Brief review of recent development of polymer chemistry in China*

Feng Xinde
(Department of Polymer Science & Engineering, Peking University, Beijing, 100871, China)

Received Aug. 28, 1999
*The Chinese version of this paper was published in Chemistry (Huaxue Tongbao), 1999, 62 (10): 1.

Abstract The development of the polymer chemistry in China in the last two decades is briefly reviewed. This article includes the following: vinyl radical polymerization, photo-induced vinyl polymerization, olefin polymerization with Ziegler-Natta catalyst and metallocene catalyst, ring-opening polymerization of cyclic ethers, lactone and lactite with rare-earth and other organo-metallic catalysts, synthesis of condensation polymers PEEK and heterocyclic polymers with high temperature resistance, fluoro-containing polymers, biomedical polymers, electron conductive polymers, and liquid crystalline polymers.
Keywords Vinyl radical polymerization and photo-induced vinyl polymerization, Ziegler-Natta and metallocene olefin polymerization, High temperature polymers, Fluoro-containing polymers, Functional polymers

Polymer science in China was born and developed after the birth of the new China, a review of which was given in the Proceeding of the Celebration of 50^th Anniversary of Chinese Chemical Society^[1]. Polymer chemistry research involves various polymerization reactions and processes of synthetic polymers. In addition to the production of engineering plastics, rubbers, fibres, coatings and adhensives, there have been major developments in the fields of engineering plastics, composites and special polymers, and functional polymers since the 80's. The research field is therefore expanded. This article emphasizes on free radical polymerization of olefins, photo-induced initiation and photo-curing polymerization, Ziegler-Natta (Z-N) coordination polymerization and metallocene-catalyzed olefin polymerization, ring-opening polymerization of cyclic ethers, lactone and lactite and polymerization mechanism, with a brief review of PEEK obtained through condensation polymerization, heat-resistant heterocyclic polymers and fine polymers including fluoro-containing, biomedical, electron conductive, and liquid crystalline polymers

1. OLEFIN MONOMERS AND POLYMERIZATION OF a -OLEFINS
Olefin monomers such polar monomers having vinyl structure can undergo free radical polymerization, while ethylene and α-olefins mainly undergo coordination polymerization, and dienes such as butadiene and isoprene can undergo not only free radical but also coordination and anion polymerization. The five-year NSFC-supported Major Project "Polymerization of Olefins and Dienes-Mechanism, Kinetics and Structural Regulation" was completed in April, 1992, which was led by Xin-De Feng and Jia-Cong Shen and carried out by researchers from several universities and institutes including Peking University, Jilin Univ., Zhongshan Univ., Changchun Institute of Applied Chemistry of Chinese Academy of Sciences, Zhejiang Univ., Shanghai Jiaotong Univ. and Fudan Univ.^[2] The major results from the project were reported in IUPAC International Symposium on Olefin and Vinyl Polymerization and Functionalization Reaction, Mechanism and Industrial Application held in Hangzhou on October 14-18, 1991.^[3] They include: raising olefin and diene free radical polymerization and coordination polymerization reaction theory to the molecular design standard, reaching the goal of controlling polymerization reaction and product structure; inventing a number of highly active initiator systems for free radical polymerization, charge transfer photopolymerization systems and highly efficient supported catalytic systems for coordination polymerization,^[2] developing new polymerization processes, enlarging the scope of monomers, developing new kinds of polymers, elucidating polymerization reaction mechanisms, calculating various polymerization reaction rate constants by following the polymerization process through ESR,^[6] formulating kinetic theory, developing product structure control methods, and providing theoretical basis and basic data for polymerization reactions and optimization of polymer products. The project produced more than 220 papers and won many national and provincial awards.

