Optical Activity in Polymers

Optical activity in biopolymers has been known and studied well before this phenomenon was observed in synthetic polymers. Homo polymerization of vinyl monomers does not result in structures with asymmetric centers (The role of the end groups is generally negligible). Polymers can be formed and will exhibit optical activity, however, that will contain centers of asymmetry in the backbones [73]. This can be a result of optical activity in the monomers. This activity becomes incorporated into the polymer backbone in the process of chain growth. It can also be a result of polymerization that involves asymmetric induction [74, 75]. These processes in polymer formation are explained in subsequent chapters. An example of inclusion of an optically active monomer into the polymer chain is the polymerization of optically active propylene oxide. (See Chap. 5 for additional discussion). The process of chain growth is such that the monomer addition is sterically controlled by the asymmetric portion of the monomer. Several factors appear important in order to produce measurable optical activity in copolymers [76]. These are: (1) Selection of comonomer must be such that the induced asymmetric center in the polymer backbone remains a center of asymmetry. (2) The four substituents On the originally inducing center on the center portion must differ considerably in size.(3)The location of the inducing center mustbeclose tothe polymer backbone.(4)Thepolymerizationreaction mustbe conducted at sufficiently low temperature to insure stereo chemical selectivity. An example is a copolymerization of maleic anhydride with optically active l-a-methylbenzyl vinyl ether. The copol ymer exhibits optical activity after the removal of the original center of asymmetry [77]. An example of an asymmetric induction from optically inactive monomers is an anionic polymerization of esters of butadiene carboxylic acids with (R)-2-methylbutyllithium or with butyllithium complexed with (-)methyl ethyl ether as the catalyst. (This type of polymerization reaction is described in Chap. 4) The products, tritactic polymers exhibit small, but measurable optical rotations [78]. Also, when benzofuran, that exhibits no optical activity, is polymerized by cationic catalysts like aluminum chloride complexed with an optically active co catalyst, like phenylalanine, an optically active polymer is obtained [77]. By contrast, an example of formation of enantioselective polymer from achiral monomers, where the chirality is inherent in the main chain is polymerization of 1,5-hexadiene with an optically active catalyst [77]. The catalyst precursors are (R,R)or (S,S)-[ethylene-1,2-bis(Z5-4,5,6,7-tetrahydro-1 indenyl)zirconium (1,10-by-1-naphtholate). The product is an optically active version of poly (methylene-1,3-cyclopentane):

This polymer is highly isotactic and contains 72% trans rings. n It was also observed that conjugated polymers that are also electrical conductors (see Chap. 10) exhibit optical activity that depends critically on their structural organization [78]. Thus, strong chiroptical properties can be obtained from substituted polythiophene [79] (Chap. 10) with optically active side chains, especially when the monomers are coupled within the polymer in a regio regular head-to-tail fashion. Actually, optical activity of these materials is only found when the polymers are aggregated at low temperature, in poor solvent, or in solution cast films. This contrasts with other optically active polymers, like polypeptides, poly(l-alkynes) and poly isocyanates that show an optically active conformation of the main chain in the absence of supramolecular association. In addition, it was shown that a repetitive inversion of optical activity in films can be obtained by warming and cooling cycles, where the cooling rate determines the handedness of the associates [78]. A similar result concerning inversion of chirality has been found in solution, depending on the composition of a binary solvent mixture.

اخر الاخبار

اخبار العتبة العباسية المقدسة

المجمع العلمي يقيم محطة قرآنية على طريق الزائرين بذكرى شهادة الإمام الكاظم (عليه السلام) في واسط

قسم العلاقات العامّة ينظّم برنامجًا ثقافيًّا لأكثر من 100 طالب من حفظة القرآن الكريم في النجف

قسم شؤون المعارف يختتم دورة حول تحقيق المخطوطات في جامعة كربلاء

قسم الشؤون الطبية يطلق برنامجًا تدريبيًّا لملاكاته بالتعاون مع مستشفى الكفيل التخصصي

المزيد

الآخبار الصحية

6 فواكة تساعدك على الشعور بتحسّن عند المرض

حفاظا على صحة العين.. هذا أفضل وقت لتناول فيتامين أ

الاستخدام اليومي لسماعات الأذن.. أخطر مما تتصور !

هل توقيت نشاطك اليومي يؤثر على صحة دماغك؟.. العلماء يجيبون

المزيد

الآخبار التكنلوجية

ابتكار ألماني قد يغيّر مستقبل صناعة البلاستيك

تحديثات وأرباح عبر المشاهدات.. ما الذي تخطط له "لينكد إن"؟

"الكوكايين الوردي".. مخدر جديد يثير قلق السلطات الأميركية

دراسة: تغيّرات في أسلوب القيادة قد تكشف مبكرا عن الخرف !

المزيد

مواضيع ذات صلة

Autoacceleration

Ceiling Temperature

Effect of Reaction Medium

Steric, Polar, and Resonance Effects in the Propagation Reaction

Propagation

Capture of Free Radicals by Monomers

Spherulitic Growth

Crystallization from Solution

Crystallization from the Melt

The Crystalline State

Rheology and Viscoelasticity of Polymeric Materials

Thermodynamics of Elasticity