The coordinative chain transfer co-and ter-polymerizations of myrcene, isoprene and styrene have been achieved using Cp*La(BH 4 ) 2 (THF) 2 (Cp* = pentamethylcyclopentadienyl) combined with magnesium dialkyl and aluminum dialkyl. New highly stereoregular poly(myrcene-co-styrene), poly(myrcene-co-isoprene), and poly(myrcene-co-isoprene-co-styrene) copolymers have been obtained with good yields and a wide range of compositions. The concept of coordinative chain transfer polymerization (CCTP) induced control of the microstructure of statistical copolymers could be successfully extended to 1,4conjugated dienes statistical copolymerization and terpolymerization and to a renewable monomer. In addition to significant transmetalation in the course of the polymerization, a modification of the apparent comonomers reactivity ratio was observed in the presence of a chain transfer agent. This could be combined, depending on the nature of the chain transfer agent, with either a stereospecific polymerization of both conjugated dienes or the introduction of 3,4 enchainments in the microstructure. The former 1,4-trans stereoselective homo-, co-, and terpolymerizations enable the growth of several stereoregular chains per catalyst atom, providing significant catalyst economy, at the expense of the activity however. The latter affords an easy and straightforward modification of the microstructure of conjugated dienes co-and terpolymers in terms of both composition and regioregularity. This is particularly interesting regarding long and tedious catalysts synthesis required to reach the same array of polymeric materials via a classical living approach.
RAFT polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA, 1100 g mol À1 ) was first performed using cyanoisopropyl dithiobenzoate (CPDB). The control of PEGMA homopolymerization, carried out in THF ([PEGMA] ¼ 40 wt %) at 65 C (reflux) using 2,2 0 -Azobis(isobutyronitrile) (AIBN) as initiator, was shown by the linear increase of molar masses and the low polydispersity indices (PDI) observed with conversion and also by the determination of a high chain transfer constant (C tr ¼ 9.1) for CPDB in this system. Copolymerization of PEGMA with methacrylic acid (MAA) ([PEGMA]/[MAA] ¼ 40/60) was then undertaken first in dioxane at 85 C. High conversion (89%) was obtained in 3 h without any composition drift and with a good level of control (PDI \ 1.40). When the polymerization was performed in water, a strong increase in polymerization rate was observed with almost quantitative conversion (98%) in 2 h without affecting the level of control of the final copolymers (PDI $ 1.30). These last results were tentatively explained by the formation of hydrophobic domains in which the polymerization occurred as in bulk.
End-functionalized
1,4-trans-polydienes (isoprene
and myrcene) were prepared by means of a neodymium-mediated continuous
polymerization–functionalization process. A chain transfer
stage was essential to get a high level of functionalization of the
polymer. The living 1,4-trans-stereoregular polymer
chain was reacted with benzophenone to afford up to 97% −CPh2OH end-group with isoprene, while in the case of myrcene,
using the same strategy afforded 1,4-trans-polymyrcene
with a functionalization rate of 83%. The strategy was validated with
complementary functionalization experiments using benzaldehyde and
styrene oxide, both affording 95% end-capped trans-stereoregular polydienes.
Methacrylate end-capped poly(N-acryloylmorpholine) macromonomers (PNAM-MA) of number-average molar mass of about 2400 g mol−1 have been synthesized by post-modification of PNAM chains obtained by RAFT.
Dans cet article nous proposons une approche qui permet d’accorder les qualités de conception, d’élaboration et d’administration d’une épreuve de mesure des compétences en expression orale à une situation de fort enjeu. En s’appuyant sur des exemples de certifications où la production orale n’est pas l’unique compétence évaluée, nous montrerons que les bénéfices de la mise en œuvre d’une démarche rigoureuse et d’une approche scientifique, qui confère à l’outil de mesure des compétences des qualités de validité, de fidélité, de sensibilité et d’équité, sont des garanties pour répondre de façon appropriée à la demande initiale du commanditaire et aux attentes des utilisateurs finaux. Nous examinerons également les avantages de cette démarche pour les candidats.
The interaction mechanism between polycarboxylate-type superplasticizer (PCP) and cement hydration is not fully understood and incompatibilities between concrete and additive are sometimes observed. In some cases, the fluidity tends to increase ("overfluidification") few minutes after mixing. This is a problem because the overfluidification leds to bleeding of the concrete which could be critical on job site. Our study consisted first in highlighting the phenomenon of "over-fluidification" by slump flow tests on mortar. Next, the time evolution of the rheological behaviour of cement pastes in the presence of PCP was analysed thanks to a rheometry protocol in order to quantify the phenomenon. Later on, a parametric study was undertaken using this methodology. The operating conditions such as temperature and mixing process were studied as well as the effect of PCP structural parameters and the chemical characteristics of cement. In order to understand the origin of the phenomenon, adsorption measurements of PCP on cement particles were performed in the same conditions as those in the rheological measurements. Indeed, the phenomenon of "over-fluidification" could be related to the rate of the initial adsorption and the adsorption kinetics, both of which depend on the parameters of the process, the PCP structure and the cement reactivity.
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