QUESTION 5: QUESTION 5: BASE ON CLASSIFICATION OF CHEMICAL COMPOUND OPINION: There are no
II.2. CÁC PHƯƠNG PHÁP TỔNG HỢP POLYME
II.3.8. HÌNH THÁI CẤU TẠO (HIỆN TƢỢNG ĐỒNG PHÂN-Isomerism)
II.3.8.1 Định hướng monome
II.3.8.2 Đồng phân không gian
II.3.8.3 Đồng phân hình học
DEFINITION:ISOMERISM
Two molecules are said to be isomers if they are made up of the same number and types of atoms, but differ in the arrangement of these atoms.
II.3.8.1. Định hướng monome
Phân tử etylen
Monome vinyl (Stereoisomerism in Vinyl Polymers):
R: Halogen, alkyl, nhóm chức hóa học...
Trong các polyme từ những monome trên, các gốc thế (R) có thể phân bố theo nhiều cách trên mạch cao phân tử tuỳ thuộc vào thứ tự kết hợp:
H
H H
R C C
β α
II.3.8.1. Định hướng monome
Các phân tử monome có thể kết hợp với nhau theo kiểu đầu nối đầu (, ):
nCH2 = CH CH2 CH CH CH2 CH2 CH CH CH2 R R R R R
Hoặc theo "đầu nối đuôi" (,):
nC H 2 = C H C H2 C H C H2 C H C H2 C H R R R R
Các phân tử monome có thể kết hợp với nhau một cách ngẫu nhiên, lúc thì "đầu nối đầu" lúc thì "đầu nối đuôi“
Trong đa số các polyme, hình thái sắp xếp "đầu nối đuôi" chiếm ƣu thế.
CH2 CH CH CH2 CH CH2 CH CH2 CH CH CH2 R R R R R
II.3.8.2. Đồng phân không gian
Polymerization of a vinyl monomer, CH2=CHX, where X may be a halogen, alkyl or other chemical group (anything except hydrogen!) leads to polymers with microstructures that are described in terms of tacticity.
• These arrangements are called racemic and meso
• Imagine the backbone carbon atoms of the chain to be arranged in the form of a zigzag, all.in the plane of the page; we call a pair of adjacent units (diad) where the substituent (X) is in one unit above the
plane of the page and in the other below the plane of the page, a racemic diad.
• If in the two units, the substituent groups are both on the same side of the chain, then we call it a meso
diad.
• What do we call polymers made up of such regularly arranged units?
• Not meso and racemit polymers, but isotactic and syndiotactic!
Khi tất cả các nhóm thế R nằm cùng một phía của mạch, polyme đƣợc gọi là isotactic:
Note: An isotactic chain is composed of 100% meso diads
Nhóm thế R nằm cách đều theo hai phía của mạch ta có polyme syndiotactic:
Note: A syndiotactic chain is composed of 100%
racemo diads
Khi nhóm thế R sắp xếp hoàn toàn ngẫu nhiên polyme đƣợc gọi là atactic
II.3.8.3.Đồng phân hình học
Đồng phân cis của polyizopren (cao su):
Cấu trúc trans của polyizopren là guttapecsa
CH3
H
C C
CH2 CH2
n
CH3
H
C C
CH2
CH2
n
II.3.8.4. Phổ và đặc trƣng cấu trúc của mạch phân tử polyme dạng vinyl
How do you know if a vinyl polymer is isotactic, or atactic, or whatever?
The answer will be talked in this part!!!
Now is a good point to discuss the determination of chain microstructure by spectroscopic methods.
The techniques we will discuss: nuclear magnetic resonance spectroscopy
We will focus on the basic
The interaction of light with with matter
When a beam of light is focused on a sample a number of things can happen.
•
matter
It can be reflected, or if the sample is transparent to the frequency or frequencies of the light, simply transmitted with no change in energy.
Some of the light can also be scattered and some absorbed;
•
• Light is only absorbed if its energy corresponds to the energy difference between two quantum states of the sample
The relationship between this energy difference and the frequency of the absorbed light being given by the Bohr frequency condition:
Where h is Planck's constant and v is the frequency of the light in cycles/sec (Hertz, Hz).
Isotactic form, where groups of the same type are (stereochemically) on the same side of the chain.
Here it is easy to see that the two hydrogen atoms of the CH2 groups (the little balls) are not magnetically equivalent.
One is always in the environment of- two methyI groups, while the other lies between two ester groups
In contrast, for a purely syndiotactic polymer, the CH2 protons are magnetically eqivalent, each being flanked on one side by a methyl group and on the other by an ester.
Naturally, atactic polymers have both types of CH, units.
Schematic diagram of different poly(methyl methacrylates).
•
How do you know the composition and sequence distribution of monomers in a copolymer?
Two isolated NMR lines near 3.6 and 3.9 ppm, assigned to -OCH3 and –OCH2,- groups, respectively, can be used for analysis.
Divide the areas of the two NMR lines by 3 and 2, respectively, and the copolymer composition then follows directly from Equation