#Reversible addition fragmentation chain transfer polymerization

[whats-in-a-sentence]

The concept is based on the reversible addition of radicals to carbon-sulfur double bonds in thiocarbonyl thio transfer reagents (R-S(C=S)Z (figure 26.26).

"Chemistry" 2e - Blackman, A., Bottle, S., Schmid, S., Mocerino, M., Wille, U.

[materialsscienceandengineering]

Reversible addition-fragmentation chain transfer (RAFT) ( 2 ) polymerization is a form of reversible deactivation polymerization (and therefore chain growth polymerization) wherein the molecular weight of the resulting polymers is carefully controlled through the chain transfer polymerization step. RAFT polymerization is often regarded as one of the most versatile methods for creating polymers with specifically desired structures, such as brush, comb, or star like polymers, as well as dendrimers or cross linked polymers. 

Image sources: ( 1 ) ( 2 )

[mohammedaljuaid]

Controlled radical polymerization (Brief introduction).

مقدمة

تندرج المواد البوليمرية تحت مركبات كيميائية تُسمى “

Macromolecules

" أي الجزيئات الضخمة أوال��ملاقة إلا أن البوليمرات تتميز عن بقايا الجزيئات المُدرجة تحت هذا التصنيف بأنها مكونة من جزيئات صغيرة متراصة و مترابطة بروابط تساهمية تُسمى"مونمر". التركيب الكيميائي للمونمرات يسمح لتدرج التحضيرات البوليمرية إلى صنفين:

البلمرة بالتكاثف أو البلمرة الخطية  “Step growth" 

يتميز التركيب الكيميائي للمونمرات المصنفة ضمن هذا النوع بأنها قادرة على التفاعل مع بعضها البعض لينتج البوليمر كناتج رئيسي و نزع جزيء صغيركالماء. ومثال على هذا الصنف هو تفاعل الأحماضالأمينية مع بعضها البعض وبنزع جزيء الماء من طرفي الحمض الأميني و تحضير نايلون ٦,١٠ أو٦,٦. وفي هذا النوع لا يحصل تدخل من قبل أيونات موجبة وسالبة ولا كذلك جذور حرة. 

البلمرة بالإضافة "Chain growth" 

ويلزم في التركيب الكيميائي لمونمرات أن تحتوي على رابطة مضاعفة. وجود الكترونات باي يجعلها أكثرعُرضة للهجوم من قبل الجذر الحر حيث يكون الالكترون الوحيد بالجذر رابطة سيجما قوية مع أحدى الكتروني الرابطة المضاعفة فينتج عن ذلك جذر جديد يهاجم رابطة مضاعفة في مونمر آخر و هكذا تستمرالبلمرة. 

خطوات ميكانيكية البلمرة بالجذر الحر

البدء "Initiation" .1

تُعتبر هذه الخطوة هي الخطوة الأسرع من بين الخطوات الثلاث وفيها تقوم البادئة "Initiator" بإنتاج جذور حرة بطرق مختلفة تختلف بأختلاف التركيب الكيميائي للبادئة أما بتسخين المحلول أو تسليطه بأشعةUV أو بإضافة حفاز. من المعروف بأن الجذور الحرة ذات نشاطية عالية لذلك بمجرد إنتاجها تبدأ الهجوم على المونمرات بعد الهجوم على المونمر يُصبح الناتج جذر جديداً جزيء أكبر يهجم على مونمر آخر وهكذا. 

النمو "Propagation" .2

عندما يهجم الجذر على جزيء أو اثنين من المونمر يبقى الناتج جذراً نشطاً مالم يتم تثبيط بعامل معين. هذا الجذر يستمر بالنمو عن طريق إضافة مونمر إلى سلسلته وتُسمى هذه العملية بالنمو. تختلف سرعة عملية النمو بإختلاف نوع المونمر لذلك كل مونمر يمتلك ثابت يُسمى ثابت النمو ورمزه Kp. ويُعبر هذا الثابت بأنه عدد الوحدات التي تُضاف للسلسلة النشطة في الثانية. 

