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ÒÔÏÂÊÇÐèҪעÒâµÄÓï·¨ ²»¹æÔò±ä»¯µÄµ¥¸´Êý µ¥Êý ¸´Êý criterion criteria ÒÀ¾Ý phenomenon phenomena ÏÖÏó medium media ½éÖÊ radius radii °ë¾¶ index indices Ö¸Êý appendix appendices ¸½Â¼ stimulus stimuli ´Ì¼¤ basis bases »ù´¡ synthesis syntheses ºÏ³É matrix matrices »ùÌå Novel biomaterials with improved, specific biological action are being dev eloped using polymers that are responsive to environmental stimuli. һЩÊýÓÐÌØÊâÐÔµÄÃû´Ê data, kinetics ͨ³£×÷¸´ÊýÃû´Ê research, work: ²»¿ÉÊýÃû´Ê The data in Table 1 are obtained from the reactions at different temperatu res. The kinetics of polymerization within each droplet are the same as those f or the corresponding bulk polymerization. Much research has been done to redirect the tropism of retroviruses by eng ineering the envelope glycoprotein. Most research on poly(ortho esters) has focused on the synthesis of polyme rs. һЩ½á¹¹µÄÊý <Ö÷νµÄͳһ> one of ¸´ÊýÃû´Êµ¥Êý none of ¸´ÊýÃû´Ê¸´Êý none of µ¥ÊýÃû´Ê/²»¿ÉÊýÃû´Êµ¥Êý no ¸´ÊýÃû´Ê¸´Êý no µ¥ÊýÃû´Ê/²»¿ÉÊýÃû´Êµ¥Êý a kind of µ¥ÊýÃû´Êµ¥Êý this/that kind of µ¥ÊýÃû´Êµ¥Êý these kinds of ¸´ÊýÃû´Ê¸´Êý ¸´ÊýÃû´Ê of this/that kind ¸´Êý sort Óëkind ÀàËÆ a type of µ¥ÊýÃû´Êµ¥Êý a series of ¸´ÊýÃû´Ê¸´Êý a class of ¸´ÊýÃû´Ê¸´Êý the number of ¸´ÊýÃû´Êµ¥Êý a number of ¸´ÊýÃû´Ê¸´Êý Ara-C is one of most important antitumor agents, especially used in the tr eatment of various leukaemias. One of possibilities for enhancing the pharmacological properties is to link it to a carrier. Biocompatibility is one of the main criteria for the selection of a particular biomaterial. Controlled drug delivery technology represents one of most rapid advancing areas of science in which chemists are contributing to human health care. Several models have been proposed but none appear to be generally applicable. None of the presently used ones are applied rigorously. Thus far no studies have conclusively demonstrated long-term value of any of the cardiac gene therap approaches. This type of termination is often referred to as spontaneous termination. A number of polymer backbones that are potentially degradable are detailed in the text. The number of mechanisms which have been proposed to explain the stereopecificity of Ziegler-Natta catalysts. * The number of ¸´ÊýÃû´Ê: Õû¸ö×öÖ÷ÓïʱνÓïӦΪµ¥Êý; µ«´Ë¾äµÄ¶¯´Êhave ÊÇ´Ó¾ä µÄνÓï ¶ø´Ó¾äÊÇÐÎÈÝmechanisms. ÔÚÆäËüµÄ´Ó¾äνÓïÖÐÒ²ÊÇͬÑùÈç: One of the mechanisms which have been¡¡ A series of telomers reported in table 1 were then synthesized. A diverse range of mechanisms have been developed. Polymers are a promising class of biomaterials that can be engineered to m eet specific end-use requirements. The most important class of poly(amides) for controlled release are the po ly(amino acids). The majority of poly(anhydrides) are prepared by melt-condensation polymer ization. A wide variety of natural and synthetic polymers have been used to fabrica te tissue-engineering matrices. ºóÃæÍ¨³£ÒªÓø´ÊýÃû´Ê¶Ô¿ÉÊýÃû´Ê¶øÑÔ various different most all many several one of many of kinds of a number of the number of a (wide) variety of a class of a series of a wide range of Dozens of different types of receptors are present on the average mammalia n cell. In a number of special applications (e.g. contact lenses and skin-grafting materials), additional requirements such as transparency should be considered when selecting a polymer. Many of these transmembrane proteins are receptors. Some polymers are highly crystalline primarily because their structure is conductive to packing, while others are crystalline primarily because of the secondary forces. For still other polymers, both factors may be favorable for crystallization. ÐÎÈÝ¿ÉÊýÃû´ÊµÄÁ¿´Ê a few many a number of ÐÎÈݲ»¿ÉÊýÃû´ÊµÄÁ¿´Ê a great/small/certain amount of a great/good deal of ÐÎÈÝ¿ÉÊýºÍ²»¿ÉÊýÃû´Ê¾ù¿ÉµÄÁ¿´Ê a lot of lots of Although a few polymers may be completely amorphous and a few completely crystalline, most polymers are partially or semi-crystalline in character. The use of IPNs has received a great deal of attention in recent years and different kinds of IPN have been synthesized with different compositions. The major amount of work on size distributions has been on polyesters and polyimides. A sizeable amount work has