聚丙交酯/聚乙二醇多嵌段共聚物的合成及其性能

聚丙交酯 (ＰＬＬＡ)由于具有良好的生物降解性和生物相容性 ,在医学领域已经得到了广泛的临床应用 ,近来又被制备成细胞支架大量应用于组织工程中[1,2 ] ,但由于其疏水性而造成细胞亲和性不好 .聚乙二醇 (ＰＥＧ)具有良好的亲水性 ,良好的生物相容性 ,但是ＰＥＧ是非降解性的 ,只有低分子量的ＰＥＧ可以被吞噬细胞所吞噬或透过肾滤膜而排出体外 ,因此 ,低分子量的ＰＥＧ常被用来与丙交酯 (Ｌ ＬＡ)共聚以改善ＰＬＬＡ支架的亲水性 .聚丙交酯 聚乙二醇共聚物 (ＰＬＥ)的三嵌段及两嵌段共聚物的合成及其性能的研究已被广泛报道[3～ 5] .研究…     

CONSERVATION OF THREE—POINT COMPACT SCHEMES ON SINGLE AND MULTIBLOCK PATCHED GRIDS FOR HYPERBOLIC PROBLEMS

For nonlinear hyperbloic problems,Conservation of the numerical scheme is important for convergence to the correct weak solutions.In this paper the the conservation of the well-known compact scheme up to fourth order of accuracy on a single and uniform grid is studied,and a conservative interface treatment is derived for compact schemes on patched grids .For a pure initial value problem,the compact scheme is shown to be equivalent to a scheme in the usual conservative form .For the case of a mixed initial boundary value problem,the compact scheme is conservative only if the rounding errors are small enough.For a pactched grid interface,a conservative interface condition useful for mesh fefiement and for parallel computation is derived and its order of local accuracy is analyzed.     

Strategies for parallelizing a Navier-Stokes code on the intel touchstone machines

The purpose of this paper is to predict the efficiency of the Navier-Stokes code NSS* which will run on an MIMD architecture parallel machine. Computations are performed using a three-dimensional overlapping structured multiblock grid. Each processor works with some of these blocks and exchanges data across the boundaries of the blocks. The efficiency of such a code depends on the number of grid points per processor, the amount of computation per grid point and the amount of communication per boundary point. In this paper we estimate these quantities for NSS* and present measurements of communication times for two parallel machines, the Intel Touchstone Delta machine and an Intel iPSC/860 machine, consisting of 520 and 64 Intel i860 processors respectively. The peak performance of the Delta machine is 32 Gflop. Secondly it is shown how, starting from a seven-block grid of about 5000 000 points for the Hermes space plane, a mesh of 512 equally sized blocks is constructed retaining the original topology. This example demonstrates that multiblock grids provide sufficient control over both the number and size of blocks. Therefore it will be possible to simulate realistic configurations on massively parallel systems with a specified number of processors while achieving good quality load balancing.     

A multiblock Navier–Stokes algorithm using equal-order quadratic finite elements

A new multiblock pressure-based finite element algorithm has been developed. This methodology implements quadratic interpolation for both the elemental velocity and pressure fields. A direct streamline upwinding scheme previously developedby the authors is used to model the non-linear inertia effects. Details of the algorithm and its multiblock foundation are provided along with validating test cases. The results presented clearly demonstrate the accuracy of this new approach and the differences in the pressure field for an element using quadratic versus the traditional bi linear approximation of the pressure field.     

MACROCYCLES. 9. SMECTIC MULTIBLOCK COPOLYESTERS VIA MACROCYCLIC POLYMERIZATION OF δ-VALEROLACTONE AND ε-CAPROLACTONE

Using 2,2-dibutyl-2-stanna-1,3-dioxepane (DSDOP) as cyclic initiator 1:1 copolymerizations of ε-caprolactone (ε-CL) and δ-valerolactone (δ-VC) were conducted in bulk at 80°C. A nearly equimolar incorporation and a random sequence were found by NMR spectroscopy and crystallization below 20°C was detected by DSC measurements. In a second series of experiments, the in situ formed macrocyclic copolyesters were reacted with an excess of sebacoyl chloride and after addition of silylated 4,4′,-dihydroxybiphenyl copolycondensations were performed in bulk at 230°C. These copolycondensations proceeded satisfactorily without significant transesterification and despite a biphasic character of the melt. This biphasic character resulted from the formation of a smectic LC-phase by the 4,4′-dioxybiphenyl sebacate blocks. For comparison a homopolyester was prepared from sebacoyl chloride and silylated 4,4′-dihydroxybiphenyl. The thermal properties of this homopolyester and of the LC-blocks in the copolyester were nearly identical indicating the absence of transesterification during the synthesis at 230°C. However, the properties of the LC-phase above 200–210°C did not completely agree with the data reported for the homopolyester in the literature (which are themselves inconsistent).     

Easy Access to Diverse Multiblock Copolymers with On-Demand Blocks via Thioester-Relayed In-Chain Cascade Copolymerization

Multiblock copolymers are envisioned as promising materials with enhanced properties and functionality compared with their diblock/triblock counterparts. However, the current approaches can construct multiblock copolymers with a limited number of blocks but tedious procedures. Here, we report a thioester-relayed in-chain cascade copolymerization strategy for the easy preparation of multiblock copolymers with on-demand blocks, in which thioester groups with on-demand numbers are built in the polymer backbone by controlled/living polymerizations. These thioester groups further serve as the in-chain initiating centers to trigger the acyl group transfer ring-opening polymerization of episulfides independently and concurrently to extend the polymer backbone into multiblock structures. The compositions, number of blocks, and block degree of polymerization can be easily regulated. This strategy can offer easy access to a library of multiblock copolymers with ≈100 blocks in only 2 to 4 steps.     

Visible Light Photoiniferter Polymerization for Dispersity Control in High Molecular Weight Polymers

The synthesis of polymers with high molecular weights, controlled sequence, and tunable dispersities remains a challenge. A simple and effective visible-light controlled photoiniferter reversible addition-fragmentation chain transfer (RAFT) polymerization is reported here to realize this goal. Key to this strategy is the use of switchable RAFT agents (SRAs) to tune polymerization activities coupled with the inherent highly living nature of photoiniferter RAFT polymerization. The polymerization activities of SRAs were in situ adjusted by the addition of acid. In addition to a switchable chain-transfer coefficient, photolysis and polymerization kinetic studies revealed that neutral and protonated SRAs showed different photolysis and polymerization rates, which is unique to photoiniferter RAFT polymerization in terms of dispersity control. This strategy features no catalyst, no exogenous radical source, temporal regulation by visible light, and tunable dispersities in the unprecedented high molecular weight regime (up to 500 kg mol⁻¹). Pentablock copolymers with three different dispersity combinations were also synthesized, highlighting that the highly living nature was maintained even for blocks with large dispersities. T_g was lowered for high-dispersity polymers of similar MWs due to the existence of more low-MW polymers. This strategy holds great potential for the synthesis of advanced materials with controlled molecular weight, dispersity and sequence.