Video 4: What does a polymer molecule look like?
Today, we’ll talk about what a polymer chain looks like? What are the structures of polymer chains?
We have known that a single polymer chain contains many atoms linked by single covalent bonds. The single chain may kink and coil randomly by chain bond rotations. A polymer sample consists of large numbers of chains. The bend, coil, and kink of each single chain lead to extensive intertwining and entanglement of neighboring chain, like a heavily tangled fishing line. These random coils and entanglements are responsible for the large elastic extensions displayed by rubbers.
In short, the molecular shape of a polymer chain relates to the degree of chain twisting, coiling, and bending.
今天我们讲聚合物的链结构。
我们已经知道聚合物分子链由单键连接而成。单键旋转，可使单个分子链蜷缩、缠绕。聚合物中含有大量分子链，每个单链都卷曲，并与邻近链缠绕在一起，就像纠缠、乱作一团的鱼线一样。这种无序的缠结使橡胶拥有很大的弹性伸长。 简言之，聚合物分子链的形状与链的蜷缩、卷曲、缠绕有关。
The physical characteristics of a polymer depend not only on its molecular weight and shape, but also on the structures of molecular chains.
Usually, the structures include linear, branched, crosslinked, network, and various isomeric configurations. Most of the polymers are linear.
Like this, the repeat units are joined together end to end in a single chain.
The common polymers, such as PE, PP, PVC, PSt, nylon, and PET have linear structures. 聚合物的物理特性不仅与分子量和形状有关，也与分子链的结构有关。 通常，分子链可呈线性、支化、交联、网络和各种立构体结构。大多数聚合物是线性的，单链中重复单元头-头或尾-尾相连。常用聚合物，如聚乙烯、聚丙烯、聚氯乙烯、聚苯乙烯、尼龙和聚酯等都呈线性结构。
If we call –C-C- backbone the main chain.
Sometimes, there are side-branch chains connected to the main mains. This polymer is called branched polymer. Sometimes, polymers that form linear structures may also be branched.
For example, high-density polyethylene, HDPE is primarily a linear polymer, whereas linear, low-density polyethylene, LLDPE contains very short branches, and low-density polyethylene, LDPE contains relatively short branches. The formation of side branches will reduce the packing efficiency of polymer chains, and thus decrease the polymer density.
That’s why the branched polyethylene, LDPE, LLDPE has a relatively low density than the linear polyethylene, HDPE. 我们将-C-C-骨架称为主链。
有时，主链上连有侧链，这种聚合物称为支化聚合物。有时，线性聚合物也可以有支链。例如，高密度聚乙烯HDPE，通常HDPE是线性聚合物，线性低密度聚乙烯LLDPE含有很多短支链，低密度聚乙烯LDPE含有相对更短的支链。支链会影响分子链的紧密排列、堆积，从而降低聚合物的密度。
因此，与线性聚乙烯HDPE相比，支化聚乙烯LDPE, LLDPE的密度较低。 In crosslinked polymers, there are covalent bonds between linear chains at various positions.
Many synthetic rubbers are crosslinked. In rubbers, the formation of cross-linked structures is usually called vulcanization.
交联聚合物中，线性链之间，在不同位置处存在共价键。 许多合成橡胶是交联的，橡胶形成交联结构称为硫化。
In a specific polymerization, if multifunctional monomers are used, such as cross-linkers having more than 2 double bonds, highly crosslinked structures, or three-dimensional networks are formed. The produced polymers are called network polymers. These materials have quite different mechanical and thermal properties. For example, usually, natural rubber is considered as a linear polymer. It has low strength and hardness, whereas, the cross-linked rubber, vulcanized rubber is much stronger and harder.
聚合过程中，如果使用多功能单体，如含有两个以上双键的交联剂，可形成高度交联、或三维网状结构，所形成的聚合物称为网状聚合物。
这些聚合物的机械性能、热性能与线性聚合物有很大不同。例如，天然橡胶是线性聚合物，强度和硬度较低，而交联橡胶（硫化橡胶）具有很高强度和硬度。 For polymers having more than one side group or pendant group, they may have different spatial arrangement of the side group R. We call the polymer isomers, stereoisomers or geometric isomers.