been published on the copolymerizations of vari ous cyclicmonomers. ÒÔϽṹµÄνÓïµÄÊý ÓÉǰһ¸ö¾ö¶¨ , as well as¡¡ ÓɽϿ¿½üνÓﶯ´ÊµÄ¾ö¶¨ not only¡¡but also¡¡ either¡¡or¡¡ neither¡¡nor¡¡ Èý¡¢¹Ú´Ê ¹Ú´ÊÊÇ×îÒ×·¸´íÎóµÄµØ·½Ò»¶¨Òª×¢Òâ·ñÔò±à¼µÄ¸ºµ£¼«ÖØÒòΪÿ¾ä»°¶¼»áÓõ½¹Ú ´Ê×îÆÕͨµÄ¹æÔòÈçÏ µ¥ÊýÃû´Êǰһ°ãÒª¼Ó¹Ú´Êa , the µÈ µ«ÓÐЩÇé¿ö¿É²»¼Ó ¸´ÊýÃû´Êǰһ°ã¿É²»¼Ó¹Ú´ÊÒ²¿É¼Óthe, these, those. The molecular weight of a polymer is of prime importance in its synthesis and application. The rate constant can be expressed by an Arrhenius type relationship K = A e -E/RT Where A is the collision frequency factor, E the Arrhenius activation energy, and T the Kelvin temperature. A plot of lnK versus 1/T allows the determination of both E and A from the slope and intercept, respectively. When a water-insoluble or only slightly water-soluble monomer is added, a very small fraction dissolves and goes into solution. A large but still small portion of the m onomer enters the interior hydrocarbon part of the micelles. The largest portion of the monomer is dispersed as monomer droplets. ±»ÐÞÊεÄÃû´ÊǰҪ¼Óthe: There has been a rapidly increasing effort in the last ten to fifteen year s toward the synthesis of new polymers with markedly higher temperature resistance than those previo usly available. The catalysts usually must be prepared at the low temperatures since most of them become heterogeneous when prepared at or warmed up to the temperatures above abou t -40 . ËÄ¡¢´Ó¾ä·Ö´Ê²»¶¨Ê½ ÕâЩÊÇд×÷ʱ³£ÓÃÇÒ±ØÐëÓõ½µÄ½á¹¹Ò»¶¨ÒªÕÆÎÕÔÚѧÊõÂÛÎÄÖдӾä·Ö´Ê²»¶¨Ê½Í¨ ³£ÓÃÀ´×÷ÐÞÊÎ Applications in the relatively new field of tissue engineering, where poly mers are used to assist regeneration of three-dimensional tissue structures, are more and more int egrated with biological demands. There are three general synthetic approaches that have been used to produc e polymers with increased thermal stability. If certain symmetry requirements are met, the molecules are able to pack i nto an ordered, lattice arrangement and crystallization occurs. The temperature at which this occu rs is Tm. There are at least two mechanisms through which the infectivity of a vecto r can be blocked. Biodegradable polymers containing ¨A hydrolysable ¨@ groups in their chain s, which are susceptible to biodegradation to low molecular weight, nontoxic products, have also been considered for controlled drug delivery systems. Polymers possessing high strength, solvent and chemical resistance, and se rviceability at high temperatures would find a variety of uses. The challenge facing cell and molecular biologists is to decipher how cell ular events occur. A general problem encountered in many of the polymerization systems has been a difficulty in obtaining polymers of sufficiently high enough molecular weight to have reasonable strength properties. Another approach to obtain rigid polymer chains is by the synthesis of spi ro polymers. Îå¡¢±êµã Ó¢ÎÄÖÐÒ»¾ä»°½áÊøÒªÓþäºÅ×¢Òâ±ÜÃâ¹ý³¤µÄ¾ä×ÓÕâµãÓ뺺ÓïÖг£ÓöººÅµÄϰ¹ß²»Í¬£¬Çë×¢ÒâÒÔϽṹµÄ±êµã ¡¡ such as A and B ¡¡ ¡¡ such as A, B and C ¡¡ ¡¡ , such as A, ¡¡ ¡¡ , for example, ¡¡ ¡¡ , for example ¡¡ ¡¡ ,e.g., ¡¡ ¡¡ ,i.e., ¡¡ ¡¡, respectively. Áù¡¢ÕÆÎÕһЩѧÊõÂÛÎij£ÓõĶÌÓï½á¹¹¿É´ó´ó½µµÍд×÷µÄÄÑ¶È In order to meet all of these criteria, the vector must deliver the gene a ccurately. To overcome this problem, it would be advantageous to pseudotypelentivira l vectors with targeted MLV envelopes. Besides identifying suitable proteases for targeting, the success of this protease-targeting strategy relies on the accessibility of the linker to protease cleavage. We have focused our investigations on the in vitro and in vivo biological response of a mouse cell line. On the contrary, the homotelomer derivative 11 exhibits no substantial los s of cytotoxicity compared to free Ara-C. After more than three decades of development, in which numerous polymers have been used to replace body parts, clinical success is still relatively rare. In some cases, polymeric materials for drug delivery must satisfy addition al requirements, such as environmental responsiveness (e.g. pH- or temperature dependent phase or volume transformations.) Owing to the nature of these interactions, surface modification strategies have been used to optimize specific surface properties. Several approaches to