Nylon 6 polymerization in the solid state

The postcondensation of nylon 6 in the solid state was studied. The reactions were carried out on fine powder in a fluidized bed reactor in a stream of dry nitrogen in the temperature range 110-205°C and during 1-24 h. The solid-state polymerization (SSP) did not follow melt kinetics, but was found to be limited by the diffusion of the autocatalyzing acid chain end group. Factors thought to influence SSP were studied, e.g., heat treatment, starting molecular weight, and remelting. Surprisningly, heat treatment had little effect, but the starting molecular weight had a strong effect on the reaction rate. The higher the starting molecular weight, the faster the reaction. This could be explained as a changing concentration distribution of the reactive groups in the solid state on SSP. The kinetics of the SSP had more than one region, and the rate of reaction for conversions of over 30% could be expressed as - dc/dt = k(c/t), where k is a dimensionless constant independent of temperature with a value of 0.28. The integrated form has the form - In(c/co) = k In(t/), where co is the acid end-group concentration at the start, t is the reaction time, and is the induction time. The value of is both dependent on the starting concentration co and the reaction temperature and has an activation energy of 105 kJ/mol

Novel "green" catalysts for controlled ring-opening polymerization of lactide

Syntéza polylaktidu (PLA) polymerací za otevření kruhu cyklického monomeru (ROP) může být uskutečněna různými způsoby. Literatura uvádí více než 100 katalytických systémů, jejichž pomocí lze polylaktid a jiné biodegradabilní alifatické polyestery získat. Například organokovové katalyzátory na bázi Sn, Zn, Al atd. se po splnění své polymerační funkce stávají kontaminanty a pro humánní implantáty je použití takového materiálu diskutabilní. V současné době jsou v centru výzkumné pozornosti nové N-heterocyklické karbenové katalyzátory. Tyto „metal-free“ katalytické struktury jsou schopné reprodukovatelně řídit syntézu polymerů předem definované molekulové hmotnosti s definovanými koncovými skupinami a nízkou polydisperzitou, která je charakteristická pro živý průběh polymerace. Nabízí se možnost syntézy blokových kopolymerů a různorodých makromolekulárních architektur. Předložená diplomová práce se zabývá studiem polymerace cyklického monomeru D,L-laktidu katalyzované N-heterocyklickým karbenem. Polymerace byly vedeny v přítomnosti benzylalkoholu jako iniciátoru v roztoku THF. Byl sledován vliv složení reakčního systému monomer – iniciátor – katalyzátor. Dále byly připraveny polymery opticky čistého L-laktidu s makroiniciátory PEG s Mn = 1000 a 2000 g/mol. Střední číselná molekulová hmotnost (Mn) a polydisperzita (PDI) byly stanoveny pomocí GPC. Definovatelnost koncových skupin vybraných polymerů byla prokázána pomocí 1H NMR.The synthesis of polylactide (PLA) by ring-opening polymerization (ROP) of cyclic monomer can be realized by different routes. More than 100 catalysts for the synthesis of polylactide and other biodegradable aliphatic polyesters are published in the literature. For example organometallic catalysts based on Sn, Zn, Al etc. after finishing polymerization function became contaminants and using obtained polymer material in human body is controversial. At present, the research is focused on novel N-hererocyclic carbene catalysts. These metal-free catalysts are able to produce polymers with controlled molecular weight, narrow polydispersity, end-group fidelity with high reproducibility as well as to synthesize the block copolymers and complex macromolecular architectures, which is characteristic for living polymerization system. This diploma thesis is focused on study of polymerization of cyclic monomer D,L-lactide catalyzed by N-hererocyclic carbene. Polymerizations were carried out at the presence of benzylalcohol as initiator at THF. We were focused on the influence of composition of reaction system monomer – initiator – catalyst. Polymers of optically pure L-lactide with macroinitiators PEG with Mn of 1000 a 2000 g/mol were prepared as well. Number average molecular weight (Mn) and polydispersity index (PDI) was determined by GPC. 1H NMR was used to prove end-group fidelity.