For example, for polypropylene, PP, all the side groups, methyl groups are situated on the same side of the main chain. This isomer is called isotactic PP.
If the side groups alternate sides of the chain regularly, that is, one methyl group is on this side of the main chain, another is on the other side of the chain. This is a syndio-tactic PP.
Or, the side groups are positioned randomly on both sides of the main chain. This is an atactic PP.
聚合物链含有不止一个侧基时，侧基R可有不同的空间排布，这种聚合物称为立体异构体、空间立构体或几何立构体。
例如，聚丙烯，所有侧基-甲基都位于主链一侧时，称为全同聚丙烯。
侧基交替位于主链两侧时，即，一甲基位于主链一侧，而另一甲基位于主链另一侧，如此规则排布，称为间同聚丙烯。
侧基随机分布于主链两侧时，称为无规聚丙烯。
For polymers, such as polyisoprene, their repeat units have a double bond between carbon atoms. The methyl group and H atom may be positioned on the same side of the double bond. This is called a cis-structure, and the resulting polymer, cis-polyisoprene, is natural rubber. When the methyl group and H atom reside or are located on opposite sides of the double bond, the polymer is a trans-isomer.
聚异戊二烯中，重复单元中含有-C=C-双键。甲基和H位于双键同侧时，称为顺式结构，顺式聚异戊二烯是一种天然橡胶。甲基和H位于双键两侧时，聚合物为反式异构体。
Up to now, we have talked a lot about polymers composed of only one repeating unit in their chains. These polymers are known as homopolymers.
If more than one repeating units are contained in a polymer chain, these polymers are called copolymers.
For example, the copolymer of styrene and acrylonitrile, contains 2 repeating units come from styrene and acrylonitrile, respectively.
Another example is the copolymer of ethylene and propylene. It contains repeating units from monomers of ethylene and propylene, since these monomers are polymerized in the same reactor.
以上所讲聚合物分子链中都只含有一个重复单元，这类聚合物称为均聚物。 如果聚合物分子链中含有不止一个重复单元，该聚合物称为共聚物。 例如：苯乙烯-丙烯腈共聚物，含有苯乙烯和丙烯腈两重复单元。
单体乙烯和丙烯在同一反应器中聚合，得到乙烯-丙烯共聚物，该共聚物含有乙烯和丙烯两重复单元。
The repeating units on the copolymer chain may be arranged in various sequences along the backbone chain, including:
Random copolymer- The repeating units are arranged randomly on the main chain. Alternative copolymer - There is an ordered, alternating arrangement of the two repeating units along the polymer chain. The two repeat units alternate chain positions. The third one is block copolymer - The chain consists of relatively long sequences, blocks of each repeating unit. Identical or same repeat units are clustered in blocks along the chain.
The forth one is graft copolymer - Sequences of one monomer or repeating unit are “grafted” onto a backbone of polymer chain.
Besides the above, more complex structures of polymer chains, such as comb shaped, graft copolymer with block copolymer brushes, or star-shaped, dendrimer-like structures could be formed by living or controlled polymerization or other advanced techniques.
共聚物中的重复单元可沿主链以不同序列排布，所得共聚物包括： 无规共聚物：重复单元在主链上随机排布;
交替共聚物：两重复单元沿主链有序、交替地排布，即：重复单元位置交替; 第三种共聚物为嵌段共聚物，主链含有相对较长的重复单元序列，相同重复单元
集中在一起。
第四种共聚物为接枝共聚物，一种单体或重复单元序列接枝在主链上。 此外， 聚合物链还可形成更复杂的结构，如通过活性/控制聚合物或其他先进聚合方法，可获得梳形、含有嵌段共聚物刷的接枝聚合物、星形、树枝状结构等。 Briefly, the shape and structure of polymer chains affect or determine the physical characteristics of polymers. That’s why we want to design, or tailor the structure of polymer.
小结：聚合物链的形状和结构影响或决定聚合物的物理性质。正因如此，
需要设计、调控聚合物链结构。
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