Block copolymers by transformation of living anionic polymerization into controlled/“living” atom transfer radical polymerization

A method for the transformation of living anionic polymerization (LAP) to controlled/“living” atom transfer radical polymerization (ATRP) is reported and utilized for the preparation of block copolymers. The macroinitiators, polystyrene and polystyrene-block-polyisoprene containing the 2-bromoisobutyryl end group (PS-Br, M_n = 12 200, M_w/M_n = 1.04; PS-b-PIP-Br, M_n = 16 800, M_w/M_n = 1.03), were prepared by LAP of styrene and styrene/isoprene, correspondingly, and suitable termination agents. These compounds were used as macroinitiators for controlled/“living” ATRP to prepare block copolymers with methyl acrylate (PS-b-PMA), butyl acrylate (PS-b-PBA), methyl methacrylate (PS-b-PMMA), a mixture of styrene and acrylonitrile (PS-b-P(S-r-AN)) and also chain extension with styrene (PS-b-PS and PS-b-PIP-b-PS). The block copolymers were characterized by means of size exclusion chromatography, ¹H NMR and FT-IR spectroscopy.     

Synthesis of a propene—methyl methacrylate diblock copolymer via “living” coordination polymerization

A new diblock copolymer of propene and methyl methacrylate (MMA) was prepared by adding MMA during the living coordination polymerization of propene together with the soluble catalyst V(acac)₃/Al(C₂H₅)₂Cl at ?78°C and subsequently by raising the polymerization temperature up to 25°C. The homopolymerization of MMA with this catalyst exhibits the characteristics of a living polymerization at the initial stage of polymerization. From the copolymerization of MMA with styrene, it was concluded that the polymerization of MMA occurs via a radical intermediate. It is suggested that a propene-MMA diblock copolymer is formed via the transformation of a living polypropylene end to a radical end.     

Controlled/“living” radical polymerization of styrene catalyzed by FeCl2/succinic acid

A new ligand, succinic acid, was successfully used for atom transfer radical polymerization. The reaction was carried out at 40 to 100°C in bulk with α‐bromoethyl benzene as the initiator and FeCl₂/(succinic acid) as the catalyst system. The molecular weight of the resulting polymer increases with increasing monomer conversion, however, it is somewhat lower than the theoretical value. The polydispersity index is relatively low (M_w/M_n = 1.30) even though there may be a possible chain transfer reaction during the polymerization. The enthalpy of the equilibrium between active species and dormant ones was calculated to be 2.15 kcal/mol.     

Synthesis and structure of a “living” copolymer of propylene and 1,5-hexadiene

A soluble V(acac)₃/Al(C₂H₅)₂Cl system was found to polymerize 1,5-hexadiene to give a living polymer in toluene at ?78°C. Hydrogenation of poly(1,5-hexadiene) was conducted to determine the chain microstructure. The ¹³C NMR analysis of hydrogenated poly(1,5-hexadiene) indicated that the poly(1,5-hexadiene) is composed of alternating units of 1,3-cyclopentylenemethylene and 1-vinyltetramethylene. In addition, living copolymers of propylene and 1,5-hexadiene were prepared and characterized by analysis of the ¹H and ¹³C NMR spectra. A chain propagation mechanism of living poly(1,5-hexadiene) is proposed.     

Molecular weight distribution of “living” polymers generated by a bifunctional initiator

Different rates of initiation and propagation, e.g., in the case of living polymerization, lead to considerable deviations from a POISSON distribution. Using a bifunctional initiator the distribution obtained is composed of a single distribution and a coupled distribution. The maximum value of x?_w/x?_n never exceeds 1.4 even for a ratio k_p/k_i = 10⁶.     

Bone “mass” and the “mechanostat”: A proposal

The observed fit of bone mass to a healthy animal's typical mechanical usage indicates some mechanism or mechanisms monitor that usage and control the three longitudinal growth, bone modeling, and BMU-based remodeling activities that directly determine bone mass. That mechanism could be named a mechanostat. Accumulated evidence suggests it includes the bone itself, plus mechanisms that transform its mechanical usage into appropriate signals, plus other mechanisms that detect those signals and then direct the above three biologic activities. In vivo studies have shown that bone strains in or above the 1500–3000 microstrain range cause bone modelling to increase cortical bone mass, while strains below the 100–300 microstrain range release BMU-based remodeling which then removes existing cortical-endosteal and trabecular bone. That arrangement provides a dual system in which bone modeling would adapt bone mass to gross overloading, while BMU-based remodeling would adapt bone mass to gross underloading, and the above strain ranges would be the approximate “setpoints” of those responses. The anatomical distribution of those mechanical usage effects are well known. If circulating agents or disease changed the effective setpoints of those responses their bone mass effects should copy the anatomical distribution of the mechanical usage effects. That seems to be the case for many agents and diseases, and several examples are discussed, including postmenopausal osteoporosis, fluoride effects, bone loss in orbit, and osteogenesis imperfecta. The mechanostat proposal is a seminal idea which fits diverse evidence but it requires critique and experimental study.     