surface modification exist. In general, both chemica l and physical modifications of the polymeric surface may significantly increase the biocompatibility. A host of new polymers have been developed specially for surgical applicat ions, particularly drug delivery. For roughly the past three or four decades, biology has been undergoing a revolution toward a quantitative mechanistic understanding of cell and organism behavior. They offer improved mechanical properties and ease of synthesis. Specific gel systems are described in more detail in the following sections. Perhaps the strongest motivation for polymeric delivery systems is that many of the more recently discovered molecules require delivery at a very localized level due to their mechanism of action. A particular challenge in addressing materials issues for tissue engineeri ng is that the biological processes are not yet understood well enough to allow a clear set of design parameters to be specified a priori. Over $20 billion is spent annually on therapeutic management of CHF treatm ents alone but the overall outcome is not highly successful. Unless tissue-specific gene targeting techniques have been developed, intr avenous injection of recombinant viral vectors is unlikely to be the choice of gene delivery due to systemic infection and low efficiency of gene transfer. In most instances, there is some molecular weight range for which a given polymer property will be optimum for a particular application. The control of molecular weight is essential for the practical application of a polymerization process. Solid polymers differ from ordinary, low molecular weight compounds in the nature of their physical state or morphology. On the other hand, the highly flexible chain leads to very low values of Tg and Tm. Molecular symmetry of polymer chains also plays an important role in deter mining Tg and Tm. The reason for this is that the extent of crystallinity developed in a pol ymer is both kinetically and thermodynamically controlled while the melting temperature is only the rmodynamically controlled. This explains clearly why even the moderately high degree of syndiotactici ty present in a polymer such as PMMA is not enough to allow it to crystallize extensively. Polymers vary widely in their mechanical behavior depending on the degree of crystallinity, the degree of crosslinking, and the values of Tg and Tm. The general order of thermodynamic stability of different sized ring struc tures is, thus, given by 3,4,8 to 11 < 7,12 and larger < 5 < 6 There are two important aspects with regard to the control of molecular we ight in polymerizations. The rate constants for these reactions are orders of magnitude greater tha n those for the corresponding reactions of the diacid or diester reactants. The discussions until this point have been concerned with the polymerizati on of bifunctional monomers to form linear polymers. Of the polymers discussed in the previous section, only the polyimides hav e been far into the desired ¡ This corresponds to a two- or three-fold rate increase for a 10¡ãC tempera ture increase. While the H values vary over a wide range for different monomers, the S va lues are insensitive to structure ¨Cbeing relatively constant within the range of 20-30cal/K mol. Some polymerizations show a rapid rise in Rp followed by a rapid decrease to the steady-state polymerization rate. Emulsion polymerization refers to a unique process employed for some radic al chain polymerization. To a large extent, the molecular weight and polymerization rate can be var ied independently of each other. The system undergoes a very significant change after only a few percent of the total monomer has been converted to polymer. As a consequence the monomer droplets are no longer stable and will coales ce if agitation is stopped. Each of the curves in Fig.1 is relatively typical of emulsion polymerizati on behavior. The mechanism is not well understood due to the experimental difficulties involved in the study. The monomers have been arranged in Table 1 in their general order of react ivity. The classification of a ring-opening polymerization as a chain or step pol ymerization can be made on two bases. 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