الإنهاء "Termination" .3

بشكل عام هذه الخطوة تؤدي إلى توقف السلسلة البوليمرية النشطة على التمدد و إضافة وحدات مونمرية أخرى للسلسلة. لسوء الحظ هذه العملية قد تتم بأي لحظة من لحظات البلمرة ولها طُرق مختلفة أبرزها: 

الإندماج "Combination" 

وبهذه الطريقة يلتقي جذريين من سلسلتيين نشطة ويرتبطان مع بعضهما البعض برابطة أحادية. قد تكون السلسلتين لهما نفس الطول أو أحدهما أقصر. ربما يكون أحد الجذريين سلسلة بوليمر و الآخر ليس كذلك. 

 الإنتقال لمونمر أو مذيب  

وفيه يقوم جذر السلسلة النشطة بنقل الجذر إلى جزيء مونمر أو مذيب كنزع هيدروجين بكسر الرابطة الأحادية للهيدروجين بالمونمر أو المذيب والنتيجة تكون سلسلة بوليمرية مثبطة و جذر جديد يقوم بمهاجمة مونمرات أخرى. 

Disproportionation

قد تتواجد سلسلتي بوليمر نشطة إلا أن الجذريين الحريين عوضاً عن الإرتباط مع بعضهما البعض كما في تفاعل الإندماج فأن أحد الجذريين يقوم بنزع هيدروجين من ذرة الكربون المجاورة مباشرةً للجذر فينتج عن ذلك سلسلتين مُثبطة أحدهما تحتوي على ميثايل بالنهاية و الأخرى يتفاعل جذريها المجاوريين مكونان رابطة مضاعفة. 

نزع هيدروجين من منتصف بوليمر آخر H-Abstraction 

قد تقوم سلسلة نشطة بنزع هيدروجين من منتصف سلسلة بوليمر نشط آخر فتُصبح مثبطة وغير قابلة للنمو أكثر بينما سلسلة البوليمر الأخرى يُصبح لديها جذر جديد غير الجذر الطرفي فإما يستمر كلا الجذريين بالنمو أو في حال كانت المسافة بينهما كافية يُصبح تحلق و يتكون سلسلة حلقية. 

خطوة الإنهاء تمتلك ثابت يُسمى ثابت الإنهاء Kt وهويعتمد على تركيز الجذر الموجود في جميع الطرق لإنهائية التي تمت مناقشتها سابقاً لذلك بوجود هذه الطرق تُصبح إحتمالية تثبيط و قتل السلاسل البوليمرية واردة بشكل أكبر خصوصاً بالثلث الأخير من زمن البلمرة بسبب التنافسية بين Kp و Kt. 

عيوب البلمرة بالجذر الحر 

لذلك فأن البلمرة بالجذر الحر تؤدي إلى سلاسل ذات أطوال مختلفة في المحلول الواحد. كذلك يصعب التحكم بتركيب السلسلة البوليمرية بسبب نشاط الجذرالعالي و خروجه عن السيطرة. كذلك يصعب بالغالب التأكد من تواجد المجاميع الناشئة من تفكك البادئة في بداية أو نهاية السلسلة البوليمرية وإن وُجدت فلن تكون بجميع السلاسل البوليمرية. هذه المجاميع قد تكون مهمة في تطبيقات معينة. على سبيل المثال قد تحتوي على مجاميع نشطة ضوئياً تسمح بتتبعها أو مجاميع منالممكن أن تتفاعل إختيارياً مع مجاميع معينة في مواد حيوية هالجسر الكبريتي أو مجاميع الأمين في الحمض الأميني لايسين أو بالكحولات في السكريات أو مجاميع تسمح بربط بوليمر A ببوليمر B دون الخوض في تفاعلات الجذور. وجود عينة بوليمر تحتوي على سلاسل بوليمر بمدى أطوال كبير يجعلها غير مناسبة للتطبيقات الطبية الحيوية لأن كل مدى داخل هذا المدى الواسع له خصائص مختلفة. 

ويتضح من ذلك بأن البوليمرات المُحضرة بالجذرالحر قد لا تتناسب مع العديد من التطبيقات بينما لابأس بها في تطبيقات صناعية معينة. كذلك تحِد منتحضير بوليمرات بتراكيب مختلفة قد يكون لها تطبيقات جديدة أو تساهم بشكل أفضل. 