A Stochastic model for dynamics of FtsZ filaments and the formation of Z-ring

Understanding the mechanisms responsible for the formation and growth of FtsZ polymers and their subsequent formation of the $Z$-ring is important for gaining insight into the cell division in prokaryotic cells. In this work, we present a minimal stochastic model that qualitatively reproduces {\it in vitro} observations of polymerization, formation of dynamic contractile ring that is stable for a long time and depolymerization shown by FtsZ polymer filaments. In this stochastic model, we explore different mechanisms for ring breaking and hydrolysis. In addition to hydrolysis, which is known to regulate the dynamics of other tubulin polymers like microtubules, we find that the presence of the ring allows for an additional mechanism for regulating the dynamics of FtsZ polymers. Ring breaking dynamics in the presence of hydrolysis naturally induce rescue and catastrophe events in this model irrespective of the mechanism of hydrolysis.Comment: Replaced with published versio

Early Stages of Homopolymer Collapse

Interest in the protein folding problem has motivated a wide range of theoretical and experimental studies of the kinetics of the collapse of flexible homopolymers. In this Paper a phenomenological model is proposed for the kinetics of the early stages of homopolymer collapse following a quench from temperatures above to below the theta temperature. In the first stage, nascent droplets of the dense phase are formed, with little effect on the configurations of the bridges that join them. The droplets then grow by accreting monomers from the bridges, thus causing the bridges to stretch. During these two stages the overall dimensions of the chain decrease only weakly. Further growth of the droplets is accomplished by the shortening of the bridges, which causes the shrinking of the overall dimensions of the chain. The characteristic times of the three stages respectively scale as the zeroth, 1/5 and 6/5 power of the the degree of polymerization of the chain.Comment: 11 pages, 3 figure

Template-directed biopolymerization: tape-copying Turing machines

DNA, RNA and proteins are among the most important macromolecules in a living cell. These molecules are polymerized by molecular machines. These natural nano-machines polymerize such macromolecules, adding one monomer at a time, using another linear polymer as the corresponding template. The machine utilizes input chemical energy to move along the template which also serves as a track for the movements of the machine. In the Alan Turing year 2012, it is worth pointing out that these machines are "tape-copying Turing machines". We review the operational mechanisms of the polymerizer machines and their collective behavior from the perspective of statistical physics, emphasizing their common features in spite of the crucial differences in their biological functions. We also draw attention of the physics community to another class of modular machines that carry out a different type of template-directed polymerization. We hope this review will inspire new kinetic models for these modular machines.Comment: Author-edited final version of a review article published in Biophysical Reviews and Letters [copyright World Scientific Publishing Company]; publisher-edited electronic version available at http://www.worldscientific.com/doi/abs/10.1142/S179304801230008

Non-steady scaling model for the kinetics of the photo-induced free radical polymerization of crosslinking networks

Recently, a semi-empirical scaling model was introduced to account for the free-radical polymerization kinetics of acrylated urethane precursors in the solid-state. By describing the radical initiation process in more detail, the kinetic model is extended herein towards general free-radical crosslinking irrespective of the initial physical state of the multifunctional precursors. Effects referred to as radical trapping and caging in the literature are clearly specified and a closed-form expression with a limited number of adjustable parameters is obtained which can be compared to experimental kinetics. In particular, the relation between polymerization rate and functional conversion can be reduced to expressions with three and four parameters in the limits of "solid-state" and "steady-state" kinetics, respectively. In the case of photoinduced free-radical polymerization and within the slow decomposition regime of the initiator, the single parameter with an explicit dependence on the incident light intensity is predicted to behave proportionally. The model is validated by comparing the relevant expressions to original calorimetric data for the free-radical photopolymerization kinetics of different acrylate urethane precursors at two temperatures, providing illustrations for solid-to-solid and liquid-to-rubber transformations. Careful monitoring of the effect of light intensity corroborates the expected scaling and additionally offers reliable estimates for the kinetic coefficients of propagation and termination

Poly(2-cyclopropyl-2-oxazoline): from rate acceleration by Cyclopropyl to Thermoresponsive properties

The synthesis and microwave-assisted living cationic ring-opening polymerization of 2-cyclopropyl-2-oxazoline is reported revealing the fastest polymerization for an aliphatic substituted 2-oxazoline to date, which is ascribed to the electron withdrawing effect of the cyclopropyl group. The poly(2-cyclopropyl-2-oxazoline) (pCPropOx) represents an alternative thermo-responsive poly(2-oxazoline) with a reversible critical temperature close to body temperature. The cloud point (CP) of the obtained pCPropOx in aqueous solution was evaluated in detail by turbidimetry, dynamic light scattering (DLS) and viscosity measurements. pCPropOx is amorphous with a significantly higher glass transition temperature (T(g) similar to 80 degrees C) compared to the amorphous poly(2-n-propyl-2-oxazoline) (pnPropOx) (T(g) similar to 40 degrees C), while poly(2-isopropyl-2-oxazoline) piPropOx is semicrystalline. In addition, a pCPropOx comb polymer was prepared by methacrylic acid end-capping of the living cationic species followed by RAFT polymerization of the macromonomer. The polymer architecture does not influence the concentration dependence of the CP, however, both the CP and T(g) of the comb polymer are lower due to the increased number of hydrophobic end groups