2. SYNTHESIS OF POLYOLEFINS^[3,7-13]
Ethylene slurry polymerization using highly efficient supported titaniun catalysts has been studied in our country (Beijing Chemical Engineering Institute, Zhongshan University) since the 70's, and the obtained polyethylene with high molecular weight (2´ 10⁶-5´ 10⁶) is a good engineering plastic. Around the 80's, the propylene bulk polymerization was carried out in laboratories and on a pilot scale (Beijing Chemical Engineering Institute, Institute of Chemistry of Chinese Academy of Sciences, and Zhongshan University). The ethylene, propylene and long chain olefins were synthesized using supported titanium catalysts (Zhejiang University, Institute of Applied Chemistry of Chinese Academy of Sciences). During the middle and latter parts of the 80's, the LLDPE obtained by ethylene and butene copolymerization in gas phase was launched at Zhongshan University and Shanghai Petroleum Chemical Institute. Rare earth catalysts had been developed for synthesis of cis-polybutadiene and polyisoprene.
    Since the 90's, large-size spherical polypropylene particles have been synthesized using new catalysts ( Beijing Chemical Engineering Institute) and butadiene polymerization studied by use of the Mo, Fe-based catalyst systems (Qingdao Chemical College). In late 90's, metallocene catalysts (Zr and Ti system) are widely used in China for polypropylene and iso-, syndio- and atactic polypropylene production, with the products having narrow molecular weight distribution (Institute of Chemistry of Chinese Academy of Sciences, Institute of Petroleum Chemical Science, Zhejiang University, Zhongshan University). The metallocene-polyolefins with wide or double peak molecular weight distribution were also reported (Institute of Applied Chemistry, Chinese Academy of Sciences). The engineering plastic syndiopolystyrene and thermoplastic styrene/ ethylene copolymer have been synthesized using half-titanllocene catalysts on a pilot scale (Zhongshan University, Shanghai Petroleum Chemical Institute).
    Recently several universities and institutes have studied post transition metal (Ni, Pd, Fe, Co, etc.) catalysts for olefin polymerization. It may well be the future of the polyolefin synthesis in our country.

3. PHOTOPOLYMERIZATION OF VINYL MONOMERS AND PHOTORESIST
The photopolymerization usually refers to the polymerization of vinyl monomers under irradiation of light in the presence or absence of an initiator. The term can sometimes be used to mean the crosslinking and the grafting reactions taking place on the macromolecule or polymer chain under exposure of light. The first reported monomer, which undergoes photopolymerization, is vinyl bromide (VB).^[15] It absorbs light with wavelength >300 nm.
    The rate of photopolymerization without an initiator is very low except for some solid state monomers. In the early 80's, Cao et. al found that the electronegative monomers such as acrylonitrile, acrylic (or methylacrylic) esters photopolymerize easily in the presence of aromatic amines,^[4] which is usually known as the inhibitor for radical polymerization, and verified that the aromatic amine forms exciplex with electronegative monomers under irradiation at first, then the charge transfer occurs from donor (aromatic amine) to acceptor (monomer) in the cage of the exciplex, and finally the proton transfer takes place immediately following the charge transfer.^[16] The generated free radicals initiate the polymerization of the monomer.
Photolithography is the key process of microelectronic technology and the photopolymer used in the conventional photolithography is known as photoresist. The photoresist film functions as etching resister. The pattern achieved in the photolithography process is based on the solubility difference of photoresist film between the exposed and unexposed areas after irradiation. In the 80's, Chinese researchers invented an all-dry pattern transfer technique, which was called development-free vapor photography (DFVP).^[17] The advantages of DFVP over the conventional process include the omission of three steps (prebaking, development, and postbaking), high resolution, and high aspect ratio. The photopolymers used in DFVP do not function as etching resister, but as photo-inducing etching accelerator. The pattern transfer of DFVP is based on the accelerator concentration difference between exposed and unexposed areas after irradiation,which causes great difference in etching rate to generate patterns. The reaction of SiO₂ with HF vapor at high temperature occurs only in the presence of catalysts known as etching accelerator.^[18]

4. POLYMERIZATION WITH RARE-EARTH CATALYSTS^[19-21]
By using rare earth compounds aboundant in China to form new type of catalysts in place of traditional Z-N catalysts, great success in the polymerization of dienes and the rubber production have been achieved in the Changchun Institute of Applied Chemistry of Chinese Academy of Sciences since the 60's. Since 1981, polymers with characteristic structures and properties have been synthesized by use of rare earth catalysts in Zhejiang University.
    Ziquan Shen et al. studied the room temperature polymerization of ethylene by using rare earth coordination catalysts, and succeeded in making highly cis-(98%-100%) polyethylene and its film with good thermal stability and antioxidation properties. Furthermore, polymerization, kinetics, mechanism, structure and property of phenyl ethylyne and the like were studied.
    Since 1985, Ziquan Shen et al. have found that rare earth coordination catalysts are not only highly efficient catalysts for the ring-opening polymerization of ethylene oxide, propylene oxide, chloro propylene oxide, propylene sulfide, chloro propylene sulfide, etc. to obtain high molecular weight homopolymers and copolymers which can not be made otherwise, but can also be used as the catalysts for ring-opening homopolymerization and copolymerization of lactide and ε-caprolactone monomers. In addition, it is found that rare earth coordination catalysts can make CO₂ and alkene oxide efficiently copolymerize to make high molecular weight polycarbonates; they can also be used as catalysts for the homopolymerization of styrene and its copolymerization with acrylonitrile, maleic anhydride, and diethenyl benzene. Furthermore, being highly active, they can make polar monomers such as methyl methacrylate and n-butyrate polymerize, forming steroregular polymers.