Psychiatric “Co-occurrence”? I'll Stick with “Comorbidlty”

Lilienfeld et al's critique of the use of the term comorbid in psychopathological research is critically examined. They are incorrect in asserting that the original use of the term in medical epidemiology was intended to restrict it to disease entities. Furthermore, the ways in which medical comorbidity can be understood apply equally well to understanding the comorbidity of psychiatric or psychological disorders. Just as it has been useful to use the general term psychiatric disorders to include symptom patterns and syndromes and few If any true diseases, so to it is useful to apply the generic term of comorbidity to the joint occurrence of psychiatric disorders. Because methodological factors can sometimes contribute to artifactual comorbidity which is uninformative about the disorders being studied is no reason to abandon the term comorbidity in psycho-pathological research.     

Fine structural differences in “fast” and “slow” muscle fibers of the crab

The muscle fibers in the accessory flexor muscle of the crab were examined with the electron microscope. The limiting membrane of the muscle fiber invaginates at many points to form an internal extrafibrillar membranous system between the muscle fibrils. Two types of muscle fiber were found. In one, the fibrils are well separated from each other by an extensive membranous system, the sarcomere lengths are short, and the filaments in the A band are thick. This muscle fiber is the same as the fast fiber previously found physiologically. The other type of muscle fiber has large fibrils poorly separated by a relatively scant system of internal membranes. It has relatively long sarcomeres and comparatively thin filaments in the A band. This type of fiber is the same as the slow fibers described physiologically. Fibers intermediate in type are also found. Nerve terminals with typical synaptic vesicles occur and appear similar on both types of muscle fiber under the electron microscope. Membrane thickenings of nerve ending and muscle fiber also occur at the terminals on both types of muscle fiber. The nerve terminals on the fast fiber are much more easily found and hence more numerous than on the slow fiber. A comparison is made between the invertebrate and vertebrate fast and slow fibers, and some physiological implications of these findings are discussed.     

“Hypotyrosinemia” in Phenylketonuria

It has been postulated that the significant incidence of learning disabilities in well-treated patients with phenylketonuria (PKU) may be due, in part, to reduced production of neurotransmitters as a result of deficient tyrosine transport across the neuronal cell membrane. Hypotyrosinemia has been reported in treated and untreated PKU but virtually no data are available. We decided to examine this in our patient population and to compare it with the published norms, patient data from our hospital clinical biochemical laboratory database, and a group of normal children and adolescents in a private pediatric practice. We found that the mean nonfasting plasma tyrosine in 99 classical PKU patients was 41.1 μmol/L, in 26 mild (atypical) PKU patients 53.3 μmol/L, and in 35 non-PKU mild hyperphenylalaninemia patients 66.6 μmol/L. This compared to nonfasting plasma tyrosine levels in 102 non-PKU subjects of 64.0 μmol/L in our hospital biochemistry database, 69.1 μmol/L in 58 volunteers in the private office practice, and 64–78.8 μmol/L in infants, children, and adolescents in the literature review. Our data support the previously undocumented statements in the literature that plasma tyrosine levels are low in PKU.     

Mechanical and thermal behaviour of networks with “flexible” and “rigid” chains

“Rigid-rod” telechelics were synthesized via repetitive Diels-Alder reaction of biscyclohexadienes and bis(1,2,4-triazoline-3,5-diones). The bistriazolinediones were used in slight excess. The rigidity of the building blocks and the molecular weight were varied. Various amounts of these reactive telechelics are used to crosslink cis-1,4-polybutadiene. The mechanical and the thermal properties of these two-component networks were investigated by stress-strain, differential scanning calorimetry and dynamic-mechanical measurements. The occurrence of microphase separation between the “rigid” and the “flexible” components is discussed to explain the properties of these materials.