The bulk polymerization of styrene at 125 °C in the presence of
a PS−TEMPO adduct was
studied with respect to the polymerization rate and the concentration
of free TEMPO as a function of
time, where PS is polystyrene and TEMPO is
2,2,6,6-tetramethylpiperidinyl-1-oxy. The results
were
perfectly consistent with the proposed kinetic scheme which assumes the
existence of a stationary state
with respect to both polymeric and nitroxyl radical concentrations and
predicts that the polymerization
rate of the nitroxide-mediated system is independent of the adduct
concentration, being equal to the
polymerization rate of the adduct-free system, i.e., the rate of
thermal polymerization in the case of the
styrene system studied here. The equilibrium constant K
for the PS−TEMPO reversible reaction was
estimated to be 2.1 × 10-11 mol L-1 on
the basis of the dilatometric and electron spin resonance
data.
This value of K was indicated to be large enough to set
the system under control. This work thus shows
that in order for the “living” radical polymerization mediated by a
stable nitroxyl radical (SNR) to proceed
successfully, a constant supply of initiating radicals (by, e.g.,
thermal initiation) to make up for the loss
of polymer radicals due to irreversible termination is essential as
well as the frequent reversible
combination of polymeric and nitroxyl radicals. The total number
of initiating radicals to be supplied in
this way may be small compared with the number of polymer−SNR adducts
so that they have no important
influence on the molecular weight and its distribution of the
product.
Tissue biocompatibility of cellulose and its derivatives was examined in two in vivo tests, one for absorbance by living tissue and one for foreign body reaction. The samples examined were regenerated celluloses and cellulose derivatives: methyl cellulose, ethyl cellulose, aminoethyl cellulose, hydroxyethyl cellulose, and cellulosic polyion complexes. The in vivo absorbance by living tissue was found to depend on the degree of crystallinity and the chemical structure of the sample. The foreign body reaction was relatively mild for all the samples examined, showing that cellulose can be converted to biocompatible materials by physical and/or chemical transformation.
This is the first report of the synthesis of a well-defined
glycopolymer by free radical
polymerization.
N-(p-vinylbenzyl)-[O-β-d-galactopyranosyl-(1→4)]-d-gluconamide
(VLA), a styrene derivative with an oligosaccharide moiety, was polymerized in
N,N-dimethylformamide solution at 90 °C by
the nitroxide-mediated free radical polymerization technique.
Acetylated VLA gave polymers with a
molecular weight from about 2000 to 40 000, an
M
w/M
n ratio of about 1.1
in all cases, and a conversion
of up to about 90%, where M
w and
M
n are the weight- and number-average molecular
weights.
Indispensable for this success were (1) the use of
di-tert-butyl nitroxide (DBN) rather than other
nitroxides
such as TEMPO, (2) the acetylation of VLA, and (3) the use of a radical
initiator DCP (dicumyl peroxide)
as an accelerator. DBN provided a well-controlled polymerization
of VLA at 90 °C (VLA becomes unstable
at higher temperatures, e.g., >120 °C). The acetylation
effectively prevented the chain transfer that
leads to dead polymers and broad polydispersities. DCP remarkably
accelerated the rate of polymerization
(the rate of chain extension), which otherwise was impractically slow,
without causing any appreciable
broadening of polydispersity.
Dense and uniform layers of a biologically active carbonatecontaining hydroxyapatite can be formed on various kinds of organic polymers by the following biomimetic method. First, a substrate is set in contact with particles of CaO-Si0,-based glass soaked in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma for forming the apatite nuclei on the substrate. Second, the substrate is soaked in another solution highly supersaturated with respect to the apatite, e.g., with ion concentrations 1.5 times those of SBF (IJSBF) for making the apatite nuclei grow on the substrate in situ. The induction period for the apatite nucleation, which is defined as the time of the first treatment required for forming enough of the apatite nuclei to make the continuous layer after the second treatment, was almost 24 h for most of the examined polymers. The adhesive strength of the formed apatite layer to the polymers was as high as 3 to 4 MPa for poly(ethy1ene terephthalate), poly-ether sulfone, and poly (vinyl alcohol) hydrogel. This type of apatite-organic polymer composite is expected to be useful for repairing not only living hard tissues but also soft ones.
The dissociation rate constant kd related to the homolytic cleavage of the C-ON bond formed between a polystyrene (PS) and 2,2,6,6-tetramethylpiperidin-l-oxyl (TEMPO) is determined by adopting the gel permeation chromatography peak-resolution method to the styrene polymerization with a PS-TEMPO adduct as a probe and the radical initiator tert-butyl hydroperoxide. The result was given by the Arrhenius equation, k, = A exp(-E/(RT)) withA = 3.0 x 1013 s-l and E = 124 kJ * mol-'.
A dense, uniform, and highly biologically active bone-like apatite layer can be formed in arbitrary thickness on any kind and shape of solid substrate surface by the following biomimetic method at ordinary temperature and pressure: First, a substrate is set in contact with particles of bioactive CaO-SiO2-based glass soaked in a simulated body fluid (SBF) with inorganic ion concentrations nearly equal to those of human blood plasma so that a number of apatite nuclei are formed on the substrate. Second, the substrate is soaked in another solution with ion concentrations 1.5 times those of SBF (1.5SBF) so that the apatite nuclei grow in situ. In the present study, organic polymer substrates were treated with glow-discharge in O2 gas atmosphere, then subjected to the above-mentioned biomimetic process. The induction periods for the apatite nucleation on all the examined organic polymers were reduced from 24 to 6 h, with glow-discharge treatment. The adhesive strengths of the formed apatite layer to the substrates increased from about 4 to 10 MPa for poly(ethylene terephthalate) and poly-ether sulfone, and from 1 approximately 2 to 6 approximately 7 MPa even for poly(methyl methacrylate), polyamide 6 and polyethylene. It is supposed that highly polar groups such as carbonyl, ester, hydroxyl, and carboxyl ones formed by glow-discharge treatment increased the affinity of a silicate ion with the substrates to decrease the induction period, and also increased the affinity of the apatite with the substrate to increase the adhesive strength.(ABSTRACT TRUNCATED AT 250 WORDS)
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