5. SYNTHESIS OF PEEK BY CONDENSATION POLYMERIZATION^[22-25]
PEEK resin was special-purpose engineering plastic first developed in the early 80's and produced exclusively by ICI of England. In the period of Seventh Five-Year Plan, Zhongwen Wu et al. undertook the task of developing PEEK resin and finished the laboratory research work. In the period of Eight Five-Year, experimental product line was begun. Major performance parameters of self-made PEEK all reach the standards of similar products abroad. The goals for the Ninth Five-Year Plan is to complete the pilot test on a production scale of 30 T/Y, and this has been finished with the selling price lower than that of international market. Meanwhile, achievement has also been made in the study of condensation polymerization mechanism and structure-property relations. In addition to PEEK, new kinds of polymers have also been developed such as PEEKK, PEBEK, cyclic polyacryl etherketone, fluoro-containing polymers, and liquid crystalline polymers.

6. THE SYNTHESIS OF HEAT-RESISTANT HETEROCYCLIC POLYMERS^[26-28]
The work of Fengcai Lu and co-workers on heterocylic polymers covers: in basic research, they designed synthetic routes from considerations of molecular structures, improved synthetic methods and synthesized new monomers and more than one hundred new heterocylic polymers including polyphenylquinoxaline, polyphenyl-1,2,4-triazine, poly-1,3,5-triazine polypyrrolone, polyimide, polyamide-imide and poly-Schiff base containing bisthiazole rings. Regarding the polymer property, they examined high temperature water-resistant property, the catalytic activity of polymer complexes, ferromagnetism of chelate polymer, electrical conductivity, ultrafiltration property, pervaporization property, gas separation property, adsorbability of noble metals, reversible photochromism and lyotropic behavior, etc. On the mechanistic aspect, they proved through quantum chemistry calculations and experiments in their study of high temperature hydrolytic reaction that the well known heat-resistant polyimide (Du Pont Kapton-H film) becomes completely hydrolyzed, while polyphenylquinoxaline possesses outstanding high temperature hydrolytic resistance, thus showing that the mechanism of high temperature hydrolysis of heat-resistant polymers differs from that of thermal decomposition.^[26] They also studied the high temperature hydrolysis and electronic structure characterization of heat-resistant heterocyclic polymers. By using a simple method -- pyrolysis at 1200^oC in nitrogen, several good electrical insulating heterocyclic polymers were converted into conductors with a room temperature electric conductivity reaching 10² S/cm. For the first time, they employed high resolution pyrolysis-gas chromatograph/mass spectrometry and thermogravimetry to examine the composition and distribution of thermal degradation products of four polyphenyl-1,2,4-triazines and eight polypyrrones in relation to their structures, and studied the mechanism of their thermal degradations.
    Four such studies were supported by the NSFC. Three of them have been listed as research achievements in the Annual Report (1995, the Science Press). Their study also yielded numerous materials including new ablation resistant laminates, high temperature-high pressure water resistant electrical insulating polymers, vacuum deposition of metal on resin coatings, radiation resistant materials, heat-resistant gas separation films, etc. Lu and co-workers have received many awards for their great contribution.