 البلمرة الحية "Living polymerization"

قبل عام ١٩٥٦ لم يكون هنا سوى بلمرة بالجذر الحر وكان يُعتقد بأن التحكم بالجذور الحرة أمر ميؤس منه ويجب تقبل هذه الحقيقة حتى قام العالم Szwarc بتحضير بولي ستايرين بواسطة البلمرة الأنيوية.تختلف ��لبلمرة الأنيونية عن البلمرة بالجذر الحر ليس فقط بنوع البادئة و أن الذي يهاجم الرابطة المضاعفة وينمي السلسلة البوليمرية هو الشحنة السالبة بدل الجذرالحر بل كذلك لا تتواجد أي عملية من عمليات الإنهاء السابق ذكرها و بالتالي فأن السلاسل البوليمرية تنمومعاً و بنفس الطول تقريباً ولذلك قيمة PDI تقترب إلى١ بدل من إن تكون عالية مثل البلمرة بالجذر الحر. 

( ملاحظة قيمة PDI هي ناتج قسمة Mw على Mn وكلما إقتربت من الواحد يعني بأن جميع السلاسل لها نفس الوزن الجزيئي و أطول. في الجزيئات الصغيرة لأن أوزانها الجزيئية واحدة تكون PDI لهم تساوي ١دائماً).

أُعتبرت هذه الطريقة ثورة في مجال البوليمر وتمتحضير العديد من البولميرات بالبلمرة الكاتيونية والأنيونية لمونمرات كانت تتبلمرة بالجذر الحر ودراسة إختلاف خصائصها عند بلمرتها بالجذر الحر وبالبلمرة الحية. 

لذلك كان التساؤل كيف يتم تطبيق مفهوم living polymerization على الجذور بدل الشحنات ؟ لأن الشحنات المتشابهة تتنافر وليست بنشاطية الجذرالعالية لذلك عملية الإنهاء إحتماليتها قليلة خلال البلمرة والسلاسل متساوية تقريباً. ومن هذا التساؤل ظهر مايُسمى بـ البلمرة الحية "المتحكمة" للجذر (Controlled “living” radical polymerization). 

البلمرة الحية للجذر " Living radical polymerization "

العامل المهم في البلمرة الحية هو التقليل قدر الإمكان من تركيز الجذر الحر. فكما ذُكر سابقاً بأن عملية الإنهاء تعتمد على تركيز الجذر الحر وإلا فأن السلاسل البوليمرية ستتعرض للتثبيط و التوقف عن التمدد.والهدف في البلمرة الحية للجذر هو ألا تتوقف السلاسل البوليمرية بالنمو. لذلك يتم إيقاف بعض السلاسل النشطة مؤقتاً و ترك البعض الآخر ينمو والعملية بينتوقف نشاط الجذر مؤقتاً و تنشيطه عملية عكسية هدفها التقليل من تركيز الجذور الحر. 

 هناك طرق كيميائية عديدة يُستخدم فيها البلمرة عن طريق التحكم بالجذور الحرة للحصول على بوليمرات لها خصائص مشابهة لتلك المُحضرة بالبلمرة الكاتيونية أو الأنيونية. من أوائل و أشهر  تلك الطرق إستخداماً :

  Nitroxide mediated polymerization(NMP) 

هذه الطريقة يُستخدم فيها جذر NO• يقوم هذا الجذربالتفاعل مع جذر بوليمري نشط فيثبطه ثم عند زياد ةدرجة الحرارة تنكسر الرابطة معطية جذر NO• وسلسلة بوليمرية نشطة جذرياً تستمر بمهاجمة المونمرات النمو. هذه الطريقة تسمح بالتحكم بالجذرالحر لكن من عيوبها إستهلاك طاقة حرارية. 

Reversible addition-fragmentation chain transfer (RAFT) 

عامل RAFT هو جزيء يحتوي على مجموعة وظيفية تُسمى thiocarbonylthio  يحتوي  هذا العامل على ذرتي كبريت أحداهما مكونة رابطة مضاعفة مع ذرة كربون مرتبطة بعامل ثابت Z والأخرى مرتبطة بنفس الكربون برابطة أحاديةومرتبطة بمجموعة R. تقوم سلسلة بوليمرية نشطة بالهجوم على الرابطة S=C مكونة جذر على ذرة الكربون التي تقوم بكسر الرابطة من ذرة الكبريت الأخرى مكونة رابطة مضاعفة على الجهة المقابلة وجذر نشط يهاجم المونمرات لتكوين سلسلة بوليمرية وهكذا. عند الهجوم على هذا العامل فأن الجذرالكربوني و الرنين الحاصل تختلف ثباتيته بإختلاف المجموعة Z لذلك تختلف هذه المجموعة بإختلاف نوع المونمر و المذيب المُستخدمين. 

