全文获取类型
收费全文 | 5393篇 |
免费 | 143篇 |
国内免费 | 21篇 |
专业分类
化学 | 3751篇 |
晶体学 | 69篇 |
力学 | 73篇 |
数学 | 234篇 |
物理学 | 1430篇 |
出版年
2022年 | 28篇 |
2021年 | 32篇 |
2020年 | 58篇 |
2019年 | 63篇 |
2018年 | 51篇 |
2017年 | 34篇 |
2016年 | 86篇 |
2015年 | 88篇 |
2014年 | 115篇 |
2013年 | 255篇 |
2012年 | 250篇 |
2011年 | 266篇 |
2010年 | 154篇 |
2009年 | 158篇 |
2008年 | 284篇 |
2007年 | 239篇 |
2006年 | 261篇 |
2005年 | 248篇 |
2004年 | 243篇 |
2003年 | 191篇 |
2002年 | 200篇 |
2001年 | 150篇 |
2000年 | 160篇 |
1999年 | 89篇 |
1998年 | 62篇 |
1997年 | 66篇 |
1996年 | 64篇 |
1995年 | 42篇 |
1994年 | 67篇 |
1993年 | 90篇 |
1992年 | 96篇 |
1991年 | 74篇 |
1990年 | 64篇 |
1989年 | 56篇 |
1988年 | 76篇 |
1987年 | 73篇 |
1986年 | 74篇 |
1985年 | 105篇 |
1984年 | 94篇 |
1983年 | 39篇 |
1982年 | 69篇 |
1981年 | 74篇 |
1980年 | 67篇 |
1979年 | 60篇 |
1978年 | 52篇 |
1977年 | 64篇 |
1976年 | 52篇 |
1975年 | 39篇 |
1974年 | 49篇 |
1973年 | 30篇 |
排序方式: 共有5557条查询结果,搜索用时 15 毫秒
991.
Naofumi Nakamura Kimihiro Suzuki 《Journal of polymer science. Part A, Polymer chemistry》1997,35(9):1719-1731
A kinetic study was carried out on the acetalization reaction of 1,3-butanediol, as a model compound for poly(vinyl alcohol) (PVA), in water, under acidic conditions. Since these equilibrium constants of ketalization reaction of 1,3-butanediol and ethylene glycol are so small, the kinetic parameters were estimated from the hydrolysis reactions of the corresponding ketals. It was made clear that these reactions proceed in the reversible bimolecular reaction, and the heat of reaction and activation energy are nearly equal to that of PVA. The rate constants of hydrolysis reaction (k′s) of model compounds were calculated on the basis of value of acetone ketal, Hammett-Taft's equation log k′s/k′so – 0.54(n – 6) = ρ*σ* was established, and the value of ρ* was obtained (3.60), which coincided with the value of PVA. Therefore, it was made clear that the hydrolysis reactions of acetals and ketals are electrophilic reaction (SE II reaction) and the step of rate determination is the formation of hemiacetal and hemiketal. The rate constants of hydrolysis reaction of 1,3-butanediol acetals and ketals were approximately 10–20 larger, and those of ethylene glycol were approximetly 50–80 larger except for ketals, and those of ethanol were roughly 2000–10,000 larger compared with that of high-molecular weight compound (PVA). It can be well explained that these differences in the rate constant depend on their entropy and the mobility of molecules. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1719–1931, 1997 相似文献
992.
993.
994.
995.
996.
997.
998.
999.
Takeru Iwamura Ikuyoshi Tomita Masato Suzuki Takeshi Endo 《Journal of polymer science. Part A, Polymer chemistry》1999,37(4):465-472
The anionic polymerization of N‐acryloyl‐N′‐(4‐methylbenzoyl)urea (1) was carried out at 80°C for 24 h in DMF, DMSO, acetonitrile, or toluene by t‐BuOK or DBU (3 mol %) as an initiator to obtain polymer 3 in a good yield. The structure of 3 was dependent upon the initiator used, in which t‐BuOK selectively conducted the hydrogen‐transfer polymerization, while DBU partially induced the vinyl polymerization (16–20%). Likewise, N‐acryloyl‐N‐methyl‐N′‐(4‐methylbenzoyl)urea (2, i.e., an N‐methylated derivative of 1) was subjected to the hydrogen‐transfer polymerization. Although the yield of the polymer was lower in comparison with 1, the structure of the obtained polymer 4 was similarly governed by the initiator. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 465–472, 1999 相似文献
1000.