7. FINE POLYMER---FLUORINE--CONTAINING POLYMERS
The study on organofluorine chemistry in China was started in the late fifties in order to meet the defense needs. At first, efforts were made on polymers of tetrafluoroethylene and 1,1-difluoroethylene, as well as their copolymers with other monomers, including the syntheses of monomers, polymerization, the characterization and processing of polymers etc., and various fluorine-containing plastics and elastomers were made for a wide range of applications. The synthesis of perfluoroalkanesulfonic acid resin was then studied.
    In recent years, fluorine-containing polymers such as water-soluble polymers, polyacrylates and aromatic polyesters have been studied, and studies on polymeric self-assembly has become one of the focuses in polymer science. Based on the fact that fluorocarbon chains possess higher hydrophobicity than the corresponding hydrocarbon analogs, introduction of a small amount of fluorocarbon-containing comonomer into the hydrophilic backbone of water-soluble polymers induces stronger association between fluorocarbon groups, which thus imparts unique rheological properties as well as other advantages to these hydrophobically modified water-soluble polymers. A variety of fluorocarbon-containing hydrophibically modified water-soluble polymers have been synthesized, including ionic, nonionic, and fluorocarbon group end-capped ones. Characterization of these polymers by use of viscosity, light scattering, and fluorescence spectrum confirms the fact that very strong association between fluorocarbon chains exists in these polymer solutions.
    With perfluoroalkylation reaction initiated by sodium dithionite being used as a key step, some new fluorine-containing monomers such as fluorine-containing acrylates and polyfluoroalkyl substituted hydroquinones were synthesized. The polymers obtained from the polymerization of polyfluoroalkyl substituted hydroquinones and terephthaloyl chloride showed better thermal stability and improved processing properties.^[30,31] Sodium dithionite can also initiate the polymerization of a ,b -diiodoperfluoroalkanes and dienes to yield the corresponding polymers.

8. FINE POLYMER---BIOMEDICAL POLYMER
Since the late 70's, Peking University and Nankai University have started researches in the field of biomedical polymers. In the period of Ninth Five-Year Plan, a NSFC-supported major project had been organized and headed by Bing-Lin He and Ren-Xi Zhuo, with much work done in the field and excellent research results obtained. The "Biomedical Polymer" project won the First Prize of Science and Technology Progress Prize of the Ministry of Education (1998). For instance, Xin-De Feng et al. designed and synthesized a series of segmented poly (ether urethanes) and their ceric ion-initiated graft copolymers.^[32] These polymers show good nonthrombogenic property; A kind of biodegradable polymer with zero-order degradation kinectics was prepared by ring-opening copolymerization of DL-lactide and e -caprolactone. The copolymers thus obtained can be used as drug controlled release materials.^[33,35] Based on the principle of molecular recognition, Bing-Lin He et al. designed and synthesized polymeric absorbents for blood purification. Such absorbents can be used not only to clean up liver-failure,^[35] renal-failure and endogenous substances accumulated in the bodies of patients suffering from autoimmune disease through hemoperfusion,^[36] but also for the treatment of patients of hypnotics toxication (over 1000 cases of cure in clinical trial); In the research on the modification and immobilization of enzymes with natural or synthetic polymers, some efficient reaction methods have been developed which can enhance the loaded level and at the same time maintain the high bioactivity.^[37] Ren-Xi Zhou et al. not only designed and synthesized a wide variety of biodegradable polyphosphates for controlled drug delivery, but also developed 4-dimethylaminopyridine (DMAP) as a catalyst for phosphorylating polycondensation in solution to increase the polymerization rate and the average molecular weight of the polyphosphates.^[38] It was found that enzyme-catalyzed ring-opening polymerization of cyclic phosphate can be achieved by using lipase, such as porcine pancreas lipase.^[39] The immunological properties of some phosphorous-containing polymers were also investigated for the first time.^[40]
    Si-Cong Lin et al. proposed the "Maintaining Normal Conformation Hypothesis" for the design of the surface structure of nonthrombogenic materials, and developed surface grafting reactions for polyurethane, polysiloxane and polyalkene. Many new materials with good nonthrombogenic properties have been synthesized.^[41] All of the research achievements mentioned above have been well received internationally. But most importantly, it laid down a sound foundation for the development of biomedical polymers in our country. After the Biomaterial Congress convened in Japan, an international Post-Symposium on Polymeric Biomaterials was held in Kunming, China in 1988. In addition, several international symposiums on biomedical polymers have been held in Tianjin, Guilin, Wuhan, Kunming and Xian in the 80's and 90's.
    At present, there are ten more universities and research institutes engaged in studies on biomedical polymers in our country. Their research interests cover almost all aspects of the field.

