboryl)dimethylglyoximato]cobalt(II) (COBF) at 60 degrees C was conducted. Using appropriate Mark-Houwink-Kuhn-Sakurada constants for polyTRIS, the chain transfer constant (C-s) for COBF was found to be similar to 1400 in toluene solution. This low Cs value, as compared to the value reported for methyl methacrylate polymerization (similar to 3.5 . 10(4)), is only partly explained by a diffusion-controlled chain transfer reaction in the methacrylate series of monomers. A study on the influence of conversion on the molecular weight distribution indicated significant broadening and bimodality, consistent with reversible catalyst poisoning and chain transfer to the macromers. High oxygen solubility in TRIS is hypothesised to play a role in the reversible catalyst poisoning making it difficult to obtain a controlled reaction under normal free-radical polymerization reaction conditions

Comparison of the Mayo and chain length distribution procedures for the measurement of chain transfer constants

The Mayo and chain length distribution (CLD) procedures for the measurement of chain transfer constants are outlined and compared, and it is found that both procedures are essentially equivalent and yield reliable results. It is shown that both procedures are relatively insensitive to changes in initiator concentration under practical circumstances and that the most reliable results are obtained with the Mayo procedure using the weight-average molecular weight and with the CLD procedure using the slope in the ln(number distribution) at the peak molecular weight. The CLD procedure is discussed in more detail, and it is shown that as long as termination is not a significant chain stopping event, the ln(number distribution) should be a straight line. Therefore, the limiting slope of such a plot can be reliably obtained, preferably from the peak molecular weight region, as previously discussed by Mead and Mead [Macromolecules 1996, 29, 7727]. However, when termination is significant, the limiting slope is only reached at high molecular weights, and it is unlikely that such slopes can be reliably obtained from experiments

Atom transfer radical polymerization in the presence of a thiol: more evidence supporting radical intermediates

Octanethiol, a free-radical chain transfer agent, was added to the atom transfer radical polymerization of methyl methacrylate. The molecular weight and polydispersity data of the polymers produced were consistent with the known chain transfer behaviour of octanethiol in conventional free-radical polymerization. These results, which clearly indicate a radical nature of the propagating species, can be explained by a simple kinetic model, which describes an otherwise "living" polymerization system in the presence of an added chain transfer agent