Atom transfer radical polymerization(ATRP) 

تُعتبر طريقة ATRP مُستوحاة من تفاعل atom transfer radical addition (ATRA) وفيها تتم إضافة هاليد الألكيل إلى مجموعة الكين مكونة ناتج هاليد الكيل ذو وزن جزيئي أكبر من الهاليد الالكيل المتفاعل. 

في طريقة ATRP يتم إستخدام هاليد الألكيل كبادئة وبهذه الطريقة يجب إستهلاك البادئة كلياً بالبداية وإلا فأن الوزن الجزيئي يُصبح أكبر من النظري. يتم تنشيط البادئة و إنتاج جذر منها عن طريق حفاز معدن. من مزايا المعدن الحفاز أن يسهل عليه التحول من حالة أكسدة n إلى حالة أكسدة بعد نزع الهاليد أخرى n+1وبذلك يدخل هذا المعدن بحالتي الأكسدة في تفاعل عكسي مع سلسلة البوليمر النشطة بالجذر و نظيرها المثبط بالهاليد القادم من المعدن بحالة الأكسدة العالية.في الغالب يتم إستخدام النحاس Cu كحفاز بسبب وفرته العالية و رخصه مقارنة بمعادن أخرى كالروثينيوم. و يُعتبر مُفضل مقارنة بالنحاس لأنه أعلى نشاطاً منه. في ميكانيكية ATRP يكون النحاس ذوالأكسدة المُنخفضة CuX

هو المنشط ودوره هو نزع الهاليد من الكربون نزعاً متساوياً بحيث ينتج جذر ألكيلي نشط يُهاجم الرابطة المضاعفة بالمونمر و يكون سلسلة نشطة و معدن بحالة الأكسدة العالية CuX2. ثم يتم تثبيط الجذرالنشط بالمعدن ذو الأكسدة العالية CuX2 وهو المثبطب واسطة الهاليد ليتكون CuX و P-X. في هذه العملية العكسية يوجد ثابتين وهما ثابت التنشيط Kact و ثابت التثبيط Kdeact وللحصول على بلمرة حية مُتحكمة لابد يكون  Kdeact > Kact  لتقليل تركيز الجذر النشط و تجنب عملية الإنهاء. وبهذه الطريقة كلما زادت نسبة تحويل المونمر تقل قيمة PDI وهي المطلوبة لتكون البلمرة حية و متحكمة "Controlled". 

يتم معرفة البلمرة إذا كانت حية أو جذر حر حركياً عن طريق رسم منحنى ln(M0/M) مقابل الزمن فتكون العلاقة خطية إذا كانت البلمرة تفاعل من الدرجة الأولى معتمداً على تركيز المونمر أما إذا المعدل يعتمد على تركيز الجذر أيضاً فأن البلمرة تكون غير مُسيطرة و تصبح جذر حر. أحياناً يحصل إنحراف طفيف في العلاقة يكون سببه كفاءة البادئة نفسها لأنها لا تُستهلك بشكل كلي بسرعة في البداية. 

هذه الطرق مكنت المتخصصين بالبوليمر من تحضيربوليمرات بأشكال هندسية مختلفة و تكوين بوليمرات بتراكيب محددة و مختلفة. بالإضافة إلى الحصول على مجاميع بنهاية و بداية السلسلة تكون مطلوبة لتطبيقات معينة أو من الممكن تعديلها للحصول على مجاميع أخرى تُساعد في توظيف السلسلة البوليمرية بطرقأخرى. على سبيل المثال، بطريقة ATRP يتم الحصول على هالوجين بآخر السلسلة يمكن إستبداله بمجموعة أزيد و ربط البوليمر بجسيمات نانوية تمتلك مجموعات الكاين على سطحها بواسطة Click reaction. 