9. FINE POLYMERS---CONDUCTING POLYMERS^[42-46]
The research on conducting polymers in China began in 1979. The main polymers studied were polyacetylene (PA) and polypyrrole (PPy). Renyuan Qian, Fosong Wang, et al. prepared PA with regular molecular and crystalline structures by means of rare earth catalyst and observed single-crystal electron-diffraction patterns from the selected regions of PA films prepared. Wang Fosong, Wang Lixiang, et al. studied the syntheses, doping and conducting properties, processing and practical applications of PAn. Soluble PAn was successfully prepared by controlling the post-treatment conditions of the ammonium persulfate process. The improved process was patented in China. In 1996, a catalytic polymerization process was developed in which H₂O₂ was used as oxidant and iron ions as catalyst.
    The ability to undergo protonic-acid doping (that is, it obtains conductivity upon reacting with an acid and becomes an insulator upon reacting with a base) is one of the important features of PAn. The doping and dedoping reactions can be carried out in aqueous, organic and gas systems, and are reversible. In 1989, Xiabin Jing et al. proposed a "Four-Ring BQ Derivative" model to describe the structure of the doped PAn. According to this model, the protonation of PAn takes place on the imine nitrogen, the positive charge carried by the proton is delocalized over 4-monomer units, and the repeating unit is composed of three kinds of benzene rings and two kinds of nitrogen atoms. This model was further confirmed by the high resolution NMR spectra of poly(2,5-dimethyl aniline) both in doped and dedoped states^[44] obtained in 1997.
    Wang Xianhong et al. prepared "self-doped Pan" by the sulfonation of PAn itself. It exhibits reasonable conductivity under nearly neutral environment. By using organic acids as doping agent, the doped PAn showed improved solubility in organic solvents and in other polymers. The PAn doped by phosphonic acids or sulfonic acids with polyoxyethylene chains can dissolve in aqueous systems^[45]. Taking advantage of the improved solubility of the doped PAn, blends of PAn with non-polar polymers or water-soluble polymers were prepared and they showed conducting threshold below 2%. Among them, the melt-blends of doped PAn with polyethylene and ABS having a conductivity of about 10^-6 S/cm can be used for anti-static purpose. The coating materials prepared by blending undoped PAn with epoxy resin or by blending doped PAn with polyurethene exhibited an anti-corrision ability comparable to zinc rich primer. A new function of PAn , i.e., the ability to prevent the adhesion of marine organism has been discovered recently, thus making it possible to use PAn as coaing materials for ship.
    Renyuan Qian, Yongfang Li, et al.^[46] studied the influence of solution pH and supporting electrolyte on the polymerization of pyrrole, and proposed a modified mechanism for pyrrole polymerization, i.e., the pyrrole monomer undergoes protonation before forming a cationic free radical and the anions in the solution are involved in the polymerization reaction. Based on the above mechanism, a kinetic equation was derived for the electrochemical polymerization of pyrrole. By examining the electrochemical redox behavior of PPy under various pH's and the changes in mass, size and conductivity of PPy after base or water treatment, they suggested that there are two different doping mechanisms for PPy, redox doping and protonic acid doping by which two corresponding structures of doped PPy are formed.

10. FINE POLYMER---LIQUID CRYSTALLINE POLYMER
The research on liquid crystal polymers in China began in the early 1970's. The First National Symposium of Liquid Crystal Polymers was held in Shanghai in 1987, which showed that the research on liquid crystal polymers in China had advanced to a high level. Since then the symposium has been held for six times. The First International Conference of Liquid Crystal Polymers (IUPAC) was held in Beijing in 1994. During the past more than twenty years, there have been a strong row of scholars and devoted researchers who concentrate their efforts on liquid crystal polymers and some creative and breakthrough work has been done. Qi-Feng Zhou, et al. designed and successfully synthesized a series of novel liquid crystal polymers, i.e., mesogen-jacketed liquid crystal polymers, which are of the side chain type but with mesogenic groups attached laterally to the main chain without or with only short spacers.^[47-49] With such a molecular constitution the mesogenic units would form a "jacket" around each chain backbone because of their high population in the backbone, and because they are bulky and rigid, the jacket would in turn force the main chain to extend thus showing much higher rigidity than otherwise. The aim of these efforts is not only to prepare a third class of liquid crystal polymers that can be polymerized by chain polymerizations as for many conventional side chain type polymers, but also to make them have the properties of rigid or semi-rigid polymers as represented by main chain liquid crystal polymers. In addition, Zhou et al. has prepared high molecular weight PPTA and studied the synthesis of polyesteramide,^[50] while Hongzhi Zhang et al. and Qixiang Zhou et al. have synthesized a series of new thermotropic copolyesters.^[51-53]

Acknowledgement: Much help in the first draft from my colleagues is greatly appreciated. We thank Prof. Dr. G.Y. Wei for his final English compilation of this article.

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