Molecular mechanics calculations on cobalt phthalocyanine dimers

In order to obtain insight into the structure of cobalt phthalocyanine dimers, molecular mechanics calculations were performed on dimeric cobalt phthalocyanine species. Molecular mechanics calculations are first presented on monomeric cobalt(II) phthalocyanine. Using the Tripos force field for the organic part of the molecule and parameters derived from the literature and subsequently optimized to describe the CoII force field resulted in a geometry that is in very good agreement with experimental data from the literature. Optimization of the dimeric structure leads to a geometry in which both phthalocyanines are separated by 3.2 Å and one of the molecules is shifted 2.38 Å in both the X- and Y-directions with respect to the other. This geometry is in excellent agreement with literature data on ß-Co(pc) crystals and with other calculated and experimental data on similar systems. All calculations were performed with three possible charge distributions in the phthalocyanine molecule and it was shown that varying the charge distribution had no significant effect on the final dimeric structure. This method provides valuable insight into the most important energetic interactions leading to dimer formation

The Importance of Chain-Length Dependent Kinetics in Free-Radical Polymerization: A Preliminary Guide

The effect of chain-length dependent propagation at short chain lengths on the observed kinetics in low-conversion free-radical polymerization (frp) is investigated. It is shown that although the values of individual propagation rate coefficients quickly converge to the high chain length value (at chain lengths, i, of about 10), its effect on the average propagation rate coefficients, kp, in conventional frp may be noticeable in systems with an average degree of polymerization (DPn) of up to 100. Furthermore it is shown that, unless the system is significantly retarded, the chain-length dependence of the average termination rate coefficient, kt, is not affected by the presence of chain-length dependent propagation and that there exists a simple (fairly general) scaling law between kt and DPn. This latter scaling law is a good reflection of the dependence of the termination rate coefficient between two i-meric radicals, kt i,i, on i. Although simple expressions seem to exist to describe the dependence of kp on DPn, the limited data available to date does not allow the generalization of these expressions

Biocompatible and thermo-responsive nanocapsules through vesicle templating

Thermo-responsive and biocompatible cross-linked nanocapsules were synthesized through dimethyldioctadecylammonium bromide (DODAB) vesicle templating. For this, firstly two random copolymers, N-vinylcaprolactam (VCL) and acrylic acid (AA), with different chain lengths but using the same monomer ratio, were synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. These anionic random copolymers were adsorbed onto cationic DODAB vesicles. Then, biocompatible and thermo-responsive nanocapsules were obtained by semicontinuous emulsion polymerization under monomer-starved conditions for both the main monomer (VCL) and the cross-linker. Although in all the cases the typical thermal behavior of PVCL-based nanocapsules was observed, hysteresis between cooling and heating cycles was observed at low temperature in the case of non-cross-linked nanocapsules. This behavior was reduced using different types and amounts of cross-linkers. In addition, transmission electron microscopy (TEM) characterization demonstrated the successful formation of nanocapsules either with short or long random copolymers. The formation of stable nanocapsules was confirmed below and above the volume phase transition temperature (VPTT) by surfactant lysis experiments through optical density and DLS measurements in all the nanocapsules synthesized. These biocompatible and thermo-responsive nanocapsules could be suitable and potentially useful as nanocarriers for drug delivery

Role of polycation promoters in the cobalt phthalocyanine-catalyzed autoxidation of thiols

The promotion effect of 2,4-ionene on the cobalt phthalocyanine-catalyzed autoxidn. of thiols was studie

Further Effects of Chain-Length-Dependent Reactivities on Radical Polymerization Kinetics

In the present paper, we finalize some threads in our investigations into the effects of chain-length-dependent propagation (CLDP) on radical polymerization kinetics, confirming all our previous conclusions. Additionally, and more significantly, we uncover some unexpected and striking effects of chain-length-dependent chain transfer (CLDTr). It is found that the observed overall rate coefficients for propagation and termination (and therefore the rate of polymerization) are not significantly affected by whether or not chain transfer is chain-length dependent. However, this situation is different when considering the molecular weight distributions of the resulting polymers. In the case of chain-length-independent chain transfer, CLDP results in a considerable narrowing of the distribution at the low molecular weight side of the distribution in a chain-transfer controlled system. However, the inclusion of both CLDP and CLDTr yields identical results to classical kinetics – in these latter two cases, the molecular weight distribution is governed by the same chain-length-independent chain transfer constant, whereas in the case of CLDP only, it is governed by a chain-length-dependent chain transfer constant that decreases with decreasing chain length, thus enhancing the probability of propagation for short radicals. Furthermore, it is shown that the inclusion of a very slow first addition step has tremendous effects on the observed kinetics, increasing the primary radical concentration and thereby the overall termination rate coefficient dramatically. However, including possible penultimate unit effects does not significantly affect the overall picture and can be ignored for the time being. Lastly, we explore the prospects of using molecular weight distributions to probe the phenomena of CLDP and CLDTr. Again, some interesting insights follow