REFERNCE

https://pubs.acs.org/doi/abs/10.1021/acs.chemrev.5b00671

https://www.sciencedirect.com/science/article/pii/S0079670007000044

https://www.cmu.edu/maty/chem/fundamentals-atrp/index.html

[rubberandplasticproducts]

Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies – Polymer Chemistry Blog

Paper of the month: Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies 02 Dec 2019 Chang et al. employ addition-fragmentation chain transfer to generate well-defined block copolymers. The use of a polymethylmethacrylate (PMMA) containing an unsaturated chain end as a macroinitiator during reversible complexation mediated polymerization has been previously reported by Goto and coworkers. Typically, such macroinitiators can also be used as macromonomers to generate branched polymers via propagation. In this work, Goto and co-workers elegantly demonstrate that the occurrence of addition-fragmentation chain transfer and propagation strongly depends on the temperature during the polymerization of styrene. Through carefully monitoring the kinetics of the polymerization of styrene, the authors discovered that propagation is predominant below 60 ̊C, consistent with previous reports. However, upon elevating the temperature (e.g. 120 ̊C), addition-fragmentation chain transfer dominates instead. This discovery then allowed access to the efficient synthesis of block copolymers with PMMA and polystyrene at high temperatures. Importantly, addition-fragmentation chain transfer was also predominant over propagation during the polymerizations of acrylonitrile and acrylates yielding well-defined block copolymers. PMMAs with different molecular weights were also investigated and the polymerization was controlled utilizing iodine transfer polymerization for styrene and reversible complexation mediated polymerization for the other monomers. Such an approach is highly advantageous due to the ease of the operation and it is expected to be a practical alternative for efficient block copolymer synthesis. Tips/comments directly from the authors: The proper purification of polymers and the careful NMR analysis were important for obtaining the accurate kinetic data. The kinetic study provided a useful idea enabling the synthesis of block copolymers of PMMA with polystyrene (PSt). Block copolymers of PMMA with PSt, polyacrylonitrile, and polyacrylates are accessible. Relatively high monomer conversions are achievable. Not only the isolated alkyl iodide but also the alkyl iodide in situ generated from iodine (I2) and azo compound can effectively be used as the initiating dormant species. The in situ method is less expensive and robust and hence can be a practically attractive Read the full article now for FREE until 10th January! Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies, Polym. Chem. , 2019, 10, 5617-5625, DOI: 10.1039/c9py01367a About the web writer Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

[healthtimetaylor]

Chrysin and docetaxel loaded biodegradable micelle for combination chemotherapy of cancer stem cell.

PMID:  Pharm Res. 2019 Oct 23 ;36(12):165. Epub 2019 Oct 23. PMID: 31646391 Abstract Title:  Chrysin and Docetaxel Loaded Biodegradable Micelle for Combination Chemotherapy of Cancer Stem Cell. Abstract:  PURPOSE: Cancer stem cells (CSCs) have been suggested to represent the main cause of tumour progression, metastasis and drug resistance. Therefore, these cells can be an appropriate target to improve cancer treatment.METHODS: A novel biodegradable brush copolymeric micelle was synthesized by the ring-opening polymerization (ROP) and reversible addition-fragmentation chain transfer (RAFT) polymerization. The obtained micelle was used for co-delivery of the anticancer drug docetaxel (DTX) and Chrysin (CHS) as an adjuvant on the CSCs originated from Human colon adenocarcinoma cell line. Cancer stem cells were enriched by MACS technique and characterized by flow cytometry analysis against CD133 marker.RESULTS: Data demonstrated that the micelles harbouring DTX@CHS had potential to reduce cancer stem cell viability compared to free DTX@CHS, single-drug formulations and the control group (p 

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[twitterfaves]

From Twitter: Light-induced self-assembly of cats using visible light mediated photoinduced electron transfer–reversible addition–fragmentation chain transfer polymerization pic.twitter.com/b6aJndIbgI— Sylvain ❄️👨🏻‍🎓 (@DevilleSy) August 30, 2019

http://twitter.com/DevilleSy

[whats-in-a-sentence]

It enables the creation of polymer chains that are more uniform in molecular mass (i.e. low polydispersity, see figure 26.21), which means that such polymers exhibit more refined properties.

"Chemistry" 2e - Blackman, A., Bottle, S., Schmid, S., Mocerino, M., Wille, U.

