第三章 天然大分子材料化学.ppt

1、天然大分子材料化学,纤维素 木质素 淀粉 天然胶 其他天然大分子,纤维素（Cellulose）,纤维素是资源丰富的天然高分子化合物(830 million tons)，主要来源于棉花和木材，除了棉花中的长纤维可以直接纺制成织物外，棉花短纤维和木材中纤维素必须经过适当的化学反应后才能形成有用之物 由其结构式可见，纤维素的每个结构单元含有三个羟基，故纤维素有很强的氢键，结晶度(60-80%)也很高，所以天然纤维素加热直至分解也不熔融，难于加工。但可以利用这些羟基的化学反应，如酯化、醚化等，破坏氢键，改变其性能,纤维素的溶解（粘胶纤维）,纤维素通过化学反应变成可溶性黄原酸衍生物，然后纺丝或成膜 2，

2、3和6位上的羟基都有可能起黄原酸化反应，实际生产中黄原酸化程度为每个重复单元大约0.5个黄原酸基团，这可足以形成纤维素溶液,纤维素的降解（degradation）,氧降解（Oxidative degradation） 碱降解（Alkaline degradation） 酸水解（Acid hydrolysis）,Oxidative degradation,Alkaline degradation,Unzipping reaction at the reducing end group leads to a weight loss, but not a loss in DP, continues

3、until it is stopped by an irreversible stabilization reaction chain scission,Enolization 烯醇化,The generally accepted mechanism for alkali catalyzed cleavage of the glycosidic linkages in nonoxidized cellulose has been derived mainly from studies of model compounds, I.e. various -D-glucoopyranosides.

4、The proposed mechanism is shown below,Acid hydrolysis,A slow scission of the glycosidic linkage is the rate-determining step Attack of the linkages occurs primarily in the disordered regions The decrease in DP is limited (length of cellulose crystallite),Cellulose derivatives,Cellulose ester Cellulo

5、se ethers Cyanoethyl（氰乙基） cellulose Deoxy （脱氧） cellulose Cellulose carbamates（氨基甲酸酯） Cellulose xanthate（黄酸盐） Cellulose sulfate esters,Cellulose esters,Inorganic esters(cellulose nitrates, cellulose phosphate) Esters of organic acids,Cellulose Triacetate（三乙酸纤维素） Secondary cellulose acetate （二乙酸纤维素） C

6、ellulose monoacetates（乙酸纤维素） Cellulose butyrate （丁酸纤维素） Mixed cellulose esters,纤维素的酯化（esterification）,纤维素的酯化产物有醋酸纤维、醋酸-丙酸纤维、醋酸-丁酸纤维和硝化纤维等 醋酸纤维为在强酸催化剂（如硫酸）存在下，在醋酸和醋酐混合液中进行乙酰化的反应，如下所示 控制适当的条件，可得到三醋酸纤维素。部分乙酰化纤维素是将三醋酸纤维素部分水解间接制成 酯化纤维素可以作为热塑性塑料，能用模压、挤出等方法加工，醋酸纤维素可用作电影胶片、涂料、塑料制品，但用量最大的是作人造纤维（又称人造丝）,Cellul

7、ose ethers,Methyl cellulose（MC，甲基纤维素） Ethyl cellulose（EC，乙基纤维素） Carboxymethyl cellulose（CMC，羧甲基纤维素） Hydroxyethyl cellulose（HEC，羟乙基纤维素） Hydroxypropyl cellulose（HPC，羟丙基纤维素） Carboxymethyl hydroxyethyl cellulose（CMHEC） Hydroxy ethyl methylcellulose（HEMC） Hydroxy ethyl ethyl cellulose（HEEC） Hydroxy propy

8、l methyl cellulose（HPMC） Hydroxy butyl methyl cellulose（HBMC）,纤维素的醚化（etherification）,可由纤维素与氯代烷在氢氧化钠作用下，反应制得纤维素醚化产物，其中甲基和乙基纤维素最为重要，其反应式如下 乙基纤维素具有耐化学试剂、耐寒、不易燃，对光与热较稳定以及能溶于廉价溶剂等优点，故可广泛地用于涂料、清漆、乳化剂、上浆剂、上光剂和粘合剂等 CMC,By-product,HEC HPC CMHEC,Cyanoethyl cellulose,Cellulose readily reacts with olefins conta

9、ining polar substituents The reaction proceeds particularly smoothly where R is H and Y, is a strongly electron-withdrawing group shuch as CN, -CONH2 or SO2R,Deoxy cellulose,Most involve displacement of a good leaving group,e.g., a tosylate（甲苯磺酰基） or mesylate（甲磺酰基）group by a halide, nitrate or sulfa

10、te via an SN2 nucleophilic substitution type reaction Halodeoxy celluloses are prepared by the nucleophilic displacement of a good leaving group by halides in a variety of solvents,X= F, Cl, Br or I,Cellulose carbamates,The reaction medium is usually pyridine but homogeneous reactions have been rece

11、ntly descibed. The degree of substituion (D.S.) of 3 is easily obtainable Reaction of cellulose with diisocyanates have been used to crosslink cellulose,Cellulose xanthate,The formation of cellulose xanthate is employed in the viscose rayon process. Commerically, a vapor phase process is used The D.

12、S. Values are low (ca. 1-1.5) as this is sufficient for solubilization of the xanthate in aqueous NaOH for the wet spinning of viscose rayon fibers,Reduction of cellulose,The most convenient method for the reduction of aldehydes and ketones is to use metal hydrides such as lithium aluminum hydride a

13、nd sodium borohydride. The carbonyl group is reduced to give primary or secondary alcohols As a result this procedure can only be applied to cellulose that have been oxidized to give aldehyde or carboxylic groups,Cellulose graft copolymer,The first method involves reacting the cellulose (via its OH

14、groups) with a performed polymer having a reactive end group In the second method, an active species (free radical or ion) is generated on the cellulose backbone, which initiates the polymerization of monomers,木质素（Lignin）,在植物界中是仅次于纤维素的天然有机高分子 其不溶于水，只能溶于强碱与亚硫酸盐溶液中 属芳香族类，其中含有羟基、甲氧基和羰基等,Lignin biopolym

15、er,Alkylation and dealkylation（烷化和脱烷） Methylolation（羟甲基化） Amination（胺化） Carboxylation and acylation（羧化和酰化） Halogenation and nitration（卤化和硝化） Hydrogenolysis（氢解） Oxidation and Reduction（氧化还原） Sulfomethylation（磺甲基化） Sulfonation（磺化） Nitroxide formation（硝基氧化） Silylation（硅烷化） Phosphorylation（磷酸化） Grafting and Oxyalkylation（接枝和烷氧基化）,Alkylation,Reaction with diazoalkanes（重氮烃类） Reaction with alcoho