The nature of the chain-length dependence of the propagation rate coefficient and its effect on the kinetics of free-radical polymerization. 1. Small-molecule studies

In this paper we summarize and analyze the currently available small-molecule data, both experimental and theoretical, that is relevant to chain-length-dependent propagation in free-radical polymerization (FRP). We do this in order to appreciate the nature of chain-length-dependent propagation, because workers are becoming increasingly cognizant of its necessity in reaching a complete understanding of FRP kinetics. We show that studies of addition in small-molecule (model) systems support a chain-length dependence (at short chain lengths i) which is described by the following functional form, which therefore can be said to be physically realistic: , where the values of C1 and i1/2 are of the order of 10 and 1, respectively. These results are supported by transition state theory, which predicts a very similar behavior for the Arrhenius frequency factor. We illustrate that in systems with low number-average degree of polymerization (DPn), this chain-length dependence can dramatically affect the observed (chain-length-averaged) propagation rate coefficient kp, which can be significantly higher than the long chain value, kp. However, this effect is only observed if the activation energy for the first radical addition is similar to that for propagation. In the case that the former is significantly higher (e.g., when choosing a less than optimal initiator or in the case of retardative chain transfer), the chain-length-dependent propagation predicted by our model will not be observed, and in fact a significant lowering of kp can in cases be expected up to relatively high DPn

The nature of the chain-length dependence of the propagation rate coefficient and its effect on the kinetics of free-radical polymerization. 1. Small-molecule studies

In this paper we summarize and analyze the currently available small-molecule data, both experimental and theoretical, that is relevant to chain-length-dependent propagation in free-radical polymerization (FRP). We do this in order to appreciate the nature of chain-length-dependent propagation, because workers are becoming increasingly cognizant of its necessity in reaching a complete understanding of FRP kinetics. We show that studies of addition in small-molecule (model) systems support a chain-length dependence (at short chain lengths i) which is described by the following functional form, which therefore can be said to be physically realistic: , where the values of C1 and i1/2 are of the order of 10 and 1, respectively. These results are supported by transition state theory, which predicts a very similar behavior for the Arrhenius frequency factor. We illustrate that in systems with low number-average degree of polymerization (DPn), this chain-length dependence can dramatically affect the observed (chain-length-averaged) propagation rate coefficient kp, which can be significantly higher than the long chain value, kp. However, this effect is only observed if the activation energy for the first radical addition is similar to that for propagation. In the case that the former is significantly higher (e.g., when choosing a less than optimal initiator or in the case of retardative chain transfer), the chain-length-dependent propagation predicted by our model will not be observed, and in fact a significant lowering of kp can in cases be expected up to relatively high DPn

Catalytic chain transfer and its derived macromonomers

An overview is given of cobalt-catalyzed chain transfer in free-radical polymerization and the chemistry and applications of its derived macromonomers. Catalytic chain transfer polymerization is a very efficient and versatile technique for the synthesis of functional macromonomers. Firstly the mechanism and kinetic aspects of the process are briefly discussed in solution/bulk and in emulsion polymerization, followed by a description of its application to produce functional macromonomers. The second part of this review briefly describes the behavior of the macromonomers as chain transfer agents and/or comonomers in second-stage radical polymerizations yielding polymers of more complex architectures. The review ends with a brief overview of post-polymerization modifications of the vinyl endfunctionality of the macromonomers yielding functional polymers with applications ranging from initiators in anionic polymerization to end-functional lectin-binding glycopolymers

The Importance of Chain-Length Dependent Kinetics in Free-Radical Polymerization: A Preliminary Guide

The effect of chain-length dependent propagation at short chain lengths on the observed kinetics in low-conversion free-radical polymerization (frp) is investigated. It is shown that although the values of individual propagation rate coefficients quickly converge to the high chain length value (at chain lengths, i, of about 10), its effect on the average propagation rate coefficients, kp, in conventional frp may be noticeable in systems with an average degree of polymerization (DPn) of up to 100. Furthermore it is shown that, unless the system is significantly retarded, the chain-length dependence of the average termination rate coefficient, kt, is not affected by the presence of chain-length dependent propagation and that there exists a simple (fairly general) scaling law between kt and DPn. This latter scaling law is a good reflection of the dependence of the termination rate coefficient between two i-meric radicals, kt i,i, on i. Although simple expressions seem to exist to describe the dependence of kp on DPn, the limited data available to date does not allow the generalization of these expressions

Diseno macromolecular par transferencia de cadena

In this overview article, a brief description and discussion is given about the controlled radical techniques which are based on chain transfer, i.e., Catalytic Chain Transfer (CCT) and Reversible Addition-Fragmentation chain Transfer (RAFT). The CCT is an incredibly efficient method for making functional low-molecular weight polymers. In fact, the products are macromonomers, which can be used as chain transfer agents to make telechelic polymers or comb polymers. RAFT polymerization is probably the most versatile of all controlled radical polymerization techniques, with the ability to make complex architectures, such as block copolymers and stars in a controlled manner under less rigorous conditions