[materialsscienceandengineering]

Ultraheavy precision polymers

An environmentally friendly and sustainable synthesis of 'heavyweight' polymers with very narrow molecular weight distributions is an important concept in modern polymer chemistry. Thanks to a new photoenzymatic process, Chinese researchers have been able to increase the range of possible monomers. As reported in the journal Angewandte Chemie, the researchers were able to obtain well-defined linear and star-shaped polymers with ultrahigh molecular weights.

Because many polymer properties depend heavily on molecular weight, it is desirable to have as narrow a molecular weight distribution as possible. Precision polymers with ultrahigh molecular weights (> 1 t/mol) would be interesting candidates for high-performance elastomers, low-concentration hydrogels, photonic materials, durable coatings, and flocking agents. However, such heavyweight polymers are not easy to produce with a uniform distribution of molecular weights. The radical polymerizations in widespread use are especially difficult to control in this respect. Modern methods, such as RAFT polymerization (RAFT: reversible addition-fragmentation chain transfer) offer a significantly higher degree of control by keeping the concentration of reactive radicals very small. A special agent reacts reversibly with the growing polymer chains to form a nonradical species. Whenever the intermediate dissociates, new active radicals are formed. This slows the reaction and results in longer, more uniform polymer chains.

Read more.

[itsmedicinesfakianakis]

Thermodynamic Parameters of Temperature-Induced Phase Transition for Brushes onto Nanoparticles: Hydrophilic versus Hydrophobic End-Groups Functionalization

Quantification of the stimuli-responsive phase transition in polymers is topical and important for the understanding and development of novel stimuli-responsive materials. The temperature-induced phase transition of poly(N-isopropylacrylamide) (PNIPAm) with one thiol end group depends on the confinement—free polymer or polymer brush—on the molecular weight and on the nature of the second end. This paper describes the synthesis of heterotelechelic PNIPAm of different molecular weights with a thiol end group—that specifically binds to gold nanorods and a hydrophilic NIPAm end group by reversible addition-fragmentation chain-transfer polymerization. Proton high-resolution magic angle sample spinning NMR spectra are used as an indicator of the polymer chain conformations. The characteristics of phase transition given by the transition temperature, entropy, and width of transition are obtained by a two-state model. The dependence of thermodynamic parameters on molecular weight is compared for hydrophilic and hydrophobic end functional-free polymers and brushes.

Thermodynamic parameters of the temperature-induced phase transition of heterotelechelic poly(N-isopropylacrylamide) brushes grafted onto gold nanorods are studied by 1H high-resolution magic-angle sample spinning NMR spectroscopy and compared to free polymer in aqueous solution. Transition temperature, entropy, and width of transition demonstrate remarkable differences depending on the molecular weight and polarity of the free, terminal end groups.

from # All Medicine by Alexandros G. Sfakianakis via alkiviadis.1961 on Inoreader http://ift.tt/2ik4Zhw from OtoRhinoLaryngology - Alexandros G. Sfakianakis via Alexandros G.Sfakianakis on Inoreader http://ift.tt/2ihWZNY

[rubberandplasticproducts]

Benchtop flow-NMR for rapid online monitoring of RAFT and free radical polymerisation in batch and continuous reactors – Polymer Chemistry Blog

Paper of the month: Benchtop flow-NMR for rapid online monitoring of RAFT and free radical polymerisation in batch and continuous reactors 30 Sep 2019 Knox et al. utilize a benchtop flow-NMR for rapid online monitoring of a range of polymerisation methodologies. To precisely engineer macromolecular materials, close monitoring of the polymerization progress is required. Therefore, real-time online monitoring provides polymer chemists the opportunity to accurately observe and optimize their reactions. To this end, Warren and co-workers utilized benchtop flow-nuclear magnetic resonance (NMR) as a very convenient and powerful tool for real-time monitoring of polymers synthesized either by controlled radical polymerization or free radical polymerization protocols. In particular, reversible addition-fragmentation chain-transfer (RAFT) polymerization was employed to polymerize acrylamides giving very high conversions in less than 10 minutes and the kinetic profile of this reaction was efficiently captured. In a second example where RAFT dispersion polymerization was monitored. In spite of the rapid polymerization rates, high temporal resolution enabled the previse determination of the onset of rate acceleration usually observed for polymerization induced self-assembly (PISA) systems. In addition to the monitoring of the aforementioned complex systems, the free radical polymerization of methyl methacrylate was also studied. In this case, the linear semi-logarithmic plot indicated the expected pseudo-first order kinetics. The results discussed here demonstrate the power of using benchtop NMR spectrometers for online flow applications where both controlled and free radical polymerizations can be employed. It is the author’s opinion that the lower price of these instruments will improve access to NMR spectroscopy while the reduced sample preparation/time taken for analysis will increase research output. Tips/comments directly from the authors: Despite the reduced field strength, detailed polymerization kinetics comparable to traditional ‘high field’ NMR can be obtained since the vinyl protons are easily resolved. Flow-NMR is a powerful tool to improve time-resolution and reduce lab workload but must be used with care – e.g. flow rate and sample cell geometry must be optimized. Hydrogenated solvents can be used with lower-field instruments, but solvent selection is important: minimising any potential solvent overlap is key to reliable data. Spectral corrections such as to the phase and baseline are crucial for reliable data – especially if using an automated system. Read the full article now for FREE until 8th November! Benchtop flow-NMR for rapid online monitoring of RAFT and free radical polymerisation in batch and continuous reactors,  Polym. Chem. , 2019, 10, 4774-4778, DOI: 10.1039/C9PY00982E About the web writer Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

[rubberandplasticproducts]

Ab initio RAFT emulsion polymerization mediated by small cationic RAFT agents to form polymers with low molar mass dispersity – Polymer Chemistry Blog

Stace et al. employ small cationic RAFT agents to produce low dispersity polymers in ab initio emulsion polymerization. Reversible addition fragmentation chain transfer (RAFT) polymerization has revolutionized the field of polymer chemistry providing access to a wide range of materials with controlled molecular weight, functionality, end-group fidelity and dispersity. In their current contribution, the groups of Moad, Keddie and Fellows joined forces to report a range of low molar mass cationic RAFT agents that allow for predictable molecular weight and dispersity in ab initio emulsion polymerization. In particular, upon utilizing the protonated RAFT agent ((((cyanomethyl)thio)carbonothioyl)(methyl)amino)pyridin1-ium toluenesulfonate and the analogous methyl-quaternized RAFT agents, 4-((((cyanomethyl)thio) carbonothioyl)(methyl)amino)-1-methylpyridin-1-ium dodecyl sulfate, styrene could be efficiently polymerized yielding polystyrene with narrow molecular weight distributions (Đ m 1.2–1.4). The authors attribute the success of ab initio emulsion polymerization with the former RAFT agent to the hydrophilicity of the pyridinium group which allows for the predominant partition of the water-soluble RAFT agent into the aqueous phase.  The RAFT agent also gives minimal retardation. In addition, by employing 4-((((cyanomethyl)thio) carbonothioyl)(methyl)amino)-1-methylpyridin-1-ium dodecyl sulfate, a “surfactant-free” RAFT emulsion can be achieved producing a low Đ m  polystyrene although the RAFT end-group was lost upon isolating the polymer. Additional preliminary experiments were also performed demonstrating that this class of RAFT agents can be broadly applicable in ab initio emulsion polymerization of a range of other more-activated monomers including acrylates and methacrylates producing low dispersity polymers while the polymerization of less activated monomers such as vinyl acetate showed good control over the molecular weight, albeit broader molecular weight distributions. The authors are currently investigating such systems to establish their full utility in emulsion polymerization and develop robust and scalable conditions for the formation of block copolymers. Tips/comments directly from the authors: There are two significant challenges in implementing successful ab initio emulsion polymerization in a high throughput platform such as the Chemspeed® Devising a protocol for vortexing/agitating so as to form, and then maintain, a stable latex. The protocol reported was the end-result of many experiments. Degassing the reaction medium. RAFT polymerization can be successfully carried out in non-degassed media.  However, for good reproducibility, optimal dispersity, high end group fidelity and acceptable polymerization rates, degassing remains important.  In conducting experiments on the Chemspeed®, it is important to make sure the media to be dispensed by the robot are degassed, and that all of the solvent lines, and the solvent used to prime and wash the syringe needles are degassed. Read the full article for FREE until 6th December! Ab initio RAFT emulsion polymerization mediated by small cationic RAFT agents to form polymers with low molar mass dispersity, Polym. Chem., 2019, 10, 5044-5051, DOI: 10.1039/C9PY00893D About the Web Writer Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

[rubberandplasticproducts]

Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies – Polymer Chemistry Blog

Paper of the month: Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies 02 Dec 2019 Chang et al. employ addition-fragmentation chain transfer to generate well-defined block copolymers. The use of a polymethylmethacrylate (PMMA) containing an unsaturated chain end as a macroinitiator during reversible complexation mediated polymerization has been previously reported by Goto and coworkers. Typically, such macroinitiators can also be used as macromonomers to generate branched polymers via propagation. In this work, Goto and co-workers elegantly demonstrate that the occurrence of addition-fragmentation chain transfer and propagation strongly depends on the temperature during the polymerization of styrene. Through carefully monitoring the kinetics of the polymerization of styrene, the authors discovered that propagation is predominant below 60 ̊C, consistent with previous reports. However, upon elevating the temperature (e.g. 120 ̊C), addition-fragmentation chain transfer dominates instead. This discovery then allowed access to the efficient synthesis of block copolymers with PMMA and polystyrene at high temperatures. Importantly, addition-fragmentation chain transfer was also predominant over propagation during the polymerizations of acrylonitrile and acrylates yielding well-defined block copolymers. PMMAs with different molecular weights were also investigated and the polymerization was controlled utilizing iodine transfer polymerization for styrene and reversible complexation mediated polymerization for the other monomers. Such an approach is highly advantageous due to the ease of the operation and it is expected to be a practical alternative for efficient block copolymer synthesis. Tips/comments directly from the authors: The proper purification of polymers and the careful NMR analysis were important for obtaining the accurate kinetic data. The kinetic study provided a useful idea enabling the synthesis of block copolymers of PMMA with polystyrene (PSt). Block copolymers of PMMA with PSt, polyacrylonitrile, and polyacrylates are accessible. Relatively high monomer conversions are achievable. Not only the isolated alkyl iodide but also the alkyl iodide in situ generated from iodine (I2) and azo compound can effectively be used as the initiating dormant species. The in situ method is less expensive and robust and hence can be a practically attractive Read the full article now for FREE until 10th January! Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies, Polym. Chem. , 2019, 10, 5617-5625, DOI: 10.1039/c9py01367a About the web writer Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

[itsmedicinesfakianakis]

A Useful Method for Preparing Mixed Brush Polymer Grafted Nanoparticles by Polymerizing Block Copolymers from Surfaces with Reversed Monomer Addition Sequence

The preparation of well-defined block copolymers using controlled radical polymerization depends on the proper order of monomer addition. The reversed order of monomer addition results in a mixture of block copolymer and homopolymer and thus has typically been avoided. In this paper, the low blocking efficiency of reversed monomer addition order is utilized in combination with surface initiated reversible addition−fragmentation chain-transfer polymerization to establish a facile procedure toward mixed polymer brush grafted nanoparticles SiO2-g-(PS (polystyrene), PS-b-PMAA (polymethacrylic acid)). The SiO2-g-(PS, PS-b-PMAA) nanoparticles are analyzed by gel permeation chromatography deconvolution, and the fraction of each polymer component is calculated. Additionally, the SiO2-g-(PS, PS-b-PMAA) are amphiphilic in nature and show unique self-assembly behavior in water.

This paper describes the utilization of the low blocking efficiency of reversed monomer addition order (i.e., the “wrong order”) in combination with surface initiated reversible addition−fragmentation chain-transfer polymerization to establish a facile procedure toward mixed polymer brush grafted nanoparticles SiO2-g-(PS, PS-b-PMAA). Additionally, the SiO2-b-(PS, PS-b-PMAA) nanoparticles are amphiphilic in nature and show unique and controllable self-assembly behavior in water.

from # All Medicine by Alexandros G. Sfakianakis via alkiviadis.1961 on Inoreader http://ift.tt/2w2s0vn from OtoRhinoLaryngology - Alexandros G. Sfakianakis via Alexandros G.Sfakianakis on Inoreader http://ift.tt/2wWpcwb